Vibrio cholerae O1.

Vibrio cholerae can be classified according to O group: strains in O group 1 (V. cholerae O1) cause cholera, whereas strains in other O groups (non-O1 V. cholerae) are generally associated with milder illness. The virulence of V. cholerae O1 is determined primarily by the presence of a protein enterotoxin, cholera toxin (CT). Strains that do not produce cholera toxin (i.e., are not toxigenic) tend to be avirulent or have reduced virulence (Morris et al., 1984). Persons with the most severe forms of cholera (cholera gravis) have profuse, watery diarrhea (Pierce and Mondal, 1974). The volume of diarrhea may exceed 1 liter per hour, resulting in rapid water depletion, circulatory collapse, and (if untreated) death. Fortunately, cholera gravis is relatively uncommon. In infections with V. cholerae O1 of the classical biotype, four or five inapparent infections or mild illnesses may occur for every apparent case. In infections with El Tor (the biotype present in the United States), 25-100 other infections may be expected for every hospitalized case (Bart et al., 1970).

It has traditionally been thought that strains of V. cholerae O1 were transmitted by fecal contamination of food or water, and this mode of transmission likely predominates in developing countries. However, there is increasing evidence that free-living strains of V. cholerae O1 have become established in the U.S. Gulf Coast environment and may be transmitted to man via consumption of raw, undercooked, or cross-contaminated shellfish (Morris and Black, 1985). The number of cases of cholera associated with these strains is relatively small (in the range of 50 cases since 1973); their significance lies in their potential for causing severe disease in otherwise healthy hosts.

Epidemiology and risk assessment

Epidemiologic investigations have associated V. cholerae O1 illness with eating crabs, shrimp, and raw oysters harvested along the Gulf Coast (Blake et al., 1980). The CDC (1989) reported three outbreaks of V. cholerae O1 involving 16 cases between 1978 and 1987 (Tables 3-2 – 3-4), and NETSU (Rippey and Verber, 1988) reported 13 cases in this same period (Tables 3-2 and 3-3). Based on these and other reports in the literature, at least 50 cholera cases appear to have been acquired in the United States since 1973 when the first recent indigenous U.S. case was reported (Blake et al., 1980; Lowry et al., 1989b; Morris and Black, 1985).

Toxigenic V. cholerae O1 has been isolated from estuarine water, from fresh shrimp, and from cooked crabs. All U.S. V. cholerae O1 isolates have been hemolytic, biotype El Tor, serotype Inaba, and all have had the same unique HindIII digest pattern on Southern blot analysis. Because of the occurrence of cases over a period of years and in a variety of locations, many authors suggest that this strain has become endemic along the Gulf Coast (Morris and Black, 1985).

Although toxigenic V. cholerae O1 strains are known to be present in Gulf Coast estuaries, the percentage of shellfish that carry the organism appears to be quite low. In one study conducted by the FDA, V. cholerae O1 was isolated from 0.9% of 790 oyster lots sampled over 12 months. None of the V. cholerae O1 strains isolated was able to produce cholera toxin (Twedt et al., 1981).

Seroepidemiologic studies provide an alternative means of assessing disease burden/risk of infection, particularly for microorganisms such as V. cholerae O1 that produce a high percentage of asymptomatic infections. In a study conducted along the Gulf Coast in Texas (Hunt et al., 1988), 0.89% of persons sampled had elevated titers of both vibriocidal and anticholera toxin antibodies, the standard serologic assays used for V. cholerae infections. These assays have relatively low specificity, making the interpretation of results difficult. However, these data do raise the possibility that a small percentage of persons living along the U.S. Gulf Coast has recently been infected with toxigenic strains of V. cholerae O1 (Hunt et al., 1988).

Disease control

Because V. cholerae appears to contaminate marine animals in situ, it must be destroyed by treatment of the food. For crustacean seafood, proper cooking in primary processing (crab) or at the food service level (shrimp) and avoidance of recontamination of cooked product are recommended (Shultz et al., 1984). Studies conducted by CDC indicate that large 0 for less than 8 minutes or steamed for less than 25 minutes may still contain viable V. cholerae organisms (Blake et al., 1980), an observation that has led to a series of recommended time and temperature conditions for cooking crabs.

Toxigenic V. cholerae O1 strains can be identified rapidly in shellfish by using deoxyribonucleic acid (DNA) probes. A monitoring system that employs such probes might be useful in identifying potential "high-risk" harvesting areas. However, without further studies it is unclear how such data should be used. Given the low frequency with which toxigenic V. cholerae O1 are found, it would be difficult to justify embargoing all shellfish from a given area based solely on a positive sampling result. Using probes to screen shellfish on a lot-by-lot basis would be of little value, because results from a single crab or oyster are unlikely to be representative of the lot as a whole. Finally, epidemiological data are lacking to show how changes in the frequency of isolation of V. cholerae O1 from shellfish correlate with changes in disease occurrence in the community.

Non-O1 Vibrio cholerae ( V. cholerae of O Groups Other Than 1)

Non-O1 V. cholerae strains are ubiquitous in estuarine environments (including bays and estuaries of the U.S. Gulf, Atlantic, and Pacific coasts) and are commonly isolated from shellfish. In one study conducted by the FDA, non-O1 V. cholerae was isolated in up to 37% of U.S. oyster lots harvested during warm summer months (Twedt et al., 1981). Non-O1 V. cholerae has been associated with gastroenteritis, wound and ear infections, and septicemia (Hughes et al., 1978; Safrin et al., 1988). Gastroenteritis can occur in normal, healthy persons (Morris et al., 1990). Septicemia appears to occur primarily in persons who are immunocompromised or who have underlying liver disease; the mortality rate for persons with septicemia exceeds 50% (Safrin et al., 1988). Persons who have acquired non-O1 V. cholerae infections in the United States have almost all given a history of having eaten raw oysters before the onset of illness (Morris et al., 1981). However, the number of reported cases of non-O1 V. cholerae gastroenteritis is relatively low, suggesting that only a minority of strains are able to infect humans, or that most infections result in mild or asymptomatic illness.

Epidemiology and risk assessment

In the NETSU data base (Table 3-2), non-O1 V. cholerae is the most common bacterial cause of molluscan shellfish-associated illness. Still, only 120 cases were reported between 1978 and 1987 (Rippey and Verber, 1988). The CDC (1989) reported only two shellfish-associated outbreaks involving 11 cases during the same period (Tables 3-2 and 3-3). Given the amount of raw shellfish consumed and the frequency with which the organism is present in shellfish, the number of reported cases of non-O1 disease in the United States is much less than might be anticipated. Seven coastal area hospitals in four southern states isolated only seven specimens of non-O1 V. cholerae (including five isolates from a single outbreak) from approximately 11,000 stool cultures performed with the use of thiosulfate-citrate-bile salts-sucrose (TCBS) agar, an appropriate selective culture medium (Morris and Black, 1985). Only two non-O1 V. cholerae specimens were isolated from over 10,000 stool cultures on TCBS performed during 14 years in a Chesapeake Bay area hospital (Hoge et al., 1989).

This small number of reported cases is probably a reflection of several factors. It is likely that the majority of environmental strains lack the necessary colonization factors, appropriate toxins, or other virulence determinants to cause human disease (Morris et al., 1990). Even when infections occur, patients may often be asymptomatic or have only mild illness and, consequently, not come to medical attention. In support of the latter hypothesis, non-O1 V. cholerae strains were isolated from 13 (2.7%) of 479 persons in a cohort of physicians attending a convention in New Orleans in late September; among persons eating raw oysters, 4% had non-O1 V. cholerae in their stool. However, despite this relatively high colonization rate, only 2 (15%) of the 13 culture-positive persons were symptomatic, comparable to the overall 14% rate of diarrhea reported in the entire cohort (Lowry et al., 1989a).

Disease control

There are currently no programs attempting to limit exposure of the general population to non-O1 V. cholerae, other than recommendations that persons who are immunocompromised or who have underlying liver disease avoid the consumption of raw oysters (Blake, 1983). Exposure to non-O1 strains could be reduced significantly if oyster harvesting was confined to colder months when Vibrio counts in water are lowest (Twedt et al., 1981).

If it is assumed that most environmental non-O1 V. cholerae strains are nonpathogenic, identification of these strains in oysters has limited public health utility. It would clearly be of value if pathogenic strains could be differentiated from those that are nonpathogenic. Basic research in this area should be encouraged.

Epidemiology and risk assessment

V. parahaemolyticus is a common marine isolate, with isolation reported from water, sediment, suspended particulates, plankton, fish, and shellfish (Joseph et al., 1983). However, it is likely that only a small fraction of marine isolates are potentially pathogenic. For example, in a study by Thompson and Vanderzant (1976), only 4 of 2,218 isolates from Galveston Bay were able to produce the heat-stable hemolysin generally associated with virulence.

In Japan, V. parahaemolyticus has been implicated as the etiologic agent in 24% of reported cases of food-borne disease (Miwatani and Takeda, 1976). In the United States, V. parahaemolyticus has caused several major food-borne disease outbreaks (Barker, 1974). The CDC food-borne surveillance data from 1978 to 1987 (Tables 3-2 and 3-3) reported V. parahaemolyticus as the cause of 15 outbreaks associated with 176 cases of mostly crustacean shellfish-associated illness (CDC, 1989). No other seafood-associated illnesses from V. parahaemolyticus were reported to CDC during this time; NETSU reported 52 cases in the same period (Rippey and Verber, 1988) (Tables 3-2 and 3-3).

Outbreaks of V. parahaemolyticus have often been associated with cross-contamination or time/temperature abuse of cooked seafood. Although sporadic cases associated with the consumption of raw oysters have occurred, there does not appear to be the same strong association with consumption of raw oysters as reported for non-O1 V. cholerae. As with non-O1 V. cholerae, estimates of incidence are complicated by the need for TCBS selective media for isolation of the organism from stool cultures. In areas and studies in the United States where TCBS has been used routinely, V. parahaemolyticus has been the Vibrio species isolated most frequently from stool samples, with an isolation rate two to three times that reported for non-O1 V. cholerae (Bonner et al., 1983; Hoge et al., 1989; Lowry et al., 1989a). However, the overall disease incidence remains quite low, even in communities where shellfish consumption would be expected to be high. In a hospital-based study in Annapolis, Maryland, there was estimated to be less than 0.5 case/100,000 population/year (Hoge et al., 1989).

