Limiting Transmission Risks

As individuals, we can protect ourselves from zoonoses via simple behaviours that involve basic hygiene and an elementary understanding of the ecosystems and animals with which we come into contact. Indeed, through our daily actions, we can limit our exposure to any zoonotic agents that may occur in our environment. These actions will differ depending on the mode of transmission: direct contact with animals, environmental exposure, ingestion of contaminated food or water, or arthropod vectors (see Figure 11).

Figure 11

Individual actions for preventing infection with directly transmitted, foodborne, and vector-borne zoonoses.

Avoiding contact is the easiest way to prevent directly transmitted zoonotic diseases. For example, you should never touch a dead or injured animal with your bare hands, especially if it is a wild animal. This simple measure will protect you from physical injuries, such as those inflicted by bites, scratches, and beak or horn blows. In other cases, just avoiding any kind of touch is important. For example, hares and other species can be carriers of tularaemia, caused by the bacterium Francisella tularensis, which can penetrate bare skin. You should also avoid handling animals that are sick or behaving abnormally. One obvious example is that wild mammals infected with rabies act differently, which includes displaying less fear of humans. When interacting with pets, you should always adopt good basic hygiene, such as washing your hands after contact. In this way, you avoid introducing zoonotic agents (e.g., any helminth eggs on your pet’s coat) into the mucous membranes of your mouth, nose, or eyes. You should also avoid letting your pets lick you, a behaviour that can infect small lesions on your skin or your mucous membranes with bacteria from your pet’s oral microbiota. In work environments, direct transmission can be prevented by wearing personal protective equipment (PPE): specific clothing to don when dealing with animals; gloves for handling contaminated substances (e.g., dead animals); safety glasses to protect against splashes during pressure washing; and a filtering face mask when disposing of high-risk materials, such as abortion products. PPE can be adapted for use with pets, if necessary. Finally, you can reduce zoonotic risks by properly caring for the health of domestic animals, which includes rearing them under appropriate conditions and using targeted preventive and curative treatments when necessary.

You can also avoid consuming commonly contaminated foods, especially if you are in an at-risk category. Notably, people who are pregnant or immunocompromised should avoid eating raw milk cheeses, certain types of cold cuts, and seafood products, which often carry Listeria bacteria. These two categories of individuals are at risk of severe infections in a way that those with normally functioning immune systems are not. Another recommendation is to avoid collecting wild berries at low elevations, especially in the vicinity of travel corridors used by wild animals. Their droppings can transmit various parasites, such as Echinococcus species. For the same reason, you should carefully wash or peel any vegetables and fruits eaten raw, as they may have been contaminated by excrement. Boiling or cooking food eliminates most foodborne zoonotic agents. You should always cook meat extremely well, especially pork and poultry, to avoid consuming viruses (e.g., hepatitis E), bacteria (e.g., Salmonella or Campylobacter), and parasites (e.g., tapeworms or Toxoplasma). Take extra care when meat is barbecued, a technique that does not always cook foods through. Practicing good hygiene in the kitchen is also important. When refrigerators and work surfaces are improperly cleaned, it allows the growth of enteropathogenic microbes and can result in cross-contamination. Wooden cutting boards are a special concern because they are often used to cut meat but can be difficult to clean.

Waterborne agents can be eliminated via modern water filtration and chemical treatment methods. However, you should never drink water from sources of unknown quality. When traveling internationally, avoid drinking untreated tap or well water. You should also steer clear of ice cubes, which are not always made with drinking water. Cold buffets spread out on crushed ice are another concern. Furthermore, it is preferable that you do not swallow any water used for showering or brushing your teeth. On such trips, adopt the same practices as when you are hiking and want to drink water from ponds or streams. Before drinking any water, you can treat it yourself via filtering, boiling, ultraviolet sterilisation, the use of disinfectant tablets, or a combination thereof. Such treatments can eliminate or inactivate any pathogenic microorganisms present.

For vector-borne zoonotic diseases, the best preventive strategy is to avoid infested areas, wear clothing that entirely covers your body, and apply repellents to your body and clothes. Certain mosquitoes can develop around human dwellings, so it is important to eliminate any sources of stagnant water, even small ones, because they serve as potential breeding grounds (e.g., drainage plates under flower pots, rainwater collection containers, and/or dog water bowls). In highly infested areas, bites can be prevented by covering windows and beds with mosquito nets, to which insecticides may also be applied. To avoid tick bites, you should wear clothing that entirely covers your body. In particular, you should pull your socks up over your lower pant legs. Because ticks remain attached for several days, you should inspect your entire body after each instance of potential exposure (e.g., walks in the forest or picnics at the edge of the woods). Performing a tick check will allow you to identify any that have attached themselves to you and to remove them as quickly as possible, thus limiting the time during which they can transmit viral or bacterial pathogens. The easiest approach is to use a tick removal tool such as a “tick twister”: simply insert the tool’s hooked end between the skin and the tick’s rostrum, then twist the tool around several times. You can also use fine-tipped tweezers. Regardless of the technique, it is essential to remove any ticks quickly and completely.

Boosting the Body’s Defences

Sometimes transmission cannot be avoided. Our bodies respond to the presence of infectious agents by mounting an immune response (see p. 22). However, some people are more sensitive than others to pathogens. Immune responses vary among individuals and within individuals at different points of their lives.

From the very beginning of our lives, our immune systems are learning to deal with the microbes they encounter at the interface created by our natural barriers, namely the skin and mucosa. Thus, each of us has a particular immunological history that is determined by our environments, including exposure to different animals, vegetables, foods, cognitive and emotional experiences, and, most of all, microbes. We are surrounded by a microbial landscape, composed of both pathogens and non-pathogens, that shapes our immune responses through cross-reactions to sets of similar antigens. This developmental process is greatly enhanced by the viruses, bacteria, fungi, protozoa, and acarians that live on us and make up the microbiota associated with our intestines, skin, respiratory systems, and other organs. These microbes also serve as a barrier against invasions.

