3.1. OVERVIEW
Preventing or reducing dengue virus transmission depends entirely on control of the mosquito vectors or interruption of human–vector contact.
Activities to control transmission should target Ae. aegypti (the main vector) in the habitats of its immature and adult stages in the household and immediate vicinity, as well as other settings where human–vector contact occurs (e.g. schools, hospitals and workplaces), unless there is sound evidence that Ae. albopictus or other mosquito species are the local vectors of dengue. Ae. aegypti proliferates in many purposely-filled household containers such as those used for domestic water storage and for decorative plants, as well as in a multiplicity of rain-filled habitats – including used tyres, discarded food and beverage containers, blocked gutters and buildings under construction. Typically, these mosquitoes do not fly far, the majority remaining within 100 metres of where they emerged. They feed almost entirely on humans, mainly during daylight hours, and both indoors and outdoors.
Integrated vector management (IVM) is the strategic approach to vector control promoted by WHO (1) and includes control of the vectors of dengue. Defined as “a rational decision-making process for the optimal use of resources for vector control”, IVM considers five key elements in the management process, namely:
advocacy social mobilization and legislation – the promotion of these principles in development policies of all relevant agencies, organizations and civil society; the establishment or strengthening of regulatory and legislative controls for public health; and the empowerment of communities;
collaboration within the health sector and with other sectors – the consideration of all options for collaboration within and between public and private sectors; planning and decision-making delegated to the lowest possible administrative level; and strengthening communication among policy-makers, managers of programmes for the control of vector-borne diseases, and other key partners;
integrated approach to disease control – ensuring the rational use of available resources through the application of a multi-disease control approach; integration of non-chemical and chemical vector control methods; and integration with other disease control measures;
evidence-based decision-making – adaptation of strategies and interventions to local vector ecology, epidemiology and resources, guided by operational research and subject to routine monitoring and evaluation;
capacity-building – the development of essential infrastructure, financial resources and adequate human resources at national and local levels to manage IVM programmes, based on a situation analysis.
Control of Ae. aegypti is mainly achieved by eliminating container habitats that are favourable oviposition sites and which permit the development of the aquatic stages.
The habitats are eliminated by preventing access by mosquitoes to these containers or by frequently emptying and cleaning them, by removing the developing stages using insecticides or biological control agents, by killing the adult mosquitoes using insecticides, or by combinations of these methods.
Historically, efforts to control dengue vectors in the WHO Region of the Americas resulted in the elimination of Ae. aegypti populations from much of the neotropics by the 1970s. However, re-introductions followed, leading to the re-establishment of vector populations. Today, therefore, the main aim of most programmes is to reduce the densities of vector populations as much as possible and to maintain them at low levels. Where feasible, efforts may also be made to reduce the longevity of adult female mosquitoes by the use of insecticidal methods in order to lessen the risk of virus transmission.
In selecting the most appropriate vector control method, or combination of methods, consideration should be given to the local ecology and behaviour of the target species, the resources available for implementation, the cultural context in which control interventions are carried out, the feasibility of applying them in a timely manner, and the adequacy of coverage. Methods of vector control include the elimination or management of larval habitats, larviciding with insecticides, the use of biological agents and the application of adulticides.
3.2. METHODS OF VECTOR CONTROL
Ae. aegypti uses a wide range of confined larval habitats, both man-made and natural. However, it may not be feasible or cost-effective to attempt to control the immature stages in all such habitats in a community. Some man-made container habitats produce large numbers of adult mosquitoes, whereas others are less productive. Consequently, control efforts should target the habitats that are most productive and hence epidemiologically more important rather than all types of container, especially when there are major resource constraints. Such targeted strategies require a thorough understanding of the local vector ecology and the attitudes and habits of residents pertaining to the containers.
3.2.1. Environmental management
Environmental management seeks to change the environment in order to prevent or minimize vector propagation and human contact with the vector-pathogen by destroying, altering, removing or recycling non-essential containers that provide larval habitats. Such actions should be the mainstay of dengue vector control. Three types of environmental management are defined:
Environmental modification – long-lasting physical transformations to reduce vector larval habitats, such as installation of a reliable piped water supply to communities, including household connections.
Environmental manipulation – temporary changes to vector habitats involving the management of “essential” containers, such as frequent emptying and cleaning by scrubbing of water-storage vessels, flower vases and desert room coolers; cleaning of gutters; sheltering stored tyres from rainfall; recycling or proper disposal of discarded containers and tyres; management or removal from the vicinity of homes of plants such as ornamental or wild bromeliads that collect water in the leaf axils.