Disease control

The impact of major food-borne outbreaks due to V. parahaemolyticus can be limited by emphasizing avoidance of cross-contamination and time/temperature abuse of cooked shellfish or of seafood that will be consumed raw. Illness due to V. parahaemolyticus is typically associated with multiplication of the organism to high numbers on the food either in the raw state or after cooking. Seafood eaten shortly after adequate cooking is generally safe (Bradshaw et al., 1974). Control has thus been aimed at ensuring good handling and processing practices whereby the seafood is held at low temperature (<5°C) and recontamination of the cooked product is avoided.

Specific counts of V. parahaemolyticus on seafoods have been proposed as an index of hazard, but this method has not been widely used because of cost, lack of specificity for pathogenic strains, and time lag (3-5 days) between sampling and results.

The DNA probes for identification of hemolytic (Kanagawa-positive) strains are available (Nishibuchi et al., 1985). With nonradioactively labeled oligonucleotide probes it may be possible to identify potentially pathogenic strains within a matter of hours. However, no effort has been made to make such probes commercially available or to assess their utility as public health tools.

Epidemiology and risk assessment

Like other Vibrio species, V. vulnificus appears to be part of the normal bacterial flora of estuaries along the U.S. Gulf, Atlantic, and Pacific coasts. Although data are limited, some studies reported that close to 100% of oyster lots harvested in some areas during warm summer months have been positive for V. vulnificus, and in one study, 11% of the blue crabs harvested from Galveston Bay during summer months had the organism in their hemolymph (Davis and Sizemore, 1982; Tamplin, 1990).

Occurrence of V. vulnificus-induced primary septicemia is significantly associated with eating raw oysters (Tacket et al., 1984). However, susceptibility appears to be limited to certain high-risk groups, particularly those with hemochromatosis, cirrhosis, hematologic or other disorders associated with immunosuppression, renal failure, and diabetes (Table 3-6). Wound infections are associated with contamination of wounds with seawater. Although wound infections can occur in otherwise healthy persons, deaths associated with wound infection have occurred almost exclusively among persons in the same high-risk groups mentioned above.

TABLE 3-6

Risk Factors for Primary Vibrio Septicemia.

Between 1978 and 1987, NETSU reported 100 cases of V. vulnificus infections (Tables 3-2, 3-3, and 3-7) (Rippey and Verber, 1988). V. vulnificus infections have not been reported to CDC because outbreaks involving two or more individuals have not occurred. The increased number of cases reported to NETSU in the last two years probably reflects better isolation methods and a more rigorous screening for the organism by public health laboratories. However, the mortality rate has remained high (Table 3-7).

TABLE 3-7

Vibrio vulnificus Cases Associated with Oysters, 1978-1987.

The incidence of V. vulnificus infections in coastal states appears to be 0.5 case/100,000 population/year, with primary septicemia accounting for approximately two-thirds of the cases. Based on reported cases, the incidence in the Florida panhandle was estimated to be 0.4 case/100,000/year (Klontz et al., 1988). Johnston et al. (1985) estimated an incidence of 0.8/100,000/year in Louisiana coastal parishes. The estimated incidence in a hospital-based study in Annapolis, Maryland was 0.5 case/100,000/year (Hoge et al., 1989). These estimates are clearly lower than might be expected, given the frequency with which the organism has been identified in shellfish and the amount of shellfish consumed in these areas. This may reflect differences in virulence among strains, the effect of infectious dose (i.e., the minimum number of organisms needed to cause disease), or the critical importance of host susceptibility in the disease process.

Disease control

The incidence of V. vulnificus infections associated with ingestion of raw oysters by high-risk individuals (Table 3-6) could be reduced by cooking the product before eating. If a dose-response relationship exists, disease incidence might also be reduced by reducing the counts of V. vulnificus in shellfish by restricting harvesting to times when vibrio counts in harvesting areas are low (i.e., when water temperatures are <20°C) or by monitoring vibrio counts carefully in summer months. A recent study by Cook and Ruple (1989) indicated increases of one to three orders of magnitude in subsequent storage of shellstock at above 20°C. Rapid cooling of shellfish after harvest may decrease the initial bacterial counts, and careful avoidance of time/temperature abuse during handling and shipment would limit subsequent multiplication of the organism (Cook and Ruple, 1989). Low-dose gamma irradiation of live shellstock and fresh or frozen seafood products has been shown to be extremely effective in the elimination of Vibrio species (Giddings, 1984; Kilgen et al., 1988). Rapid diagnostic techniques are available for V. vulnificus, based both on agglutination reactions with specific antisera (Simonson and Siebeling, 1986) and on DNA probes (Morris et al., 1987a). However, these techniques have not been used as regulatory tools, and their exact role in disease control remains to be determined.

An ability to differentiate potentially pathogenic strains from strains without recognized virulence would clearly be of help in designing appropriate control strategies (Simpson et al., 1987). Similarly, there is a need to better understand the interaction between the organisms and oysters, both before and after harvesting. A better understanding of the host response to V. vulnificus would allow more careful delineation of groups at high risk for infection (Table 3-6). An educational program defining the risks and explaining prevention by cooking and proper handling should be directed to this group through physicians, health organizations, and other public sources. Education of seafood producers, seafood processors and handlers, health professionals, and consumers regarding groups at high risk is also extremely important. Further studies in these areas must be encouraged as part of any overall plan for control of this pathogen.

Other Vibrio Species

Several other Vibrio species have been associated with human illness, including V. mimicus (Shandera et al., 1983), V. hollisae (Morris et al., 1982), V. fluvialis (Huq et al., 1980), and V. furnissii (Brenner et al., 1983) (Tables 3-2, 3-3, and 3-5). Illness from these species has generally been associated with seafood or shellfish consumption. However, the incidence of infection with these organisms appears to be quite low, and they are of much less importance as human pathogens than V. cholerae, V. parahaemolyticus, and V. vulnificus.

V. fluvialis, V. mimicus, and V. furnissii all appear to be naturally present in the marine environment. Infection with V. mimicus and V. hollisae has been associated with consumption of raw shellfish, especially raw oysters (Morris et al., 1982; Shandera et al., 1983). It is very difficult to estimate the incidence of infection with these organisms, because appropriate selective media for isolation are seldom used (except for V. hollisae, which may be best seen on blood agar) and because of difficulties with Vibrio taxonomy. Although they contain a number of inherent biases, data on the number of strains of each species sent to CDC for identification provide some basis for comparing relative frequencies of isolation. In a period during which CDC received 136 non-O1 V. cholerae isolates for identification, 39 V. mimicus isolates were received, 32 V. hollisae , 15 V. fluvialis, and 16 V. furnissii (Farmer et al., 1985). Between 1978 and 1987, NETSU (Rippey and Verber, 1988) reported five cases of V. mimicus, five cases of V. hollisae, and five cases of V. fluvialis (Tables 3-2 and 3-3).

The risk of infection with these organisms can likely be reduced by strategies similar to those used for other Vibrio species: that is, by using good manufacturing practices (GMPs) involving immediate and proper refrigeration, by possibly limiting shellfish harvesting to cooler months, by monitoring Vibrio counts in summer months for management purposes, by using low-dose irradiation for processing raw seafoods or live shellstock, and by advising persons in high-risk groups to avoid consumption of raw shellfish.

Trematodes

Only digenetic trematodes are known to produce disease in humans, and there are no data to suggest that any of these organisms represent a substantive health problem in the United States.

Clonorchis sinensis, or the Chinese liver fluke, is transmitted by eating raw or undercooked freshwater fish containing cyst stages of the organism. The organism is endemic in east Asia in a zone extending from Japan to Vietnam. Heterophyes, an intestinal fluke, is transmitted by eating fresh-or brackish-water fish (frequently mullet) in a raw, salted, or dried condition. The organism is a common parasite in the lower Nile valley near the Mediterranean coastline, and has been reported in the Orient and in western India. Metagonimus yokogawai , which closely resembles H. heterophyes, may be present in freshwater trout in the Orient, the maritime provinces of the USSR, northern Siberia, and the Balkans. Paragonimus westermani, the oriental lung fluke, is acquired by eating freshwater crabs or crawfish that are raw or have been pickled in brine, vinegar, or rice wine. The organism is endemic in east Asia, including Japan, Korea, the Philippines, and Taiwan.

Transmission of food-borne illness due to trematodes is prevented by adequate cooking and probably by freezing seafoods (Healy and Juranek, 1979).

Cestodes

Diphyllobothrium latum and D. pacificum are the only cestodes associated with parasitic infections from seafoods (Table 3-8). However, there are inadequate data to estimate their incidence in the United States. Cases have been reported among Jewish cooks who sampled gefilte fish during its preparation.

D. latum is acquired by eating raw freshwater fish containing appropriate plerocercoids (Healy and Juranek, 1979). The organism is indigenous throughout many parts of the USSR; the Baltic Sea countries; central and southeastern Europe; Lake N'gami, Africa; northern Manchuria and Japan; and New South Wales, Australia. In the Americas it is found in northern Minnesota, extensive areas of Canada and Alaska, and in the lakes of southern Chile and Argentina.

Control of disease caused by cestodes is dependent on thorough cooking of fish from endemic areas; cysts are also killed by freezing at -10°C for 24 hours (Healy and Juranek, 1979).

Nematodes

Disease from Anisakis simplex arises in humans when improperly cooked, smoked, or raw fish are eaten (Cheng, 1976). Anisakids are usually found in herring less than 12 centimeters (cm) long. Although the medical literature contains few reports of anisakiasis in the United States, over 3,000 cases are reported annually from Japan (Fontaine, 1985).

Whitish to clear larval worms approximately 1 cm long are found in the posterior body cavity of fish, coiled and encapsulated (McGladdery, 1986; Smith and Wooten, 1975). About 2-3% of the total bore into muscle. Some of these may be loosely encapsulated. Worms are usually seen on fresh fillets or about anal areas in the round of the fish. Up to 30% of European herring may be infected in a single sample, but estimates of total numbers with actual fillet infection are as low as 0.001%. It has been suggested that the prevalence of muscle infection is related to the intensity of body cavity infection. Fish with eight or fewer organisms in the body cavity seldom show fillet infection.