In particular, the diverse antigens we encounter from our time in utero through our early childhoods seem to teach our immune systems to better tolerate intruders. These interactions influence our subsequent immunological ability to respond to the pathogens we encounter and have an impact on the degree of immune dysfunction. It is thought that allergic, autoimmune, and inflammatory diseases, and even certain cancers, are increasing in prevalence in Western countries because infants and children are exposed to lower levels of microbial diversity (i.e., the hygiene hypothesis). The world’s last hunter-gatherers have the richest microbiota, while humans dwelling in large Western cities have the poorest. Thus, we might derive benefits from early exposure to microbial agents, including the potential pathogens transmitted by animals.

Once this period of immunological malleability has passed, our immune responses display dramatic variability and are influenced by many factors, including our disease history, age, gender, physiological status (e.g., pregnancy), and genetics (see sidebar p. 26).

Are there ways to bolster the immune system? There are many commercial products that claim to do so. However, scientists have frequently failed to find evidence that certain plants or drugs affect the quality of our immune responses. The immune system is complex, as are the factors that interact with it. Thus, it can be quite confusing to understand how “immune enhancement” could happen. One theoretical way to prevent disease could be to modulate the microbiota’s composition, by administering microorganisms with physiological benefits (i.e., probiotics) or dietary fibres that promote the growth and development of specific microorganisms (i.e., prebiotics). Beneficial effects have been seen in people with certain diseases. However, it remains unclear at present how well these food supplements more broadly reinforce the intestinal flora of healthy people. Indeed, their effectiveness seems to depend on many factors, including microbial strain, preparation method, and individual physiology. It is crucial that their use be medically supervised.

As individuals, vaccination is the most effective strategy we can adopt to protect ourselves against zoonotic pathogens. It is mostly available for certain zoonotic diseases, generally those caused by viruses. Vaccination works by stimulating the body’s immune defences without causing disease. The process involves injecting a small quantity of foreign matter, either from the target pathogen or a close relative, before the person has encountered the infectious agent in question. Consequently, the body adds the pathogen to its “memory banks”, allowing a rapid specific protective response by the immune system in the case of future infection by the pathogen. A key step in vaccine development is identifying antigens that can serve as vaccine targets, a task that is easier for viral diseases than for bacterial diseases. It is extremely complex for parasitic diseases, which is why no vaccines against parasites have been developed to date.

Vaccines are medical treatments that must be prescribed by a physician, who will take into consideration a person’s individual health concerns and risks. In Europe, vaccines against certain zoonoses are recommended for people whose work results in specific health risks and for travellers visiting places where transmission rates are high. For example, vaccination against rabies (a viral disease) is recommended for veterinarians, bat biologists, staff at animal shelters, and slaughterhouse workers. It is also a good idea for those travelling to countries whose dog populations have a high prevalence of rabies. Additionally, vaccination against leptospirosis (a bacterial disease) is recommended for people who regularly come in contact with potentially contaminated water during work or leisure activities. However, the antibodies elicited by vaccination do not protect against all the Leptospira serogroups, which is a major limitation of this vaccine. Vaccinating dogs helps protect their owners because infected dogs excrete leptospires in their urine.

For diseases transmitted among humans, the value of vaccination is largely rooted in population-level herd immunity (see p. 31), including to zoonotic pathogens such as SARS-CoV-2 or influenza A(H1N1)pdm09, which caused the 2009 flu pandemic. If a large percentage of a population becomes immune, disease transmission dynamics are disrupted, reducing the risk of infection for those who remain vulnerable. Using modelling, we can estimate the level of vaccination needed to prevent a disease from spreading, a figure that is disease specific but that climbs with pathogen transmissibility. Thus, the individual decision to become vaccinated is an action that we take for the collective good.

Medical Treatment

When transmission has occurred and resulted in illness, medical treatment may become necessary. Depending on the symptoms, physicians may or may not face difficulties in arriving at a diagnosis. As a patient, it is essential to provide details regarding the disease’s onset and context to help determine whether it could be a zoonosis.

Once a disease has been identified, the doctor can potentially prescribe an etiological treatment to eliminate the pathogen: antibiotics for a bacterial infection, anthelmintics for worms, antifungals for a fungal infection, or antivirals for certain viral infections. A disadvantage of these treatments is that they can disrupt our microbiota because they affect not only the pathogen, but also other microbes of the same type. Physicians may request laboratory analyses to determine how sensitive the pathogen is and thus customise the treatment to avoid provoking resistance. At times, it is possible to use highly specific treatments. For example, a person bitten by a rabid dog will be given emergency care involving the injection of an anti-rabies serum containing specific antibodies (i.e., immunotherapy) with a view to blocking the virus from reaching the nervous system. The person will also be vaccinated after the fact, in the hopes that an immune response can develop faster than the virus can spread.

Other forms of treatment are used to ease the disease’s symptoms (e.g., fever and/or pain). Some zoonoses require complex, long-term treatments. For example, alveolar echinococcosis may necessitate extensive surgery and/or prolonged chemotherapy.