Changes to human habitation or behaviour – actions to reduce human–vector contact, such as installing mosquito screening on windows, doors and other entry points, and using mosquito nets while sleeping during daytime.
The choice of approach should be effective, practicable and appropriate to local circumstances. Actual or potentially important container types that cannot be removed from the area should be dealt with in situ. summarizes the main actions used to control immature Aedes larval habitats.
Environmental management actions to control immature stages of Aedes aegypti.
Improvements in, and maintenance of, urban infrastructure and basic services contribute to the reduction in available larval habitats since large Ae. aegypti populations are often associated with poor water supply and inadequate sanitation and waste disposal services.
3.2.1.1. Improvement of water supply and water-storage systems
Improving water supplies is a fundamental method of controlling Aedes vectors, especially Ae. aegypti. Water piped to households is preferable to water drawn from wells, communal standpipes, rooftop catchments and other water-storage systems. However, potable water must be supplied reliably so that water-storage containers that serve as larval habitats – such as drums, overhead or ground tanks and concrete jars – are not necessary. In urban areas the use of cost-recovery mechanisms such as the introduction of metered water may actually encourage household collection and storage of roof catchment rainwater that can be harvested at no cost, resulting in the continued use of storage containers. Traditional water storage practices may also persist even when reliable supplies are available. The installation of reliable piped water supplies in houses should therefore be accompanied by a communication strategy that discourages traditional storage practices.
3.2.1.2. Mosquito-proofing of water-storage containers
Water-storage containers can be designed to prevent access by mosquitoes for oviposition. Containers can be fitted with tight lids or, if rain-filled, tightly-fitted mesh screens can allow for rainwater to be harvested from roofs while keeping mosquitoes out. Removable covers should be replaced every time water is removed and should be well maintained to prevent damage that permits mosquitoes to get in and out.
Expanded polystyrene beads used on the surface of water provide a physical barrier that inhibits oviposition in storage containers from which water is drawn from below via a pipe and from which there is no risk of overflow. These beads can also be placed in septic tanks, which Ae. aegypti sometimes exploits.
3.2.1.3. Solid waste management
In the context of dengue vector control, “solid waste” refers mainly to non-biodegradable items of household, community and industrial waste. The benefits of reducing the amount of solid waste in urban environments extend beyond those of vector control, and applying many of the basic principles can contribute substantially to reducing the availability of Ae. aegypti larval habitats. Proper storage, collection and disposal of waste are essential for protecting public health. The basic rule of “reduce, reuse, recycle” is highly applicable. Efforts to reduce solid waste should be directed against discarded or non-essential containers, particularly if they have been identified in the community as important mosquito-producing containers.
Solid waste should be collected in plastic sacks and disposed of regularly. The frequency of collection is important: twice per week is recommended for housefly and rodent control in warm climates. Integration of Ae. aegypti control with waste management services is possible and should be encouraged.
It is also important to provide information on these activities to encourage and promote them. Globally, recycling is on the increase. This practice places value on many items previously classified as waste products, leading to growth in the recycling market and profit for both small and large-scale businesses as a consequence. But although recycling can contribute to significant economic improvements, the recycling market can potentially impact dengue vector populations. For there to be an impact, however, containers of importance must have value in the marketplace, be it real (e.g. plastics or tyres for recycling) or created (e.g. beverage container deposit laws), and advertising and promotion must be sustained.
Used tyres are common and sometimes highly productive larval habitats that may warrant special attention in urban areas. Discarded tyres should be collected, recycled or disposed of by proper incineration in waste transformation facilities (e.g. incinerators, energy-production plants, or lime kilns fitted with emission control devices). Regulation of the sale of new tyres mandating the payment of an additional deposit and return charge may also be an incentive for better management and disposal of old tyres. Tyres can be recycled in a variety of ways, including for use as shoe soles, flooring, industrial rubber gaskets or household hardware (e.g. buckets, rubbish bins). Industrially shredded tyres can be incorporated into road surfacing materials. Sanitary regulations may require that whole tyres are buried in a separate area of a landfill to avoid their rising upwards under compaction and disrupting soil cover.
3.2.1.4. Street cleansing
A reliable and regular street cleansing system that removes discarded water-bearing containers and cleans drains to ensure they do not become stagnant and breed mosquitoes will both help to reduce larval habitats of Ae. aegypti and remove the origin of other urban pests.