From 1958 to 1980, 13 cases were reported in the United States, for a mean of 0.59 case/year; however, 37 cases have been reported since 1980 (4.1 cases/year) (Deardorff et al., 1986; McKerrow et al., 1988).

Pseudoterranova dicipiens and Contraceacum closely resemble A. simplex . Both have been identified in pinnipeds in the Atlantic and northern Pacific. Cases in the United States associated with P. dicipiens have been reported on the West Coast, resulting from consumption of salmon and Pacific rockfish (Sebastes) (Kliks, 1983; Margolis, 1977; Myers, 1979).

The risk of infection from nematodes is reduced when fish are gutted soon after capture; salting may also decrease incidence. However, neither smoking nor light salting of fish normally affects larvae. Freezing at -20°C for 72 hours or heating above 55°C for 10 seconds kills adult parasites (Khalil, 1969). Because infection of fillets may be related to body cavity burdens, subsamples that cannot be immediately frozen or gutted could be checked for numbers of parasites. If the number exceeds 15, the fish should be banned for human consumption (Bier, 1976; Bier et al., 1987).

Other Nematodes

Angiostrongylus cantonensis is acquired by eating raw snails (the intermediate host for the organism). Crabs, prawns, and fish that have eaten infective snails can serve as transport hosts. Heavy infections cause an eosinophilic meningoencephalitis. Epidemics and sporadic infections occur most commonly in the South Pacific, Southeast Asia, and Taiwan (Alicata, 1988; Kliks et al., 1982). In recent years the parasite has spread to the Caribbean, and in 1986 it was reported in rat and snail populations in Puerto Rico (Anderson et al., 1986).

Gnathostoma spinigerum is acquired by eating raw or undercooked freshwater fish. The organism presents a clinical picture of larval migrans, a granulomatous lesion, or a stationary abscess. Infections have been reported from eastern and Southeast Asia, India, and Israel (Healy and Juranek, 1979).

Hepatitis Type A (Enterovirus Type 72)

Hepatitis A virus is a member of the family Picornaviridae. Of the nearly 200 human-adapted picornaviruses, 69 inhabit the enteric tract (Gerba, 1988; White and Fenner, 1986). Enteroviruses of the family Picornaviridae include the following species:

Of these enteroviruses, only HAV has been documented as a cause of seafood-associated illness, and has been isolated from infected seafoods and waters contaminated with human feces. For this reason, the other enteroviruses are not discussed here.

All of the enteroviruses are resistant to an acidic pH, proteolytic enzymes, and bile salts in the gut. Hepatitis A is less acid stable than other enteroviruses, but more heat stable, surviving at 60°C for 4 hours (White and Fenner, 1986). HAV is more chlorine resistant than indicator bacteria and the other enteroviruses, with the exception of Norwalk virus (Grabow et al., 1983; Keswick et al., 1985; Peterson et al., 1983). Hepatitis A is one of the most serious seafood-associated viral infections, causing a protracted and sometimes severe disease, but its mortality is only approximately 0.6%.

Hepatitis A virus is spread by the fecal-oral route. It is hyperendemic in countries that are overcrowded, and have inadequate sanitation and poor hygiene. Most infections in these communities occur in childhood and are subclinical. In more developed countries the disease is seen most often between the ages of 15 and 30 (Cliver, 1988; White and Fenner, 1986).

Contaminated food or water and person-to-person contact are the main routes of transmission of HAV. Each year, 20,000 to 30,000 cases are reported to the CDC. Of these cases, approximately 140 are known to be due to foods (0.5% of the total). Most of these food-borne outbreaks are due to inappropriate personal hygiene of infected individuals (Cliver, 1988). Outbreaks can also occur due to inadequate cooking of contaminated foods and human fecal contamination of drinking water supplies, swimming waters, and shellfish growing waters.

In the 1950s the first documented case of shellfish-associated HAV occurred in Sweden (Cliver, 1988). The first shellfish-associated case in the United States was documented in the 1960s (Cliver, 1988;, Gerba, 1988; Richards, 1985). Richards (1985) reported approximately 1,400 cases of molluscan shellfish-associated HAV from 1961 to 1984. Between 1978 and 1987, the CDC food-borne surveillance system (CDC, 1989) reported nine outbreaks involving 125 cases of seafood-related HAV (Tables 3-2–3-4); however, two of the outbreaks involving 92 cases were due to contamination from food handlers (Table 3-4); NETSU reported approximately 45 cases of shellfish-associated HAV in the same period (Rippey and Verber, 1988) (Tables 3-2 and 3-3). Richards (1985) noted that the incidence of molluscan shellfish-associated HAV has decreased in the last decade, although it is difficult to describe the trend of reported HAV infections in the United States with a single, unidirectional line. There is currently no valid indicator of human enteric viruses such as HAV in shellfish growing waters.

Caliciviruses and Astroviruses

Caliciviruses, like all enteric viruses, are found in human feces and are responsible for many cases of gastroenteritis. Transmission routes of calciviruses are the same as those discussed for other enteric viruses. Infection by calicivirus tends to cause a mild gastroenteric illness. No seafood-associated outbreaks due to these enteric viruses have been reported to CDC or NETSU (CDC, 1989; Rippey and Verber, 1988).

Unclassified Viruses

Unclassified viruses include poorly characterized agents of gastroenteritis such as Norwalk and Norwalk-like agents, Snow Mountain agent, small round viruses (SRVs), and non-A, non-B enteral hepatitis (Gerba, 1988; White and Fenner, 1986). Norwalk and Norwalk-like viruses resemble caliciviruses and are considered possible members of this family of viruses by some scientists (White and Fenner, 1986).

Outbreaks of viral gastroenteritis due to the Norwalk agent have been associated with swimming in waters contaminated with human sewage, eating food or drinking water that is fecally contaminated, and consuming uncooked or partially cooked shellfish harvested from estuaries contaminated with human fecal material. The first documented shellfish-associated outbreak of gastroenteritis involving Norwalk virus occurred in 1979 in Australia, where more than 2,000 people were involved (Gerba, 1988). Since then, many outbreaks of Norwalk or Norwalk-like viral gastroenteritis have been reported in the United States (CDC, 1989; Morse et al., 1986; Richards, 1985, 1987; Rippey and Verber, 1988). Norwalk and Norwalk-like viral illnesses associated with shellfish are a continuing problem; reported incidents have increased during the last decade (Richards, 1985). Between 1978 and 1987, NETSU reported 11 shellfish-related cases of Norwalk gastroenteritis and 71 cases of Snow Mountain agent (Rippey and Verber, 1988) (Tables 3-2 and 3-3). CDC (1989) reported two outbreaks involving 42 shellfish-associated cases of Norwalk and related viruses (Tables 3-2 and 3-3). Richards (1985) reported an outbreak of shellfish-associated gastroenteritis involving 472 cases in Louisiana. Norwalk was suspected but not serologically documented. Richards (1985) also reported more than 6,000 shellfish-associated cases of unspecified gastroenteritis during the past 50 years. It is probable that many of these are of viral etiology, possibly Norwalk or Norwalk-related agents. More than 75% of these cases have been reported since 1980, which may indicate increased awareness and reporting of shellfish-related illnesses or an actual increase in infection rates. Norwalk virus is more resistant to chlorine than bacterial indicators and other enteroviruses (poliovirus and reovirus), and there is no indicator for human enteric viruses in shellfish or their growing waters (Grabow et al., 1983; Keswick et al., 1985; Peterson et al., 1983).

The etiologic agent of enteral or epidemic NANB hepatitis is unknown and may represent a group of related viruses rather than a single agent. It has been suggested that the NANB agent could be an enterovirus – possibly a different serotype of HAV (Overby et al., 1983). Enteral NANB hepatitis can be more severe than HAV, and infection by the NANB agent is associated with a high incidence of cholestasis. However, only one case of shellfish-associated NANB was reported to NETSU from 1978 to 1987 (Tables 3-2 and 3-3); CDC did not report any seafood-associated NANB in the same period.

Enteral NANB hepatitis is transmitted mainly by sewage-contaminated water and sporadically by person-to-person contact. In the Middle East and Africa, it appears to be endemic (Overby et al., 1983). Cliver (1988) reported water-associated outbreaks in India, Africa, the USSR, and Mexico. Gerba (1988) reported enteral NANB hepatitis cases associated with consumption of raw shellfish in the United States. One incident of shellfish-associated NANB and 1,645 cases of shellfish-associated nonspecified (i.e., unknown etiology) hepatitis were reported by NETSU between 1978 and 1987 (Rippey and Verber, 1988) (Tables 3-2 and 3-3). No cases of NANB or unspecified hepatitis reported to CDC between 1978 and 1987 were associated with seafood consumption. If an unspecified type of hepatitis is accurately reported as associated with seafood consumption, it would most likely be hepatitis A or enteral NANB. However, accurate case histories for an infection with such a long incubation (4 to 6 weeks) are usually difficult to obtain.

Disease Control for Human Enteric Viruses

Proper classification of shellfish growing waters based on valid human enteric virus indicators, as well as implementation and maintenance of proper treatment and disposal of sewage to prevent human enteric virus contamination of shellfish harvesting waters, are the most effective measures to deter raw shellfish-associated infections by these viruses. New sewage treatment methods may have to be developed. Norwalk virus and HAV are more resistant to chlorine than bacterial indicators (Escherichia coli, Streptococcus faecalis, and acid-fast bacteria) and other enteroviruses (poliovirus and reovirus), and there is no indicator for human viruses in shellfish or in growing waters. The only potential viral indicator that shows similar chlorine resistance is f2 bacteriophage. The specifications for disinfection of drinking water – free chlorine residue of 1-2 milligrams per liter (mg/L) for 1-2 hours at pH less than 8 and turbidity less than 1 unit – may not be sufficient to totally inactivate all HAV and is not sufficient for Norwalk virus (Grabow et al., 1983; Keswick, 1985; Peterson et al., 1983).

Effective enforcement to eliminate recreational and illegal ("bootlegging") harvesting and sale of raw molluscan shellfish from known sewage-contaminated growing areas must be developed and adequately funded.

Hepatitis A virus could also be controlled by vaccination, because infection results in permanent immunity. Although the U.S. Army is conducting live vaccine trials for HAV at this time, no licensed vaccine is available to the general public. Passive immunization with gamma globulin following known exposure to HAV is currently the primary method of control and prevention (Cliver, 1988; White and Fenner, 1986).