Anticipating Risks Through Monitoring and Assessment

Disease surveillance programmes aim to collect reliable, real-time data to detect pathogens as early as possible, describe pathogen distributions in space and time, or verify pathogen presence or absence. More specifically, information on epidemiological indicators is systematically collected and analysed over spatial and temporal scales. These indicators may be focused on humans, domestic animals, wild animals, or environmental factors. They may estimate very different metrics: the number of deaths; the occurrence of certain non-specific syndromes (e.g., abortion or fever); cases reported by medical doctors or veterinarians; isolation of particular pathogen strains; the occurrence of genes related to virulence or antibiotic resistance; environmental factors associated with the presence of certain vectors; and incidents of food safety non-compliance. To strengthen surveillance efforts, it is especially important to develop participatory systems that directly involve different communities (e.g., everyday citizens, consumer groups, livestock associations, networks of practicing veterinarians, and professional solidarity funds). Such work should account for these stakeholders’ specific concerns while firmly establishing collaborations rooted in the humanities, life sciences, and social sciences. These individuals act as boots on the ground because they are on the front lines of surveillance.

Surveillance efforts are event-based (i.e., passive) when they bring together pre-existing public health data and planned (i.e., active) when they carry out research and collect new data via targeted work. Examples of event-based surveillance in France include networks that record and analyse mortality data, such as the Observatory for Farm Animal Mortality (OMAR) or the Epidemiological Surveillance Network for Birds and Wild Terrestrial Mammals (SAGIR). The European equivalent of SAGIR is EWDA. Furthermore, there are official records of animal disease cases, which were reported as required by law. The main limitation of this approach is that it is relies solely on reported cases, which are not necessarily representative of actual cases in the field. Some planned surveillance efforts use sentinel animals (see sidebar p. 118) to determine pathogen occurrence or distribution.

Zoonotic disease surveillance takes place at multiple scales. In France, surveillance in humans is identical for zoonotic and non-zoonotic diseases. Surveillance in animals is carried out by the National Animal Health Epidemiology Platform (ESA). Founded in 2011, ESA participates in international surveillance efforts. It also develops, adapts, and runs several surveillance tools aimed at various zoonotic and animal diseases. Also contributing to surveillance efforts is the National FoodChain Health Surveillance Platform (SCA), which was created in 2018. At the global level, surveillance relies on close collaborations between various international organisations, notably the OIE, WHO, and FAO, which receive funding from the World Bank and the United Nations Development Programme (UNDP). In particular, countries must immediately contact the OIE if they observe any of the organisation’s listed diseases. Furthermore, in 2006, the Global Early Warning System (GLEWS) was established to improve disease detection and jointly manage emerging risks at the ecosystem-human-animal interface. However, its visibility has declined since December 2018 because its website is no longer updated. As illustrated during the COVID-19 pandemic, a key component of surveillance is the availability of effective diagnostic and screening methods (see p. 26). Indeed, pathogens are more easily introduced and spread when infrastructures for monitoring public health and conducting laboratory diagnostics are lacking. Such resources are particularly crucial when dealing with emerging diseases, which require the rapid development and application of laboratory methodologies. For example, when SARS-CoV-1 emerged in 2002–2003, precious time was lost during the search for the causative agent because investigators first thought they were identifying a Chlamydia species and were then convinced that they were dealing with a new type of influenza (see p. 144). Part of OIE’s work is to coordinate a worldwide network of accredited reference laboratories that diagnose major zoonotic and animal diseases.

Surveillance networks are essentially surveillance webs made possible by the contributions of many partners, each acting as a link in the monitoring chain. The latter is made up of multiple components: sample collection, laboratory analysis, data compilation and analysis, result synthesis, knowledge production, and information diffusion. Effective surveillance therefore requires complex infrastructure and multifarious human and technological resources. Long-term access to these resources can be a weakness in surveillance systems because it relies on consistent funding, the availability of qualified personnel, and stakeholder diligence in reporting information to health authorities. It is essential to shore up system resilience should economic or health crises arise, with a view to avoiding a snowball effect. The COVID-19 pandemic provides a clear example of how vulnerable surveillance systems can be. The crisis led to population lockdowns and the remobilisation of human and financial resources across the globe. Another example is the PREDICT surveillance and early warning programme. It was created in 2009 by the US Agency for International Development (USAID) and was terminated by the Trump administration in September 2019, just prior to the emergence of SARS-CoV-2. In addition, it is essential to be able to evaluate the technical effectiveness, public health utility, and social acceptability of surveillance systems if we wish to ensure their improvement.

To properly analyse zoonotic risks, high-quality surveillance is crucial. It yields information that will ultimately allow decision-makers to implement appropriate management strategies. Here, the term risk refers to the likelihood of an adverse event occurring, taking into account its deleterious consequences. In this case, the hazard is a threat to public health caused by one or more zoonotic agents. For a given hazard, risk is estimated using a four-step assessment approach: 1) the hazard’s probability is calculated; 2) the probability of exposure to the hazard is determined; 3) the negative effects of the hazard are quantified; and 4) using qualitative or quantitative methodology, the hazard’s probability of occurrence and harmful consequences are characterised for a given population. Depending on available data, modelling can be employed to various ends. For example, it can facilitate comparisons of different management scenarios. Risk assessment is a multidisciplinary scientific tool that draws upon published peer-reviewed research and other sources, notably expert opinions. The results should be interpreted in plain language that is accessible to all stakeholders. It is important to state all uncertainties and assumptions and to address how they may have influenced the final result. This transparency, alongside transparency regarding conflicts of interest, is an essential part of ensuring that the assessment is sound and that the recommended management strategies are consistent. In France, for instance, risk assessments related to animal health and zoonotic risks are performed by the National Agency for Food, Environmental and Occupational Health & Safety (ANSES), which submits its opinions and recommendations to the competent authorities as well as making them public. In the United Kingdom, such risk assessments are carried out by the Department for Environment, Food, and Rural Affairs (DEFRA). The EU equivalent is the European Food Safety Authority (EFSA). These agencies were created following several health crises (e.g., related to contaminated blood supplies, “mad cow” disease, foot and mouth disease, and dioxin). Their goal is to monitor and analyse risks independently of the governmental authorities tasked with risk management.