3.2.1.5. Building structures
During the planning and construction of buildings and other infrastructure, including urban renewal schemes, and through legislation and regulation, opportunities arise to modify or reduce potential larval habitats of urban disease vectors, including Ae. aegypti, Culex quinquefasciatus and An. stephensi. For example, under revised legislation in Singapore, roof gutters are not permitted on buildings in new developments because they are difficult to access and maintain. Moreover, property owners are required to remove existing gutters on their premises if they are unable to maintain them satisfactorily.
3.2.2. Chemical control: larvicides
Although chemicals are widely used to treat Ae. aegypti larval habitats, larviciding should be considered as complementary to environmental management and – except in emergencies – should be restricted to containers that cannot otherwise be eliminated or managed. Larvicides may be impractical to apply in hard-to-reach natural sites such as leaf axils and tree holes, which are common habitats of Ae. albopictus, or in deep wells. The difficulty of accessing indoor larval habitats of Ae. aegypti (e.g. water-storage containers, plant vases, saucers) to apply larvicides is a major limitation in many urban contexts.
As Ae. aegypti often deposits eggs in water-storage containers, the larvicides should have low toxicity to other species and should not significantly change the taste, odour or colour of the water.
The International Programme on Chemical Safety (IPCS) has assessed the toxicity of the active ingredients methoprene, pyriproxyfen and temephos and those in Bacillus thuringiensis serovar israelensis (Bti) to determine their safety for use as mosquito larvicides in drinking-water at dosages that are effective against Aedes larvae. However, the safety of the active ingredients in the final formulation varies from product to product and requires further study, as does the possible microbiological contaminants in formulations of Bti. WHO's Guidelines for drinking-water quality (3) provide authoritative guidance on the use of pesticides in drinking-water. Understandably, placing chemicals in domestic water, particularly drinking-water, is often viewed with suspicion and may be unacceptable in some communities.
3.2.2.1. Target area
Productive larval habitats should be treated with chemicals only if environmental management methods or other non-chemical methods cannot be easily applied or are too costly. Perifocal treatment involves the use of hand-held or power-operated equipment to spray, for example, wettable powder or emulsifiable-concentrate formulations of insecticide on larval habitats and peripheral surfaces. This will destroy existing and subsequent larval infestations in containers of non-potable water, and will kill the adult mosquitoes that frequent these sites. Perifocal treatment can be used to treat containers, irrespective of whether they hold water or are dry at the time of application. The internal and external walls of containers are sprayed until they are covered by a film of insecticide, and spraying is also extended to cover any wall within 60 cm of the container. Perifocal treatment thus has both larviciding and residual adulticiding characteristics. This method is suitable only for collections of non-potable water (such as in large piles of tyres or discarded food and beverage containers).
3.2.2.2. Insecticides
lists the mosquito larvicides that are suitable for application to non-potable water containers. For treatment of drinking-water, temephos and methoprene can be applied at dosages of up to 1 mg of active ingredient (a.i.) per litre (1 ppm); pyriproxyfen can be applied at dosages up to 0.01 mg a.i. per litre (0.01 ppm) and Bti at1–5mg per litre
WHO-recommended compounds and formulations for control of mosquito larvae in container habitats.
3.2.2.3. Application procedures
Hand-operated compression sprayers are suitable for applying liquid insecticides to larger larval habitats. Knapsack sprayers are also suitable, especially for delivering wettable powder formulations. A syringe or pipette can be used for treating indoor flower vases and ant traps. Granule and certain other solid formulations are applied directly by (protected) hand to confined larval habitats or by a convenient standard measure (e.g. a dessert spoon or teaspoon). When treating containers of drinking-water, sufficient insecticide should be added for the volume of the container even if the container is not full of water (e.g. 1 g of 1% temephos granules for 10 litres of container volume).
3.2.2.4. Treatment cycle
The treatment cycle will depend on the species of mosquito, seasonality of transmission, patterns of rainfall, duration of efficacy of the larvicide and types of larval habitat. Two or three application rounds carried out annually in a timely manner with proper monitoring of efficacy may suffice, especially in areas where the main transmission season is short.
3.2.2.5. Precautions
Extreme care must be taken when treating drinking-water to avoid dosages that are toxic for humans. Label instructions must always be followed when using insecticides.
3.2.3. Chemical control: adulticides
Methods of chemical control that target adult vectors are intended to impact on mosquito densities, longevity and other transmission parameters. Adulticides are applied either as residual surface treatments or as space treatments.