Salmonella

Outbreaks of seafood-borne salmonellosis were reported to CDC between 1978 and 1987 (Tables 3-2 – 3-4 and 3-9); however, no cases of salmonellosis associated with shellfish consumption were reported to NETSU in the same period (Tables 3-2 and 3-3). Typhoid fever, due to infection by S. typhi, is of only historic importance as a seafood-related hazard in the United States, but continues to occur in foreign countries, potentially impacting the safety of imported seafood products.

TABLE 3-9

Seafood-Associated Salmonella Outbreaks, 1978-1987.

Salmonellae (nontyphoidal) have been isolated from a variety of fish and shellfish. The FDA Pathogen Surveillance Program documented sporadic Salmonella contamination of domestic cooked shrimp and imported raw lobster, raw shrimp, and miscellaneous fishery products in fiscal year 1988. In a study in Florida, approximately 20% of oysters, clams, and crabs were contaminated with Salmonella, with positives found during all seasons. However, no salmonellae were recovered from samples of mullet, the only free-swimming species studied (Fraiser and Koburger, 1984). The public health impact of these findings remains to be determined.

Salmonella contamination of foods of animal origin is not uncommon, and human infections associated with a wide range of food vehicles are frequently reported. Estimates of infectious doses for Salmonella range from less than 100 to more than 1,000,000 organisms; characteristics of the host, the organism, and the vehicle are all important mitigating factors in the number of Salmonella needed to infect and cause disease.

Epidemiology and risk assessment

For 1973-1986, an average of 55 food-borne outbreaks of nontyphoidal Salmonella infections, affecting a total of 3,944 persons, were reported each year to the CDC Foodborne Disease Outbreak Surveillance Program. During the same 14 years, an annual average total of 32,957 and 35,490 total Salmonella cases were reported through the laboratory-based Salmonella surveillance system and the Morbidity and Mortality Weekly Report (MMWR), respectively.

From 1978 to 1987, CDC reported six seafood-borne outbreaks involving 147 cases. Three of these outbreaks involving 80 cases were shellfish associated (Tables 3-2 – 3-4 and 3-9). Two shellfish-associated outbreaks of confirmed nontyphoidal salmonellosis were reported by NETSU between 1894 and 1988: a 100-case outbreak in Florida in 1947 due to contaminated oysters, and a 22-case outbreak in New York in 1967 associated with oysters imported from England. No cases of shellfish-associated Salmonella infections were reported to NETSU from 1978 to 1987 (Tables 3-2 and 3-3) (Rippey and Verber, 1988). Two outbreaks of nontyphoidal salmonellosis associated with salmon prepared by a caterer have been reported (Cartwright and Evans, 1988). The salmon was epidemiologically and microbiologically implicated, and multiple food-handling errors were identified. The origin of contamination of the fish was not determined, although raw chicken prepared at the same time as the salmon was also positive for S. montevideo, the outbreak serotype. Consumption of raw shellfish harvested from sewage-polluted waters also has resulted in Salmonella infections (Flowers, 1988a). Although CDC and NETSU food-borne surveillance data and literature reports indicate that seafood is a much less common vehicle for Salmonella than are other foods such as chicken or red meat, fish and shellfish may be responsible for at least a small proportion of the total number of Salmonella cases that occur each year in the United States. However, current data are inadequate to make any attempt at estimating attributable risk.

A number of years have passed since raw molluscs were associated with an outbreak of typhoid fever in the United States (Bryan, 1980; Rippey and Verber, 1988). One seafood-associated outbreak involving 25 cases was reported to CDC in 1973, but shellfish were not implicated as the vehicle. The last shellfish-associated typhoid outbreak reported in the NETSU data base occurred in 1954 (Rippey and Verber, 1988). The risk of typhoid fever associated with imported seafood may vary, depending on the rates of infection in the country of origin and the degree of pollution of harvest waters. Recent sporadic cases have been reported in consumers of raw shellfish in the Mediterranean area (Caredda et al., 1986; Torne et al., 1988) and, based on the incidence of typhoid fever in local populations, the risk associated with products imported from emerging nations would appear to be elevated (Parker, 1984).

Disease control

The microbial quality of harvest waters does not appear to be a good predictor of nontyphoidal Salmonella contamination, because oysters removed from closed and open beds had the same level of contamination (4%), and no correlation was observed between the presence of E. coli or fecal coliforms and Salmonella in finfish or shellfish (D'Aoust et al., 1980; Sobsey et al., 1980). Salmonellae do not appear to grow well in oysters during storage and are not selectively retained during depuration or relaying, so contaminated oysters can be purified (Son and Fleet, 1980). As with other pathogens, the risk of infection can also be minimized by ensuring that shellfish are cooked and handled properly.

Epidemiology and risk assessment

Contaminated food is becoming increasingly recognized as an important vehicle of L. monocytogenes (Ciesielski et al., 1987; McLauchlin, 1987). In recent years, Listeria has been implicated in outbreaks involving contaminated cabbage in Nova Scotia, fresh cheese in California, and pasteurized milk in Boston (Fleming et al., 1985; Linnan et al., 1988; Schlech et al., 1988). An outbreak of perinatal listeriosis was tentatively attributed to consumption of raw finfish and shellfish in New Zealand, although it is not clear whether patients consumed raw seafood more often than persons who did not become ill (Lennon et al., 1984). Catfish fillets are often contaminated with L. monocytogenes from their environment, but no human cases of listeriosis have been attributed to the consumption of catfish. It has been difficult to estimate the extent of listeriosis in the United States; prior to 1986, listeriosis was not reportable. A recent study identified 229 cases from the 34 million persons living in the study areas, providing minimum estimates of 1,600 culture-confirmed cases of listeriosis and 400 deaths per year for the United States (Schwartz et al., 1988). A case control study performed in conjunction with the active surveillance project identified consumption of raw hot dogs and undercooked chicken as risk factors for listeriosis; questions on seafood consumption were not included on the questionnaire (Schwartz et al., 1988).

An active listeriosis surveillance project being conducted by CDC and selected state and local health departments includes questions on seafood consumption by cases and controls, and may help define the relative importance of seafood in the epidemiology of human listeriosis. The FDA found L. monocytogenes in seven of eight categories of seafood cultured in fiscal year 1988 (FDA, 1988). Therefore, L. monocytogenes may be an unrecognized cause of food-borne enteric disease.

Disease control

L. monocytogenes is not easily inactivated by environmental influences, such as sunlight or freezing and thawing. It is found in soil, vegetation, sewage, and streams. Its prolonged presence in soil and on vegetation does not appear to depend on continual reintroduction but, rather, results from Listeria's ability to exist as free-living forms on plants and in soil.

Temperatures for the growth of L. monocytogenes range from 1 to 45°C, with the optimum between 30 and 37°C. At 4-5°C its growth rate is 50 times slower than at 35-37°C, but the organism can still grow. The optimum pH for growth is near neutral to slightly alkaline pH, with a range of 5.0-9.6. L. monocytogenes is quite tolerant to salt and low water activity. It can grow in the presence of 10% sodium chloride and can survive at 25% sodium chloride (Bryan, 1979; Medallion Laboratories, 1987; Smith et al., 1987).

L. monocytogenes is fairly heat resistant, having D _150°F values ranging from 12.8 seconds in skim milk to 17.6 seconds in cream (Bradshaw et al., 1985, 1987). Internalization in phagocytes did not significantly increase heat resistance (Bunning et al., 1986). Currently, FDA requires that L. monocytogenes not be present in ready-to-eat seafood products such as crabmeat or smoked fish. The restriction does not apply to raw products that will be cooked before eating.

Clostridium botulinum

Human botulism is relatively rare in the United States. However, the control and prevention of botulism are some of the most important considerations in food processing. C. botulinum is widely distributed in soil and in the aquatic environment (Dolman, 1964; NFPA/CMI, 1984; Sakaguchi, 1979). Botulism is generally caused by consumption of food in which C. botulinum has grown and produced its toxin. The onset of symptoms of botulism usually occurs within 12-36 hours after ingestion of the food, with a range of 2 hours to 14 days. In general, shorter onset times result in more severe symptoms. Early signs may include gastroenteritis, weakness, lassitude, dizziness, and vertigo. These are followed by eye problems such as blurred vision, diplopia (double vision), dilated and fixed pupils, and impaired light reflex reaction (Sakaguchi, 1979). Fever is absent and mental processes are normal. The major cause of death is respiratory failure and airway obstruction. Because of the severity of the disease, most cases are likely to be reported, although milder cases may go undiagnosed.

Botulism is among the most serious food-borne illnesses, with a mortality among untreated patients up to 70%. Competent medical treatment can reduce this mortality rate to approximately 15%. Fortunately, botulism is a rare disease, but cases due to consumption of seafoods were reported to CDC every year from 1978 to 1987 (Tables 3-2, 3-4, and 3-10). Nearly all of the cases in this period were caused by various ethnic foods prepared under conditions of poor general hygiene by individuals who did not understand the dangers of the procedure followed.

TABLE 3-10

Seafood-Related Botulism Outbreaks and Cases, 1978-1987.

Epidemiology and risk assessment

C. botulinum is classified into toxin types A through G based on the serological specificity of the toxin produced. Types A, B, E, and F have been reported as causal agents in human food-borne botulism resulting from the consumption of foods in which bacteria have grown and produced toxins (Hauschild, 1989). Although type E is known as the "fish botulism organism," types A and B have been implicated in botulism caused by seafood (Hobbs, 1976). Nonproteolytic strains of types E and B are readily isolated from some marine sediments. Because C. botulinum produces heat-resistant spores and requires anaerobic conditions for growth, botulism has been most commonly associated with improperly processed canned food (usually home canned). Among fishery products, semipreserved items including smoked, salted, and fermented fish have been identified frequently as botulism vehicles (Eklund, 1982; Lynt et al., 1982).