Disease Prevention and Protection

There are diverse strategies for preventing zoonotic disease transmission and protecting human communities because the sources of risks and modes of transmission are diverse themselves. The criteria that determine management options are feasibility, cost, effectiveness, social acceptability, and the minimisation of negative impacts (e.g., of an environmental, economic, social, and political nature). Ultimately, our interest in reducing zoonotic risks forces us to reflect on current and future animal production systems (see sidebar p. 127) and on our relationships with animals.

Here, the term prevention refers to preventing recognised zoonotic risks. It is distinct from the term precaution, which refers to measures taken to protect against hypothetical risks (e.g., if there is uncertainty regarding a disease’s zoonotic origin). From a practical perspective, there is extensive overlap in techniques for preventing zoonoses and diseases strictly found in animals. They rely primarily on a combination of health measures and medical solutions, although social, economic, and political strategies may also be important. It is sometimes rather difficult to implement effective measures, especially in the case of zoonoses with many reservoir species or for which the mode of transmission is unclear. Human health, animal health, and ecosystem health are all linked. Theoretically, it is essential to favour approaches that integrate the medical sciences, the veterinary sciences, and ecology. However, there remains a long road between theory and practice (see sidebar p. 122).

FROM ONE HEALTH TO PLANETARY HEALTH

Research on zoonoses has highlighted the myriad interacting relationships among humans, public health, animals, animal health, and the environment. Several terms have been developed to summarise this network of links, each adopting a particular point of view. The One Health concept was first proposed in the 2010s and posited that physicians, veterinarians, and ecologists could work together to arrive at shared benefits. However, this perspective was far too anthropocentric, given that animal populations and environmental factors were largely seen as posing risks to public health. The Global Health approach incorporated the influences of globalisation but nonetheless focused exclusively on the benefits for human health. Finally, the Planetary Health approach brought in the social dimensions of health but did not yield clear recommendations. Instead of any of the above, we should espouse a view in which the intended recipients of any shared benefits are the planet, its living creatures, and its ecosystems. In 2021, the One Health High-Level Expert Panel (OHHLEP) defined One Health as follows: “[It] is an integrated, unifying approach that aims to sustainably balance and optimise the health of people, animals, and ecosystems. It recognises the health of humans, domestic and wild animals, plants, and the wider environment (including ecosystems) are closely linked and interdependent. The approach mobilises multiple sectors, disciplines, and communities at varying levels of society to work together to foster well-being and tackle threats to health and ecosystems, while addressing the collective need for clean water, energy, and air; safe and nutritious food; taking action on climate change; and contributing to sustainable development.” This definition highlights the need to develop concrete policies that foster implementation.

Figure 12

One Health concept. Source: OHHLEP (organisation under the joint aegis of the WHO, FAO, OIE, and UNEP).

Preventive Healthcare

As at the individual level, preventing zoonotic diseases at the community level is primarily based on good basic hygiene, especially when dealing with water- and foodborne pathogens. For instance, local governments must properly maintain shared public spaces and resources to limit contamination involving the faecal matter of domestic or wild animals, which harbour a variety of viruses, bacteria, and parasites. It is essential for populations to have access to high-quality drinking water (i.e., that meets microbial water quality standards). According to the WHO, 71% of the world’s population had access to safe drinking water in 2017. All the points along water distribution systems are critical because they can be contaminated by the faeces of domestic and wild animals. In France, there is a regulatory framework for managing livestock effluent that limits the contamination of waterways and groundwater. For example, regulations stipulate that manure must be kept away from waterways and cannot be spread in rainy weather. In addition, composting-based treatments are encouraged. Treatment facilities have been established upstream of distribution systems and use physical processes such as agglutination, sedimentation, and filtration to eliminate the oocysts of zoonotic protozoa such as Giardia and Cryptosporidium (see p. 52). In addition, a combination of technologies are employed to disinfect the water, the main ones being ozonation, ultraviolet purification, and chlorination. Downstream, routine monitoring is carried out on the water distributed to households to confirm the absence of coliform bacteria, which are markers of faecal contamination.

As with foodborne zoonoses, prevention largely rests on applying basic hygiene practices in the case of structures, equipment, personnel, and products. Every professional along the food chain — including farmers, slaughterhouse workers, product processors, and vendors — has a responsibility to implement preventive measures to limit the risks of contamination. Indeed, in the EU, Hygiene Package regulations specify that food-sector professionals must put in place a customised food safety plan to ensure product compliance with health standards. In addition to respecting good hygiene practices, the food safety plan must be based on hazard analysis critical control point (HACCP) principles, which involve identifying the hazards (i.e., biological, chemical, and physical) associated with specific professional activities and implementing procedures for measuring and monitoring food safety. The entire food industry, from farm to fork, is tasked with following these food safety plans and must furnish tangible results. Thus, a traceability system needs to be put into place. The regulations specify that the system must allow identifiable entities to be traced, used, or located thanks to clear record keeping. Such records must be of sufficient quality that they can be employed for rapid and targeted product recall or withdrawal in the case of a food safety problem. Traceability is particularly important for processed foods, given that product transformation can promote the spread of microbes from a single infected animal to an entire batch of food products. Veterinary services may come in to verify that foodstuffs comply with regulatory requirements, particularly with criteria for microbiological safety. In addition, veterinary services may seize organs or other parts of carcasses that display abnormalities that could pose human health risks, such as the hazardous animal parts that must be eliminated to prevent BSE transmission (see p. 105).

Consequently, preventing infections in farm animals is a key strategy for limiting zoonotic risks, especially those associated with food products. Behaviours and habits must be adopted to reduce the risk of contamination; they generally involve the separation of animals and activities according to risk type. Implementation can take the form of biosafety rules, which describe all the measures employed to reduce the likelihood of pathogen introduction and spread in different contexts (e.g., farms, agrifood facilities, slaughterhouses, veterinary clinics, or laboratories), regions, and food chains.

Biosecurity is based on five principles:

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Exclusion (external biosecurity): preventing pathogens from reaching facilities by taking such measures as quarantining or vaccinating newly arrived animals, verifying food quality, averting the potential for contact with wild animals, and wearing appropriate clothing.

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Compartmentalisation (internal biosecurity): preventing pathogen circulation by compartmentalising facilities, where specific areas are dedicated to events associated with greater health hazards (e.g., quarantine area or birthing area); movement patterns among these areas flow from least to greatest risk.

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Containment: preventing pathogen diffusion away from facilities, which involves cleaning and possibly disinfecting any equipment that leaves the premises, managing waste and effluent, monitoring animal departures, and potentially installing, as seen at certain locations, internal vacuum systems that direct air flow from outside to inside

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Protection: preventing pathogen transfer to humans, which essentially involves practicing good hygiene.

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Preservation: preventing the environmental persistence of pathogens by carefully managing waste and effluent and limiting contact with wildlife.

Obviously, biosecurity implementation must be customised to deal with the specificities of different locations. In particular, laboratories dealing with microbes that are potentially dangerous for humans, animals, or the environment must take certain precautions to protect their staff and their facilities. For example, to reduce risks, some laboratories have been set up on islands or in fairly isolated locations, outside of large urban areas (when the focus is human diseases) or outside of livestock farming zones (when the focus is animal diseases). In certain cases, negative pressure laboratories have been built: the pressure differential between the interior and exterior of the buildings prevents any air from the escaping to the outside. All fluids and waste are fully disinfected prior to disposal. Staff are sometimes required to wear full-body, air-supplied suits to avoid any risk of infection. When an accident is seen, it is because these protocols were not properly respected.

On livestock farms, manure and slurry are major sources of environmental contamination. They must be appropriately dealt with. Prolonged storage or composting leads to fermentation, which is accompanied by increased temperatures. These thermal conditions are likely sufficient to destroy most zoonotic agents, However, further research is needed to clarify the effectiveness of these treatments. In addition, it is also an essential health measure to remove dead animals and infectious materials, such as abortion or birth products.

Sometimes, governments may impose certain measures. For example, to control avian influenza, most countries require those rearing birds to implement biosecurity measures. The specific measures to be taken vary depending on bird abundance (commercial breeding operations vs. backyard poultry), bird type (poultry, game, or zoo) and facility type (commercial, backyard, enclosed building, outdoor enclosure, or aviary). In addition, governments may require poultry and pig farmers to obtain biosecurity training, notably when animals are being reared under industrial conditions. If it is suspected that poultry have been infected with highly pathogenic avian influenza, investigations will be performed that target all types of rearing operations, and adapted measures will be taken in accordance with FAO and OIE recommendations.

However, this hygiene-based approach essentially focuses on health monitoring, documented outcomes, and disinfection efforts after pathogens are detected. Unfortunately, it is not suitable for certain situations, such as dairy farms producing raw milk cheeses. Indeed, cheese processing requires a level of microbial richness that is at odds with microbe elimination. To safeguard the gastronomic heritage that is French cheeses, compromises must be made to preserve the microbial biodiversity underlying cheese production. Fortunately, other forms of animal farming are being (re-)developed, systems that are more respectful of ecosystem functioning and animals’ physiological needs. We are currently rethinking health risks and biosecurity practices.

In our globalised world, animals and products are constantly being traded. Consequently, we face the omnipresent risk of introducing pathogens into new habitats. The health and safety of all countries is thus intimately tied together, as underscored by the COVID-19 pandemic. From a collective perspective, it is crucial to recognise that commercial health and safety regulations are a key part of preventing animal and zoonotic diseases. The OIE is responsible for establishing health and safety standards that ensure health risks are limited during global exchanges of animals and animal products. Animal identification and traceability are essential tools in this work as they promote food safety and animal health (including in relation to zoonoses). All countries have limited human, technical, and financial resources. Thus, public policies prioritise certain diseases, for the most part zoonoses, based on various criteria, including current epidemiological circumstances. In the EU, infectious diseases in animals are assigned to one of five categories depending on their pathogenicity, their zoonotic potential, and their associated prevention and control measures. For instance, exotic diseases are subject to mandatory surveillance and reporting. Eradication programmes have been established for brucellosis, tuberculosis, and rabies. Methods for applying these measures are described in legislative and regulatory texts. Veterinary services are tasked with their enforcement, as part of animal health requirements. In the case of an outbreak of an exotic disease or a disease subject to mandatory reporting, the government will impose restrictions to prevent the pathogen from spreading and clear farms of health threats (see sidebar p. 129). Farmers face sanctions if they do not respect these rules. While the government financially compensates farmers, the funds received never fully cover the losses incurred. Furthermore, a herd’s value is never just monetary; it is also emotional, psychological, and genetic. That said, these measures have functioned quite well from a public health perspective. They have led to a pronounced decrease in major zoonoses transmitted by domestic ruminants, such as brucellosis and tuberculosis.