The CDC reported 38 cases of fish-associated botulism from 1978 to 1987 (Tables 3-2, 3-4, and 3-10). Of these cases, 23 were from ethnic foods prepared and consumed in Alaska, and only one involved improperly home-canned seafood. All cases occurred in the home or as a result of eating homemade seafood products. No domestic case of botulism from commercially prepared seafoods has been reported in the United States in recent years, although canned salmon from Alaska has caused two incidents in Europe. This suggests that for most seafood consumers the present risk is negligibly low, but it is high for particular ethnic groups. During the 1960s, several outbreaks of botulism occurred from consumption of improperly processed smoked fish stored under conditions permitting the growth of C. botulinum (Pace and Krumbiegel, 1973). Improved processing methods have reduced this hazard, but safety still depends largely on low-temperature (<3°C) storage of the processed product.

Disease control

Botulism may be prevented by inhibiting growth of the organism or by destroying it in the food. Canning processes are generally designed to destroy the heat-resistant spores of C. botulinum, and this process is very effective. Type E spores of C. botulinum are more heat sensitive than type A or most type B, and pasteurization processes used in seafoods are generally effective in destroying most of them. Thus, hot smoking at 82.2°C can destroy 10,000,000 spores of type E in 30 minutes (Cann and Taylor, 1979), and processes used for crabs usually eliminate type E strains. However, as a safety factor it is necessary to store such heat-treated products below 3°C to prevent germination and growth of surviving spores and production of toxin. Processes such as heavy salting or drying, which reduce water activity below 0.975 (i.e., >5% salt), and pickling to reduce pH below 5.3 are effective in preventing growth of type E C. botulinum.

Light salting and other mild forms of processing are ineffective in destroying C. botulinum spores and, if followed by vacuum packing or controlled atmosphere storage, may actually promote growth of the organism by destroying competitive bacteria. It is essential that such products be held below 3.3°C and cooked prior to eating (Emodi and Lechowich, 1969).

Heat Treatment

Heat treatment is frequently defined in terms of D- and F-values, with subscripts indicating temperature: D is the number of minutes at the indicated temperature necessary to reduce a microbial population by 90%, and F is the lethality of a process expressed as minutes at the indicated temperature. A third term Z is the temperature difference associated with a tenfold difference in killing rate. Whereas achievement of commercial sterility (as in canned fish) generally requires a 12 × D treatment, pasteurization may require lesser treatment (commonly 4-6D).

Nearly all canned seafoods available to the U.S. consumer are fully processed, commercially sterile products that are shelf stable in their original containers. Their production is controlled by FDA or state agencies working with FDA and following a Hazard Analysis Critical Control Point (HACCP) system that has proved very effective. Because fish and shellfish are low-acid foods, the major safety concern involves Clostridium botulinum whose spores are extremely heat resistant. Processors must deliver a treatment of 12D or greater, which is ensured by requiring the regulating agency's approval of process regimes and strict recordkeeping by the companies.

No cases of botulism from commercially canned seafoods have occurred in the United States in recent years. A single outbreak in Europe due to Alaska canned salmon was shown to result from improperly functioning equipment related to the use of collapsed three-piece cans. This problem has been resolved by improved maintenance and quality assurance programs and by widespread adoption of two-piece extruded cans (NFPA/CMI, 1984; Thompson, 1982).

For equal safety, imported fully processed canned seafood products must be made by processes that meet domestic FDA requirements, including process approval, adequate recordkeeping, and employment of certified retort/operator supervisors.

Semiconserved canned fishery products may contain living microorganisms because they receive milder heat treatment in pasteurization (Delmore and Crisley, 1979; Eklund, 1982; Lerke and Farber, 1971). These products, which are frequently imported, depend on acidification, use of salt, and storage at reduced temperature for their stability and are generally marketed as delicatessen items kept under refrigeration.

Pasteurization is a term that refers to a mild heating process, usually below 100°C. By definition, the term indicates that the product is not sterile and therefore may continue to harbor microorganisms. Consequently, pasteurized products must be continuously refrigerated so that surviving microorganisms will not multiply too rapidly, shortening the product's anticipated shelf life and introducing hazards. The initial microbial population greatly affects the efficiency of the process. The most important application of pasteurization in the seafood industry is the treatment of pickled crabmeat, which is packed, sealed in cans, and held under refrigeration after treatment. The National Blue Crab Industry Association has recommended a national standard that effectively provides for the destruction of potentially hazardous bacteria that could grow during refrigerator storage. The process provides the equivalent of F _85°C in excess of 31 minutes (NBCIA, 1984).

Pasteurization has been proposed for other products including shrimp (Lerke and Farber, 1971), crawfish, and smoked fish (Eklund et al., 1988). Since the FDA dropped the GMP regulations on the processing of smoked fish, there has been increasing concern regarding the potential for a botulism outbreak associated with smoked fish. The pasteurization process described by Eklund et al. (1988) has the potential to minimize the concerns associated with this product. Pasteurization was more effective for smoked fillets and steaks than for dressed fish.

Sous vide products are minimally processed foods, including seafood, that are being introduced into the U.S. market. The products are portion controlled, vacuum packaged in plastic pouches or rigid containers that are highly impermeable to oxygen and moisture, and then cooked in either a water bath or a high-humidity oven. Cooking temperatures for sous vide are usually far lower than those associated with pasteurization. Cooking may involve temperatures close to the desired maximum internal temperature for the product, and thus require a long cooking time, or products may be cooked quickly at temperatures considerably above the desired internal maximum. In either case, the principles that apply to pasteurization also apply to this processing technology. Products should be cooked to a desired F-value and cooled quickly.

The safety of sous vide products has been questioned because they are processed only minimally and do not contain preservatives to prevent microbial growth. Furthermore, the cooking process does not eliminate nonproteolytic types of C. botulinum, and there is some question as to whether it may allow other organisms such as Listeria to survive. The shelf life and safety of these products depend solely on refrigeration; therefore, it is critical that psychrotrophic pathogens not survive. In the United States, these items are presented as refrigerated, ready-to-eat, or heat-and-serve products. They are produced under controlled conditions, and caution is exercised during distribution. As of late 1989, the products were being sold only to food service establishments and were not being sold retail. This will probably change in the near future as demand increases. The FDA has limited the use of sous vide products to approved food processing operations and currently does not allow production at retail establishments such as grocery stores. It is important that they be produced under an approved HACCP program and that the principles of pasteurization be understood and applied to their production.

Modified Atmospheres and Vacuum Packaging

Modified atmosphere packaging (MAP) is defined as a process in which air is removed and replaced with other gases, usually carbon dioxide (CO₂) or a mixture of CO₂ and other gases. The atmosphere in the package will change over time because of microbial respiration and permeability characteristics of the packaging material. Controlled atmosphere packaging (CAP) is like MAP, except that the atmosphere is monitored and kept constant. For vacuum packaging, air is removed without replacement. It is well established that modified atmosphere (MA) storage of seafood products will extend the shelf life considerably by inhibiting the normal psychrotrophic microflora and reducing the spoilage rate. There is concern about the potential growth and toxin production of nonproteolytic types of C. botulinum (Garcia et al., 1987). It has been demonstrated that MA packaging or vacuum packaging coupled with temperature abuse can create conditions in which toxin production occurs before sensorially detected deterioration (Genigeorgis, 1985; Post et al., 1985). The National Marine Fisheries Service (NMFS) recommends (but does not require) that fisheries products not be MA or vacuum packaged if they are to be stored under refrigeration (Post et al., 1985). Where vacuum or MA packaging is used, NMFS has set guidelines (NOAA, 1990).

Hazard Identification

A variety of seafood-associated bacterial, viral, and parasitic agents cause illness in humans. These agents can generally be divided into three groups: (1) those that are known to cause disease in healthy adults; (2) those that usually do not cause disease in healthy adults but can cause illness in special population groups (children, immunosuppressed patients); and (3) those that are of uncertain pathogenicity for humans (Table 3-1). It should be kept in mind that these categories are not absolute and that agents could be moved from one category to another in response to new information and changing conditions. Pathogens such as Salmonella, V. parahaemolyticus, or Norwalk virus cause disease in normal, healthy hosts; however, the illness in normal hosts is generally mild and self-limited. In contrast, when persons who are immunocompromised or otherwise at high risk are infected, severe, life-threatening illness may occur (e.g., V. vulnificus).

Dose-Response Assessment

Viral agents such as Norwalk and some bacterial agents such as Shigella require a very low infectious dose to cause illness (ca. 10² CFU for Shigella and theoretically one infectious viral particle for Norwalk). These agents are often transmitted by direct fecal-oral contamination and may pose a problem if they are allowed to contaminate food items after processing. Other bacteria generally require a higher infectious dose, which may be reached by time/temperature abuse of food products. Unfortunately, in the absence of volunteer studies the infectious doses for many of the viral and bacterial pathogens discussed are unknown; and even if such studies were carried out, it could not be proved that study conditions duplicate the wide range of human responses to natural exposure. However, it is probably accurate to say that the risk of infection increases with increased dose for most pathogens and that the dose depends to a large degree on the handling of the product after harvest, during processing, and in preparation.

Exposure Assessment

The risk of exposure is dependent on a number of variables, including the type of seafood, the location, the water quality at harvest, and the way the product is handled after harvest. Risks are markedly different for each pathogen and each product, making it difficult to generalize about overall risk of exposure to infectious agents or to establish uniform water quality or product safety standards.

Infections with Vibrio species and other naturally occurring marine bacteria are generally associated with eating shellfish. Their numbers are dependent on salinity, temperature, and a variety of other factors, with temperature (>20°C) probably being the most important variable (Baross and Liston, 1970).

Of the enteric viruses, only hepatitis A virus, Norwalk virus, Snow Mountain agent, caliciviruses, astroviruses, NANB enteral hepatitis, and unspecified hepatitis have been documented to cause seafood-associated illness. With the exception of HAV contamination of ready-to-eat seafoods by food handlers, all reported cases of seafood-associated viral infections have been from the consumption of raw or improperly cooked molluscan shellfish (Bryan, 1986; CDC, 1989; Cliver, 1988; Gerba, 1988; Richards, 1985, 1987; Rippey and Verber, 1988). Human enteric viruses from naturally occurring contamination have been isolated in field studies only from molluscan shellfish and from blue crabs collected at a sludge dump in the North Atlantic. Other marine animals, including lobsters, sandworms, detrital feeding fish, and conch, have been shown to take up enteric viruses seeded into marine waters experimentally, but this has not been reported from field studies to occur naturally. These viruses are very species specific and even receptor specific for certain cells. Once released into the marine environment, their survival and persistence are based on many factors, including temperature, salinity, ultraviolet inactivation from sunlight, and the presence of organic solids or sediments. Of these factors, the most important appears to be a temperature below 10°C and the protective action of organic material (Gerba, 1988).