Preventive Medicine

For some zoonoses, preventive healthcare is inappropriate or insufficient for stopping pathogen transmission to humans. Such cases may require the use of preventive medicine, including vaccines or medications. Above, we discussed using vaccination in humans to prevent the occurrence of certain zoonoses (see p. 114). Vaccination can also be deployed to establish herd immunity in the animal populations responsible for transmission to humans, thus severing the chain of infection. This approach is used with rabies, which largely infects humans as a result of dog bites. The WHO has found that, in countries with a high prevalence of rabies, vaccinating dogs is the most efficacious and cost-effective strategy for preventing human infections. Not only does it reduce human deaths due to dog rabies, but it also diminishes the need for post-exposure treatments in the case of dog bites (see p. 103). In 2015, the WHO, OIE, and FAO held a global conference during which an international consensus was reached: 2030 was set as the target year for eliminating dog-transmitted rabies cases in humans. This goal seems feasible, even if the precise number of human rabies cases remains difficult to estimate. That said, there are challenges related to the long-term implementation and funding of the global strategic plan, which serves to illustrate the complexity of collaborative initiatives on animal and human health (see sidebar p. 122).

Similarly, human populations are protected when ruminants are systematically vaccinated against Rift Valley fever virus, which causes a zoonosis that occurred in Mayotte in 2018–2019 and that continues to crop up in Africa. Vaccinated animals no longer serve as amplifying hosts when bitten by infected mosquitoes. Human smallpox was declared eradicated in 1980 following vaccination campaigns conducted by the WHO. Similarly, the OIE announced in 2011 that rinderpest, a non-zoonotic viral disease of artiodactyls, was eradicated. It is clear from such health victories that the systematic vaccination of domestic species can bear fruit and is often better accepted by local populations than are human vaccination campaigns.

Furthermore, some degree of human protection can be afforded by treating domestic animal populations that are infected with zoonotic pathogens, as mentioned in the section about disease prevention at the individual scale. However, such strategies must be employed with caution because the broad-scale use of antibiotics, anthelmintics, and insecticides can have significant environmental consequences and promote the emergence of resistance (see p. 82).

Prevention—the Environment and Wildlife

Broad-scale actions for preventing vector-borne zoonoses mainly take the form of disinsectisation programmes, during which the egg, larval, or adult stages of arthropod vectors are eliminated from the environment. Unfortunately, spray programmes have adverse effects on non-target arthropods, and the chemicals’ effects can become magnified in primarily insectivorous animals, such as certain bird and bat species. Equivalent concerns exist even for biological control strategies, such as the use of Bacillus thuringiensis (Bti) spores to deal with mosquitoes. Tick populations can only be controlled by holistically managing ecosystem-level biodiversity. This integrated management approach is also useful in the case of other vectors.

In addition, wild species are often unnoticed reservoirs of zoonotic diseases that pass to humans, with farm animals sometimes acting as conduits. When zoonotic pathogens are harboured in wildlife, it can be more challenging to implement government-mandated control measures in farm animals. It may thus become necessary to apply strategies that promote or medically manage the health of wildlife populations. Many European countries have wildlife health surveillance programmes. However, the latter vary in scope and not all are coordinated at the national level. Consequently, in Europe, information about the health status of wildlife populations remains limited. Management measures targeting wildlife tend to be costly and poorly regulated. They must also navigate sociological complexities because different stakeholders (e.g., hunters, farmers, and naturalists) have different relationships with wildlife. Indeed, it is important to engage with stakeholders because the above broad-scale preventive measures often rely on labour volunteered by these groups of individuals. Failure to do so can result in counterproductive stalemates. In addition, certain wild species may be subject to highly different management regimes, depending on the context (e.g., species that are commercially hunted).

To eliminate zoonotic pathogens, it is largely impossible to completely eradicate all the members of a wild reservoir species, in contrast to what is done with livestock on farms. First of all, such an approach would be ethically questionable and socially unacceptable. Second, it represents a major conservation issue because destroying an animal population also means losing the genetic wealth it represents. Third, it would be challenging to carry out and would likely only be effective in the case of small, well-defined, and easily accessible populations of a known size. In France in 2006, this management strategy was applied to red deer (Cervus elaphus), a species that serves as a reservoir for bovine tuberculosis. More particularly, efforts targeted a population found in Normandy’s Brotonne forest, which was being managed for hunting purposes and was isolated by physical barriers (i.e., a river and a highway). However, even in this case, the population’s size was more than double the original estimate, which changed how the outbreak had to be handled.

Setting aside the example of population eradication, managing population densities can be a useful approach. Theoretically, it is possible to prevent a disease’s persistence and spread within wildlife by forcing the disease’s R_e below a certain threshold. However, this metric can be complex to estimate (see p. 31).

Under real-life circumstances, the culling of wildlife populations often leads to increased pathogen persistence or spread. Indeed, such efforts can disrupt ecosystem dynamics, resulting in cascading responses in animal behaviour, social structure, territoriality, migration, or reproduction. The impacts extend beyond the target species to all the other species, large and small, with which it interacts. There may be major consequences for biodiversity, and the situation can give rise to various health and conservation issues. In addition, such strategies are costly and difficult to maintain over the long term. In Europe, counterproductive effects were seen in culling campaigns focused on red foxes (V. vulpes), aiming to control alveolar echinococcosis; an Alpine ibex (Capra ibex) population affected by brucellosis; and badgers (Meles meles) occurring in proximity to bovine tuberculosis outbreaks, to name a few examples. An alternative to eliminating the entire population is to solely eliminate infected individuals. However, this approach is complicated to implement, given that it requires the use of an efficient screening test (see p. 26) that is adapted to the target species and field conditions. The animals must also be captured, which can be quite expensive, given the technical and human resources required. For example, on average, it costs around €720 to serologically screen an ibex for brucellosis, as the animal must be trapped first. This figure is €4.60 for a domestic ruminant.