Risk Characterization

Raw or undercooked shellfish appear to carry a higher risk of disease than do other types of seafood, particularly because such shellfish constitute less than 0.1% of the seafood consumed in the United States but are responsible for a large proportion of the cases reported (CDC, 1989; Rippey and Verber, 1988).

Available surveillance data from both CDC and NETSU (Tables 3-2–3-4) and clinicopathophysiologic studies suggest that food-borne diseases due to unknown etiologies, unspecified (i.e., unknown etiology) hepatitis, and certain Vibrio species (V. parahaemolyticus, V. vulnificus, non-O1 V. cholerae) represent the greatest risk for persons consuming raw molluscan shellfish.

The largest number of reported seafood-associated illnesses have unknown etiologies (Table 3-2–3-4) (CDC, 1989; Rippey and Verber, 1988). Although these are probably not all of microbiological etiology, many have onset periods and symptoms consistent with the clinical pathology of Norwalk and related viruses. However, both the CDC and the NETSU data bases report relatively few cases of confirmed Norwalk or related viral infections associated with shellfish (Tables 3-2 and 3-3) because only a few specialized laboratories are able to serologically diagnose infections with Norwalk, Snow Mountain, and related viruses. Despite the low number of documented cases of Norwalk and Norwalk-like agents in shellfish, recent evidence suggests that shellfish-associated infections with these agents occur more frequently than identified (Morse et al., 1986) and that they may ultimately be the most common shellfish-associated pathogens, especially in coldwater clams.

The risk of infection by bacterial pathogens appears to be lower than that due to viruses. Although data are limited, the incidence of infection with the more common Vibrio species in coastal states (where shellfish consumption is probably highest) is in the range of 0.5-1.0 case/100,000 population/year (Hoge et al., 1989). Although cases of V. vulnificus associated with shellfish are reported with approximately the same frequency as V. parahaemolyticus and non-O1 V. cholerae, the severity of the infection and the high mortality rate in populations at risk elevate the importance of the former as a seafood-associated pathogen (Tables 3-2, 3-3, 3-6, and 3-7). Shellfish-associated infections due to Salmonella and Shigella are approximately the same, based on CDC surveillance data (Tables 3-2–3-4) (CDC, 1989). However, cases due to these latter agents are probably underdiagnosed because the vehicle for the vast majority of cases caused by these agents is never identified.

Parasitic infections are less common than bacterial and viral infections in the United States, with Anisakis simplex and cestodes having the greatest public health significance (Table 3-2). In general, parasitic infections are concentrated in certain ethnic groups that favor the consumption of raw or partially cooked seafood from "high-risk" geographic areas.

Pathogens in Marine or Freshwater Environments

Risks associated with Vibrionaceae can be addressed at several levels. Because these organisms are a "natural" part of the bacterial flora of bays and estuaries, their presence cannot be predicted by standard indicators such as fecal coliform counts. However, their occurrence is dependent on environmental conditions (water temperature, salinity), with counts generally peaking in the late summer and early fall. The risk of infection from Vibrionaceae could be minimized by restricting summer harvesting of oysters. More realistically, a system for monitoring shellfish and harvest waters for these organisms could be established, analogous to the current system for monitoring fecal coliforms. Rapid diagnostic techniques for identification of pathogenic vibrios are available or are being developed (Kaper et al., 1988), making such a system technically possible. Monitoring data would provide an index to times when the risk of exposure to Vibrio is greatest and could serve as an adjunct to other indicators for the management of harvest waters.

Efforts can also be directed toward reducing or eliminating organisms present in shellfish after harvest. The self-cleansing of some enteric viruses and marine vibrios may not proceed at the same rate as for enteric bacteria and indicators, which suggests that current techniques for depuration may be of little benefit in limiting contamination with Vibrionaceae. New technology-enhanced depuration must be developed for eliminating naturally occurring vibrios (Rodrick, 1990). Low-dose gamma irradiation of live shellstock and fresh or frozen seafood products has also been shown to be an effective means of eliminating Vibrio species (Giddings, 1984). Dose levels of 1.0 kilogray (kGy) (100,000 rads) to 1.5 kGy (150,000 rads) will reduce high levels (10⁶) of seeded Vibrio spp. in live shellstock oysters to undetectable levels. The LD₅₀ of live shellstock oysters is 2.25 kGy (Kilgen et al., 1988). Sensory evaluation shows no significant organoleptic differences in shellstock oysters treated with 1.5-kGy gamma irradiation (Kilgen et al., 1988). Gulf Coast summer oysters naturally infected with V. vulnificus levels of 10⁴ organisms per gram of oyster meat also had no detectable V. vulnificus levels after gamma irradiation of 1.0 kGy (M. Kilgen and M. Cole, Nicholls State University, Thibodeaux, La., personal communication, 1990).

Although data on human infectious dosages are not available for all of the Vibrionaceae, it is likely that the risk of infection can also be reduced by limiting the multiplication of organisms during handling and processing. Recent studies demonstrate that there can be a striking increase in bacterial counts (including counts of Vibrio species) when shellfish are left unrefrigerated on the decks of boats during warm summer days (Cook and Ruple, 1989). Similarly, bacterial counts can increase if shellfish are not adequately refrigerated during transportation to plants and subsequent distribution. Some processors are currently using rapid cool-down techniques to minimize these risks; such techniques need to be further evaluated, and if shown to be effective, their implementation should be encouraged.

Finally, the consumer can play an important role in minimizing risks. Consumers should be educated about the need for proper refrigeration of fish and shellfish, and the dangers of cross-contamination. They should be advised about the risks of eating raw shellfish. Of particular concern are persons who are at high risk for septicemia due to V. vulnificus (persons who are cirrhotic, who have hemochromatosis, or who are in some way immunocompromised) (Tables 3-6 and 3-7). Such persons must not eat raw or undercooked shellfish.

With the development of gene probes amplified by polymerase chain reaction and other rapid, new diagnostic techniques, shellfish can be screened for the presence of some potentially pathogenic Vibrio species (Kaper et al., 1988). These probes could theoretically be used as part of a national monitoring system for shellfish. However, further studies are required to determine the ways in which data of this type can be used to protect the public health.

The risk of parasitic infection depends largely on the species of fish and shellfish and the area from which they are harvested. In general, prevention of infection is dependent on limiting the harvesting of seafood from high-risk areas; informing sports fishers of regionally important risks associated with certain species and the availability of alternate control strategies (e.g., freezing fish before raw consumption); and educating consumers on the importance of proper cooking of fish and shellfish.

Some parasitic agents, such as the anisakids, are detectable by visual inspection techniques. However, these are probably the only infectious agents that would be detected by such a system, and the very low frequency of reported disease due to these agents does not justify using public resources specifically to identify parasites in seafood.

Pathogens Associated with Pollution

Control of pollution-associated agents generally translates into identifying sewage contamination of harvest areas. The only seafood products that are regulated by microbiological indicator standards for growing water quality are molluscan shellfish. The sanitary quality of shellfish is based on an allowable standard of 14 most probable number (MPN) fecal coliforms per 100 milliliters (mL) of growing water, with not more than 10% exceeding 43 MPN fecal coliforms/100 mL (FDA, 1989a). The National Shellfish Sanitation Program (NSSP) manual of operations also requires a sanitary survey of oyster growing estuaries prior to approval for shellfishing, relaying, or depuration (FDA, 1989a). The revised NSSP recommendations (FDA, 1989b) for monitoring interstate shellfish shipments state that:

Proper growing area classification and strict adherence to good manufacturing practices (GMP's) are the principal considerations for assuring the safety of shellfish. The bacteriological market guideline of 230 MPN fecal coliforms/100 g [grams] oyster meat should no longer be used as the primary basis for embargoing or destroying shellfish shipments. Shellstock shipments should be monitored within 24 hours of entering the state, and should be considered acceptable if: shipments are properly identified (by tag, bill of lading or label) and cooled, shellstock to 50°F or less, and shucked product to 45°F or less, and if they are in compliance with all other NSSP conditions of shipment. Shellfish should be rejected if shipments are not properly identified, are inadequately cooled (shellstock shipments exceeding 60°F and shucked product shipments exceeding 50°F), or other conditions exist, e.g. decomposition and/or adulteration with poisonous and deleterious substances. Shellfish received under conditions between acceptance and rejection criteria could be examined bacteriologically, and the following guidelines applied for acceptance: fecal coliform density of not more than 230 MPN/100 g and aerobic plate count (APC) of not more than 500,000/g.

The fecal coliform standard in growing waters and the guidelines for meats were established to prevent sewage contamination of shellfish that may be consumed raw. Fecal coliforms are present in high numbers in untreated sewage and are considered "indicators" of the possible bacterial and viral human enteric pathogens that may also be found in feces. The allowable numbers of fecal coliforms in shellfish growing waters were based on the relationship between total coliform counts and the number of Salmonella typhi present during a typhoid fever epidemic in the 1920s (FDA, 1989a). A conference was called in 1925 to establish the basic concept of a national program for sanitary control of the molluscan shellfish industry. The first growing water standard was 70 MPN/100 mL total coliforms with no more than 10% of samples exceeding 230 MPN total coliforms/100 mL and freedom from direct contamination with fresh raw sewage. This indicator standard was established through epidemiological investigations from 1914 to 1925 by the states and the Public Health Service. It was believed at this time that typhoid fever would not normally be attributed to shellfish harvested from water in which "not more than 50 percent of the 1 cc [cubic centimeter] portions of water examined were positive for coliforms" (FDA, 1989a). This equated to 70 MPN total coliforms/100 mL, which is equivalent to the fecal material from one person diluted in 8 million cubic feet of coliform-free water. Later studies concluded that fecal coliforms were more accurate indicators of fecal contamination than total coliforms, and the allowable numbers were extrapolated to the current standard of 14 MPN fecal coliforms/100mL growing water, with no more than 10% of samples exceeding 43 MPN fecal coliforms/100 mL (FDA, 1989a). However, even the more specific fecal coliform indicator of contamination in water and seafood has been questioned (Cole et al., 1986; Elliot and Colwell, 1985; Kilgen et al., 1988; Matches and Abeyta, 1983).