Control campaigns often target rodents because they host various microorganisms and tick larvae, which themselves vector pathogens. Anticoagulants are the chemical compounds that tend to be deployed. Unfortunately, they can also end up killing the rodents’ predators, which undermines control efforts. In urban areas, the targets are rats and mice. However, control efforts can only be effective if human food waste is also limited, and potential refuges are addressed. In rural areas, the main rodents of concern are voles (genera Microtus and Arvicola).

Their high densities largely result from intensive agricultural activity, including landscape simplification, hedgerow removal, and the elimination of predators.

Vaccination can be a helpful management strategy for changing transmission dynamics when a disease is established within wildlife, especially when populations exist along a continuum. In the latter situation, density reduction becomes an ineffective strategy. However, mass vaccination is costly and complex to implement. First, an effective vaccine against the disease must be available and suitable for use in the target wildlife species; oral administration is often a prerequisite. Second, ideally, the vaccine must be safe for both the target species and non-target species because various animals may end up ingesting it. Meeting this requirement is no easy task. Third, orally administering the vaccine means designing baits that will appeal to the target species and that can be distributed over a very precise grid within the target zone, either on foot or by plane. Given the cost in time and resources, it is difficult to maintain immunisation efforts over the long term. Thus, this strategy should only be prioritised if it is possible to attain a level of vaccination coverage that achieves herd immunity within the population over the short term. In 2001 in France, this approach was successfully used to eradicate vulpine rabies (see sidebar p. 134). At present, research is exploring the possible use of an injectable or oral vaccine to protect badgers against bovine tuberculosis.

Sometimes, other medical strategies are utilised to disrupt the transmission dynamics of zoonotic diseases in wildlife. Administering drug-based treatments raises ethical and environmental concerns as this approach involves disseminating pharmaceutical compounds into nature. There could be negative effects on the biology of target and non-target species alike. It could also lead to the appearance of resistance (see p. 82). In addition, as in the case of vaccination, this method is costly and complex to implement. It also requires the capture and release of many animals or the distribution of numerous drug-containing baits. This approach is sometimes used in efforts to control alveolar echinococcosis. Foxes are given an anthelmintic, praziquantel. Although parasite prevalence greatly declines, the worms are not eradicated. Although flatworms rarely seem to develop resistance to pesticides, it is hard to predict the potential impacts of the compound’s broad-scale dissemination. Another medical strategy under debate is administering immunocontraceptives to dampen reservoir host reproduction and thus affect population dynamics. In the UK, this method has been proposed as a way to address the role played by badgers in bovine tuberculosis epidemiology, given societal disapproval of badger culling. However, preliminary research has suggested that the feasibility and efficacy of this approach relies on females being treated at least every two years. This method is not without its detractors, as the immunocontraceptives could potentially affect non-target species or end up in the environment, which is already filled with endocrine disruptors.

For game species, additional actions can be taken to reduce zoonotic risks. For example, in the case of bovine tuberculosis, preventive actions consist of collecting animal viscera following a hunt, such that they can be destroyed at a rendering plant, and prohibiting the supplemental feeding of wild animals (a hunting practice) near sites of disease outbreaks. However, it is possible that the opposite effect could be achieved: if supplemental feeding stops, animals from the zone where the disease is present may move to areas where feeding is maintained, thus introducing the pathogen. In theory, it would be possible to put up fences and delineate high-risk zones. However, such a strategy is complex to apply and maintain over the long term; it might also be difficult for the population to accept.

All issues considered, the most appropriate or most feasible approach is often to just leave wildlife alone and to focus instead on farm biosecurity, including efforts to limit contact between domestic and wild animals. To this end, farmers can install and maintain solid perimeter fencing, monitor self-feeders in pastures, remove salt stones from mountain pastures, block wildlife from accessing livestock watering points, and keep free-range poultry in confined areas. However, it is impossible to guarantee that no contact at all will occur. Furthermore, these measures must be implemented by farmers, who may not agree with them or have the resources required for implementation.

Another essential facet of these efforts is to effectively deal with illegal wildlife trafficking. The possession and transport of wild animals is strictly regulated (see sidebar p. 157).

Communication

In any risk analysis, the last step is typically communicating the results. The first two steps are hazard identification and risk assessment, both performed by experts. The third step is risk management, which is carried out by decision-makers. However, all four steps must sometimes take place simultaneously, notably in times of crisis, as people face new diseases associated with many unknowns. It is particularly important to communicate with the general public about health risks so that individual preventive measures can be taken in response to collective-level strategies. Individual behavioural choices are particularly important for avoiding infections with tick-borne zoonoses (see p. 109). The same is true for foodborne zoonoses. Infections frequently occur as a result of household conditions, such as poor food storage, improper cooking practices, or cross-contamination between foods. There is another set of essential tools for reducing zoonotic risks: boosting the awareness, knowledge, and training of farmers and other professionals who work with animals or animal products across a variety of industries.

Health education can take the form of governmental programmes that encourage behavioural changes aimed at reducing exposure risks or that limit the consequences of exposure at the population level. These programmes can take various forms, including communication campaigns aimed at the general public or the most vulnerable members of the public; expert advice provided by health professionals; continuing education; extracurricular courses; and food labelling. Although a range of programmes may be used, their effectiveness may remain limited. Indeed, the top-down transmission of information, from “experts” to “laypeople”, is often doomed to failure because individuals differ in how they gauge risks and relate best to outreach that is rooted in their own life experiences. This challenge becomes especially clear when communication involves “invisible threats”, like pathogens. For example, even within the community of bat researchers, perspectives on the taxon’s zoonotic risks vary greatly, depending on the specific field of study. Unidirectional messaging will only be effective when people are open to receiving it, namely because they were already receptive. That said, people can be “nudged” towards specific behaviours that promote health by fostering certain conditions. For instance, making meat thermometers available can encourage people to assess whether their meat is properly cooked. Alternatively, positioning sinks in a more accessible way can encourage people to wash their hands before they enter the lunchroom.

Historically, researchers have tended to communicate primarily with their peers. However, sharing research with the public is now of paramount importance. Furthermore, given the current paucity of financial resources for research, highlighting the relevance of one’s work to potential funding organisations has become almost vital for scientists. Therefore, communication by researchers is not necessarily altruistic. Indeed, it is always easier to find funding for scientific topics that have received media coverage and that have an apparent societal impact. To illustrate, prion research would not have received the same levels of funding without the media focus on the “mad cow crisis”.

However, communicating about health risks is a particularly delicate task, especially in crisis situations. When the term “health crisis” is used following an event such as the emergence of a new disease, it is because the government has failed to nurture a sense of security and trust within the population. The notion of a “crisis” evokes images of political and social destabilisation in urgent need of a response and carries weighty significance within the context of the media. Crises generally arise in situations characterised by uncertainty and result from differences in how everyday citizens and public-sector experts perceive existing risks. The latter are scientists who have been tasked with sharing their collective expertise with decision-makers and thus informing public policy. This work involves making effective use of all available scientific data and knowledge while remaining fully transparent with regards to any uncertainties. One of the late 20th century’s major health crises arose in relation to “mad cow” disease. In this case, the actual threat ended up falling far short of the dire predictions made (see p. 105). A similar scenario occurred in 2009, when the A(H1N1)pdm09 virus emerged, and the WHO declared an influenza pandemic (see p. 96). Some countries feared that the strain would be highly pathogenic and overreacted, given that the strain exhibited low virulence in humans. Such cases illustrate how challenging it can be to plan for the pathways, impacts, and real-time management of emerging infectious diseases.

During health crises, public authorities intend to reassure the population through their actions but often end up having the opposite impact. Such situations are examples of a “security paradox”: the more those in power seek to engender a sense of security, the more they actually generate insecurity, especially if the underlying issues are not clearly identified and explained. If the government is transparent about existing gaps in knowledge, the population retains the message that the authorities are not well informed, creating insecurity. However, if the government fails to transparently communicate about sources of uncertainty, the population feels as though it is not receiving the full story and ends up suspicious, a situation that feeds conspiracy theories. It is therefore essential for governments to remain transparent in their announcements and decisions — clearly expressing what is and is not known. Unfortunately, urgent communication is often overly rushed and insufficiently planned, which sometimes results in ambiguous or contradictory statements. As a consequence, institutions are further discredited in the eyes of the public, and distrust of public authorities deepens. For example, during the 2006 A[H5N1] “bird flu” crisis, a French government official recommended that meat be cooked thoroughly, a statement that probably contributed to the subsequent drop in poultry meat sales. However, the virus is airborne, not foodborne. Indeed, the phrasing was particularly clumsy: “there is nothing to fear, especially if the meat is well cooked”. Cooking poultry meat properly is important, but only because it eliminates other zoonotic agents, such as salmonella.

Unfortunately, media coverage of scientific results has become just another commodity. This situation is at odds with the need to deliver accurate information to the public and to ensure transparency regarding uncertainties. In fact, certain media outlets exist solely to profit off of their large audiences and thus seek to garner a maximum of attention. To this end, they tend to utilise a narrative style in which information and figures are sequentially provided with the aim of surprising, shocking, or frightening the public. They may also pass the microphone to self-proclaimed experts who deliberately seed conflict. Professor Osterhaus at the Erasmus MC Research Centre in the Netherlands teaches a workshop in which young researchers learn to prepare for a 10-minute interview with a journalist. He recommends coming up with a single sentence that conveys the key scientific message, which should be repeated over and over for the full 10 minutes. In this way, the final message cannot be edited out given that it is the same as the initial message. Indeed, comments taken out of context can easily be repeated and turn into a “truth” that reappears over and over in certain mainstream media. This outcome has become all the more likely given that some journalists obtain information (e.g., sound bites and quotes) directly from specialised new agencies. As the information’s context and source are not always verified, it is easy for errors and misunderstandings to occur. For example, several media outlets wrote headlining articles about the link between declining vulture populations and increasing human rabies cases in India. However, the journalists were treating the correlation between the two factors as fact even though no relationship had been established. Another serious problem is that certain journalists establish an equivalence between scientific findings and arguments arising from scientism and transhumanism. Indeed, it is irresponsible to spread the belief that humanity will find solutions to past and present ecological disasters and that, as a result, it is unnecessary to rethink our ways of living and the paradigm of unlimited growth upon which they are based. On the contrary, we must place practical and ethical limits on technological development. We need to take the time to fully consider the major challenges represented by climate change and biodiversity collapse.

While modern technologies facilitate access to information and allow its widespread dissemination, they also hinder higher-quality communication, which involves exchange, dialogue, respectful debate, and constructive criticism. Governance in public health is only efficient when it draws on expertise from multiple sources. It must also build reciprocal exchanges and a relationship of trust among scientific experts, everyday citizens, important third parties (e.g., non-profit organisations or labour unions), administrative bodies, and decision-makers. Guided by the humanities and social sciences, work is underway to develop these collaborative approaches to defining public policies (e.g., living laboratories). However, there is still a long way to go before these approaches are fully integrated into the policymaking status quo.