According to Garrett (1988), the United States has only three types of general regulatory microbiological indicator criteria for other seafoods:

1.

Microbiological standards

• FDA's GMP requirement that thermally processed low-acid foods packaged in hermetically sealed containers be commercially sterile
• FDA's microbiological standards for "approved," "conditionally approved," "restricted," or "prohibited" molluscan shellfish growing waters, used in conjunction with a sanitary survey as described above

2.

Microbiological guidelines

• Microbiological criteria for crabmeat and langostinos
• Defect action levels for raw breaded shrimp plants
• Bacteriological market guidelines for molluscan shellfish (revised to GMPs for proper refrigeration, handling, and tagging in 1989) (FDA, 1989b)

3.

Microbiological specifications

• Microbiological acceptance criteria contained in federal specifications for the procurement of fish and frozen shelled oysters

The National Fisheries Institute published a draft Handbook of State and Federal Microbiological Standards and Guidelines (Table 3-11) (Martin and Pitts, 1989).

TABLE 3-11

State and Federal Microbiological Standards and Guidelines.

The basic requirements for an ideal indicator have been discussed at length (Elliot and Colwell, 1985). Some of the characteristics necessary for an indicator to accurately predict the presence of sewage-associated pathogens include:

1.

specificity for the source of the pathogens (i.e., raw sewage),

2.

presence in feces in sufficiently high concentrations to allow sensitivity of detection,

3.

persistence in the aquatic environment at least as long as the pathogens,

4.

absence of growth in the aquatic environment, and

5.

availability of inexpensive methods for detection and quantification.

A number of indicator organisms have been proposed in the past for detection of fecal pollution in fresh, brackish, estuarine, and seawaters. However, no single indicator organism exists for determination of public health risk in waters or seafood (Elliot and Colwell, 1985). Some of the problems encountered with the current fecal coliform indicator standard in growing waters and the guideline for oyster meat include the following:

1.

Non- Escherichia coli fecal coliforms and even non-sewage-related bacteria may predominate in the fecal coliform population analyzed by methods approved by the American Public Health Association (APHA, 1985a,b). These non-E. coli fecal coliforms can be found in shellfish, sediments, and the water column, especially at warm temperatures (Cole et al., 1986; FDA, 1989b; Kilgen et al., 1988; Paille et al., 1987). Characterization of the non-E. coli fecal coliform population in Louisiana oysters showed that in the warm months, Klebsiella pneumoniae isolates accounted for 86% of the non-E. coli fecal coliforms and often outnumbered E. coli by 1,000 to 1 (Paille et al., 1987). These Klebsiella oyster isolates were further characterized and compared with K. pneumoniae human clinical isolates by electron microscopy, guanine:cytosine ratios, and antibiotic resistance. The results suggested that K. pneumoniae isolates from oysters were of environmental, not sewage, origin. Their seasonal variation was typical of environmental bacteria, and they did not exhibit the multiple antibiotic resistance characteristic of the clinical strains. Similar studies by the FDA agreed that Gulf of Mexico Coast oysters harvested from approved growing waters in summer months may contain excessively high levels of non-E. coli fecal coliforms and not represent an excessive health hazard (FDA, 1989a). It was concluded that fecal coliforms may not be a reliable indicator of fecal contamination in Gulf Coast oysters, especially in summer months, and that E. coli would be a better indicator for an oyster meat guideline (FDA, 1989b; Kilgen et al., 1988; Paille et al., 1987). This resulted in the adoption of a one-year interim oyster meat guideline of 230 MPN E. coli/100 g instead of 230 MPN fecal coliforms/100 g by the Interstate Shellfish Sanitation Conference (ISSC) in 1983. This is still recommended in the NSSP operations manual for shellfish from approved growing waters with excessively high fecal coliform indicator levels, but it is not an officially adopted guideline (FDA, 1989b).

2.

The fecal coliform indicator for waters does not indicate the presence of non-sewage-related naturally occurring aquatic bacterial pathogens, such as Vibrionaceae (Elliot and Colwell, 1985; Matches and Abeyta, 1983; Tamplin et al., 1982).

3.

The fecal coliform indicator does not correlate with the presence of human enteric viruses, which are the pathogens most commonly associated with sewage contamination of waters and seafood (Cole et al., 1986; Elliot and Colwell, 1985; Gerba, 1988; Kilgen et al., 1988; Richards, 1985, 1987; Sobsey, 1980).

The human enteric pathogens of main concern from sewage contamination are now enterovirus type 72 (hepatitis type A) and Norwalk or Norwalk-like gastroenteritis viruses. However, current data also indicate that a constant and predictable relationship does not exist among fecal coliform indicators, E. coli and enteric viruses in estuarine waters and shellfish.

The public health risk associated with fecal material from animal sources versus human sources is also in question. The most productive shellfish growing estuaries are often those most subject to rainfall runoff from animal non-point sources. Extensive closures due to high fecal coliform indicator counts from non-point animal sources have been identified as one of the major concerns of state regulatory agencies and industry members from coastal areas; a great deal of research is required to assess human health risks from wild and domestic animal runoff (Elliot and Colwell, 1985; Kilgen, 1989). Terrestrial mammals carry bacterial species pathogenic to humans; however, these have generally not been associated with shellfish-borne illnesses. Rather, sewage-associated human illnesses appear most frequently to have a viral etiology, and viruses tend to be species specific. Therefore, an indicator of human enteric viruses in water and in seafoods is needed. Some indicators that have been proposed include poliovirus type 1, enterococci, E. coli, coliphages, fecal streptococci, Clostridium perfringens, Pseudomonas aeruginosa, Bifidobacterium species, Rhodococcus species, Streptococcus bovis, Bacteroides phages, and F+ phages (Elliot and Colwell, 1985; Kilgen, 1989; Richards, 1985). None of the suggested indicators appears to be an adequate indicator of human health risk from enteric virus pathogens in seafoods of water, and none of them would predict the presence of naturally occurring bacterial pathogens such as the Vibrionaceae. This is an area for vital research.

Depuration (controlled purification) and relaying have been used successfully to remove enteric bacterial pathogens and indicators from molluscan shellfish (Richards, 1988). However, the self-cleansing of some enteric viruses and marine vibrios may not proceed at the same rate as for enteric bacteria. Hepatitis A virus persists far longer in oysters and clams than E. coli or poliovirus (Sobsey, 1990). Improperly depurated shellfish have been responsible for outbreaks due to enteric viruses (Richards, 1988). Inasmuch as marine vibrios may also be present far longer than standard indicators and can multiply quickly at elevated temperatures, immunocompromised individuals should not assume that depurated shellfish are safe to consume. More research is needed both to develop indicators of adequate viral depuration and to develop enhanced depuration technology to remove naturally occurring vibrios (Rodrick, 1990).

Pathogens Associated with Processing and Distribution

Methods involved in processing can increase or decrease bacterial populations present in seafood. Processing methods are amenable to monitoring by an HACCP system. However, good data are not always available to justify currently used endpoints. Further research is needed to determine the appropriateness of current quality control criteria and to demonstrate that the use of these criteria results in a decrease in the incidence of human disease.

The major problem associated with distribution of seafood products is time/temperature abuse. Some pathogenic microorganisms can grow to dangerous numbers when exposed to prolonged temperature abuse. A bacterium that may double every 30 minutes at 25°C may require 1,200 minutes at 1°C. Therefore, it is very important that seafoods be cooled quickly to achieve maximum shelf life and to maintain safety. Only four bacterial pathogens (Listeria monocytogenes, Yersinia enterocolitica, Aeromonas hydrophila, and nonproteolytic Clostridium botulinum types E, B, and F) are considered psychrotrophs and are capable of growing at refrigeration temperatures (<4°C). Of the organisms described, only the nonproteolytic C. botulinum types have been definitely implicated in seafood-related illness. Time/temperature abuse during transportation was the major factor in outbreaks in the 1960s when smoked fish was transported unrefrigerated from the Great Lakes to distant markets (Pace and Krumbiegel, 1973).

Other bacterial pathogens that can grow at near-refrigeration temperatures (between 4 and 10°C) include enteropathogenic E. coli, Staphylococcus aureus, salmonellae, and Vibrio parahaemolyticus. These microorganisms may become a problem during transportation if seafoods are not cooled properly at any time after harvest.

Proper cooling of seafoods for transportation is critical for safety. The seafood must be cooled before being placed in the distribution system. Most refrigerated or frozen product transport systems are designed only to maintain a predetermined temperature and do not have the capacity to reduce the temperature of the product significantly. Cooling can be a lengthy process with large lots of fish, and temperatures can remain in the above 10°C range for many hours, permitting the growth of potentially hazardous bacteria if the warm product is loaded into transport containers. Chilled product should be loaded at internal temperatures below 4°C and frozen products at or below -18°C.

Imports

A large part of imported seafood originates in countries with a much higher incidence of enteric disease and lower levels of hygiene and sanitation than the United States. Therefore, seafoods from these countries could be contaminated more frequently or at a higher level with disease-producing organisms than our domestic fisheries product. It is important that the same principles of seafood-borne disease control be applied to these products as to domestically produced seafoods, preferably through an equivalent inspection system. This will require intelligent assessment of the microbiological hazards likely to be particularly troublesome in different exporting countries and the risks associated with in-country handling and processing practices. Because of the high risks associated with raw molluscan shellfish, the importation of shellfish for raw consumption should be prohibited unless standards for the microbial quality of harvest waters and postharvest processing in the exporting country are fully equivalent to those in the United States.

Future Risk

The fishing industry is in a period of rapid and profound change as a result of depletion of traditional stocks, development of domestic fisheries in Third World countries, exploitation of new fish resources, technological innovation, and changing consumer demands and expectations. The changes have had both beneficial and potentially adverse effects on microbiological risk to the final consumer of seafoods. Changes in canning technology, process recording equipment, temperature control, and freezing methods (leading to rapid cooling and freezing of much seafood at the point of capture) have undoubtedly improved microbiological safety for many seafood products, although this has not been definitively demonstrated. The public demand for lightly cooked and "fresh"-type products, which is satisfied by vacuum packaging and relatively long-term holding, can lead to potentially more hazardous products as discussed in relation to the sous vide process.

Control of temperature during the handling and storage of seafoods is widely recognized as critical for microbiological safety, but newer customer presentation systems require more stringent temperature control than those to which the industry has become accustomed. Safe storage of lightly cooked vacuum packaged products for more than a few hours requires temperatures consistently below 3°C and probably nearer to 0°C if they are to be stored for more than a few days (Post et al., 1985). These stringent conditions are necessary to prevent the growth of Clostridium botulinum type E, psychrotrophic pathogens such as Listeria and Yersinia, and possibly opportunistic pathogens such as Aeromonas.

The new seafood analogue products, which are widely used as a substitute for crab and other expensive species in seafood salads or other dishes, are manufactured from Alaskan pollock and a few other fish species. These products contain other food components such as sugars, emulsifiers, and egg white and are prepared by processes that remove most of the soluble "fishy" components from the raw material. They are heat treated so that most of the naturally present bacteria are destroyed. This provides a long shelf life under good storage conditions. These virtually sterile products provide an excellent growth medium for contaminant bacteria and do not develop the characteristically unpleasant odors associated with "bad" fish, which most consumers use as a warning not to eat the product. Therefore, care must be taken to avoid cross-contamination and warming of such products.

Ethnic foods and regional considerations

The population of the United States includes many ethnic groups with different food preferences. In coastal regions this may be reflected in the patterns of fish and shellfish consumption. From a microbiological standpoint, the most important ethnic practices involve consumption of raw and fermented seafood products. Eating raw fish is most common among Americans of Asiatic origin and presents potential problems associated with parasites and Vibrio infection. As noted earlier, the increasing popularity of sushi and, to a lesser extent, seviche has spread the practice more widely among the U.S. population. So far, there has not been evidence of a major microbiological problem, but this situation must be watched carefully.

Fermented products present a more acute hazard because they are normally produced in the home by using traditional procedures that are poorly controlled. Most cases of seafood-borne botulism in the United States in recent years occurred in Alaska among native Americans as a result of consuming fermented fish and marine mammal parts (Table 3-10). Little is known scientifically about the processes used, which seem to be largely proteolytic in nature; more study is clearly required in this area to identify procedures that would be safer than those presently used.

Traditional methods of producing marinated and cured fish may also present botulism problems, as evidenced by cases in Puerto Rico and New Jersey (Table 3-10). In most such cases, proper temperature control during preparation could eliminate the problem. Education of specific targeted groups is necessary to manage these risks.

Other risks arise from the practice of consuming parts of fish, particularly intestines and other viscera, common among recent immigrants from Southeast Asia. In most cases, such dishes are eaten after cooking, which may explain the absence of major infectious disease outbreaks. However, the consumption of intestines and whole scallops has caused intoxications due to paralytic shellfish poisoning (PSP). It is unusual for scallop adductor muscle tissue to become toxic for humans, but viscera and other organs readily become toxic. Control has typically been based on measurement of toxin in the adductor muscle. Regulations and control measures designed to deal with common U.S. eating practices may not adequately protect individuals with different eating customs.

Aquaculture

The microbiological safety and promising future of the aquaculture industry has been reviewed by Ward (1989). Many of the microorganisms of concern in growing waters and abusive conditions in processing, handling, or preparation of other seafood products are also of concern in aquaculture. Two different areas of concern for aquaculture are pathogens known to cause disease in both fish and humans (e.g., Edwardsiella tarda and Aeromonas hydrophila) and the potential development of antibiotic resistance in these pathogens. The use of antimicrobials in aquaculture at both therapeutic and subtherapeutic levels is widespread (Brown, 1989). The possible development of antibiotic-resistant strains of bacteria, such as Salmonella, resulting from these feeding practices, and the subsequent transfer of such resistant bacteria on seafood products to humans are of concern. Significant levels of resistance to tetracycline compounds, some of it transferable, have been documented in A. hydrophila strains isolated from farmed catfish and their environment (DePaola et al., 1988). The development of antibiotic-resistant pathogens in aquaculture products is certainly of concern to the consumer. The expansion of aquaculture in the future will warrant research on the development and better management of antimicrobials for aquaculture (Ward, 1989).

Raw Molluscan Shellfish

Overall, when examining the potential for seafood-associated illness from microbial pathogens, several factors must be taken into consideration, including host risk factors; sources and types of microorganisms; and seafood processing, preparation, and handling procedures that either allow microorganisms to survive and grow or destroy them before consumption. Food handlers and consumers must be made aware of all these factors. Imported products may have different levels of risk. Careful surveillance is necessary to monitor these risks adequately.

1.

The greatest numbers of seafood-associated illnesses are reported from unknown etiologies clinically suggestive of Norwalk and Norwalk-like agents of human enteric viral gastroenteritis. The vast majority of these illnesses are associated with the consumption of raw molluscan shellfish taken from harvest waters contaminated with raw or poorly treated human sewage. Although these are the most common seafood-associated illnesses, they tend to be relatively mild with no associated mortality.

2.

Naturally occurring marine Vibrio species are responsible for fewer reported cases of infections from the consumption of raw molluscan shellfish, but certain species such as V. vulnificus can be associated with high mortality in persons who are immunocompromised or who have underlying liver disease.

Other Seafoods

1.

The greatest microbiological risk associated with seafood other than raw molluscan shellfish appears to be recontamination or cross-contamination of cooked by raw product or contamination during preparation followed by time/temperature abuse. This occurs mainly at the food service (postprocessing) level. The number of total reported cases associated with finfish between 1978 and 1987 is much lower than reported for raw molluscan shellfish in the same period. Available data show that Vibrio parahaemolyticus is responsible for the largest number of other seafood-associated cases of illness, followed by hepatitis A virus, Salmonella (nontyphoidal), Shigella, Clostridium perfringens, and C. botulinum, with HAV and C. botulinum being the most potentially serious of these pathogens. However, the fish-associated HAV infections reported during this time were attributed to two outbreaks due to contamination of prepared seafood by infected food handlers. Seafood-associated illnesses due to C. botulinum were confined to a small geographical area (mainly Alaska) and were associated with the consumption of improperly processed noncommercial products.

2.

Seafood-related parasitic infections are even less common than bacterial or viral infections, with Anisakis simplex and cestodes having the greatest public health impact in the United States. In general, parasitic infections are concentrated in certain ethnic groups that favor consumption of raw or partially cooked seafood harvested from high-risk geographic areas.

Specific Recommendations for Raw Molluscan Shellfish

• High-risk groups (cirrhotics, persons with hemochromatosis, persons who are immunosuppressed) must not eat raw shellfish. It is extremely important that health professionals, especially, be educated concerning food-borne hazards to this group. Proper and thorough cooking of all shellfish before consumption would eliminate microbiological pathogens and helminthic parasites. Individuals who choose to consume raw shellfish should be educated about the potential risks described previously, and how those risks or their effects can be mitigated.
• Adequate and proper treatment and disposal of sewage must be implemented and maintained to avoid contamination of harvest areas by human enteric pathogens. This may require the development of new technology for sewage treatment.
• Valid indicators for contamination of growing waters by human pathogens must be developed. Seafood-borne infections by human enteric viruses in raw and improperly cooked molluscan shellfish could be decreased significantly by the development of valid growing water indicator(s) or direct detection methodologies for human enteric viruses.
• Effective enforcement for elimination of recreational and illegal ("bootlegged") harvesting or sale of molluscan shellfish from known sewage-contaminated shellfish growing areas should be developed and adequately funded.
• Monitoring programs for Vibrio species in molluscan shellfish and growing waters during warm months, as well as support for epidemiological research, should be established.
• Means must be investigated and implemented to eliminate, or at least reduce, levels of potentially pathogenic Vibrio species in raw shellfish. This may necessitate restriction of harvest when water temperatures are high, rapid cool-down and continued chilling of products, and possibly irradiation of live shellstock and shucked products.
• Because of the high risks associated with raw molluscan shellfish, the importation of shellfish for raw consumption should be prohibited unless there is a clear equivalence of standards for harvest waters and for postharvest processing.

General Recommendations for All Seafoods

• Persons consuming raw fish or shellfish should be made aware of the potential microbial risks associated with these practices. Persons in specific high-risk groups (persons with cirrhosis, or hemochromatosis, or those who are immunosuppressed) should never eat raw seafood. Proper and thorough cooking of all seafood before consumption would eliminate the microbiological pathogens and helminthic parasites. Individuals who choose to eat raw seafood should be educated about the potential risks described previously, and how those risks or their effects can be mitigated.
• Any seafood inspection system must be designed to address microbiological hazards through the HACCP approach. This cannot be achieved by the visual or organoleptic inspection currently used for meat and poultry. Seafood inspection requires the development of valid microbiological guidelines to accurately assess potential human health risks from microbial pathogens in raw and processed seafoods; the maintenance of adequate refrigeration; the avoidance of recontamination of cooked, ready-to-eat products by raw products; and good manufacturing practices and proper sanitation. All inspection system guidelines must apply to imported as well as domestic products under memoranda of understanding.
• More research is required to develop new technology-based processing and preservation techniques that provide for safe products. Some of the new processing methods such as sous vide and modified atmosphere can potentially create conditions that favor Clostridium botulinum type E and other pathogens such as Listeria. New methodologies that produce organoleptically superior products must also ensure superior microbiological safety. Certification procedures should be developed for any new processing techniques.
• Continuous, enhanced efforts should be undertaken to educate all health professionals, food handlers, and consumers regarding the microbiological risks of seafood-borne illness and the appropriate means of minimizing such risks, including immediate and adequate refrigeration, proper cooking, avoiding recontamination of cooked products by raw products, proper sanitation, and good personal hygiene, especially at the food service level.
• A food-borne illness surveillance system sufficient for risk identification and regulatory program planning and evaluation must be developed. This system should provide a comprehensive data base that will allow statistically valid assessments of disease incidence and food/behavior risk factors for all food-borne illnesses. It is extremely difficult to assess the relative safety of seafood products accurately, or to manage seafood-borne or other food-borne risks effectively, with the available data bases.
• New or improved methodology [e.g., enzyme-linked immunoabsorbent assay (ELISA), gene probe, polymerase chain reaction] should be developed that provide for rapid identification and quantification of indicators, seafood-associated pathogens, and microbial toxins in seafoods and in harvest waters.