3.2.3.1. Residual treatment
Perifocal treatment, as described above, has both adulticiding and larviciding effects. Suitable insecticides can be applied with hand-operated compression sprayers. Power sprayers can be used to treat large accumulations of discarded containers (e.g. tyre dumps) rapidly. Care must be taken not to treat containers used to store potable water.
3.2.3.2. Space sprays and their application
Space spraying is recommended for control only in emergency situations to suppress an ongoing epidemic or to prevent an incipient one. The objective of space spraying is the massive, rapid destruction of the adult vector population. However, there has been considerable controversy about the efficacy of aerosol insecticide applications during epidemics of dengue and yellow fever. Any control method that reduces the number of infective adult mosquitoes, even for a short time, should reduce virus transmission during that time, but it remains unclear whether the transient impact of space treatments is epidemiologically significant in the long run. There is no well-documented example of the effectiveness of this approach in interrupting an epidemic. Nevertheless, if space spraying is used early in an epidemic and on a sufficiently large scale, the intensity of transmission may be reduced, which would give time for the application of other vector control measures that provide longer-term control, including larviciding and community-based source reduction. Thus, if disease surveillance is sensitive enough to detect cases in the early stages of an epidemic, and if the resources are available, emergency space spraying can be initiated at the same time as source reduction measures and larviciding are intensified.
Not only insecticide susceptibility but also droplet size, application rate and indoor penetration of the insecticide are all crucial to the efficacy of this method for controlling Ae. aegypti. Indoor penetration of an insecticide depends on the structure of the building, whether doors and windows are left open during spraying and, when applied from vehicle-mounted equipment, residential block configuration, the route of the spray vehicle and meteorological conditions. Where indoor penetration of droplets is likely to be poor, indoor application with portable equipment will be more effective against Ae. aegypti. However, rates of coverage are much lower and accessibility may be difficult, particularly in large cities.
Vector populations can be suppressed over large areas by the use of space sprays released from low-flying aircraft, especially where access with ground equipment is difficult and extensive areas must be treated rapidly. Indoor penetration of insecticide droplets is again a critical factor for efficacy. In applying space sprays from the air, careful consideration must be given to meteorological conditions, especially wind speed at spray height and at ground level, and to the droplet size spectrum obtained at the flying speed of the aircraft. For all aerial spraying operations, clearance must be obtained from the civil aviation authority. For safety reasons, populated areas must usually be sprayed from twin-engined aircraft. Modern aircraft are fitted with global positioning systems so the exact position of the aircraft while the insecticide is being applied can be accurately recorded.
Target area
Since total coverage can rarely be achieved during ground applications, space spraying should focus on areas where people congregate (e.g. high-density housing, schools, hospitals) and where dengue cases have been reported or vectors are abundant. Selective space treatment up to 400 metres from houses in which dengue cases have been reported is commonly practised (and is sometimes also referred to as “perifocal spraying”). However, by the time a case is detected and a response mounted, the infection is likely to have spread to a wider area. Thorough planning is required to ensure that adequate resources (equipment, insecticide, human and financial resources) can be deployed in a timely manner to ensure proper coverage. Only if resources permit should area-wide treatment be considered.
Insecticides
lists the insecticides that are suitable for space spraying as cold aerosols or thermal fogs. The choice of insecticide formulation for space spraying in and around dwellings should be based on its immediate environmental impact and the compliance of the community. Only insecticide products with high flash-points should be used for thermal fogging. Space-spraying formulations are usually oil-based, as the oil carrier inhibits evaporation of small fog droplets. Diesel fuel has been used as a carrier for thermal fogging agents, but it creates thick smoke, has a strong smell and creates oily deposits, which may lead the community to reject its use. Water-based formulations are also available, some of which contain substances that prevent rapid evaporation. Label instructions should always be followed when using insecticides.
Selected insecticides for cold aerosol or thermal fog application against mosquitoes.
Application procedures
Space sprays can be applied either as thermal fogs at 10–50 l/ha or as ultra-low-volume applications of undiluted or slightly diluted technical-grade insecticide in the form of a cold aerosol of droplets of controlled size (15–25 μm) at a rate of 0.5–2.0 l/ha. Portable or vehicle-mounted thermal or cold-fog generators can be used for ground application. If the target area exceeds 1000 ha or cannot be covered by ground equipment within 10 days, aerial cold fog application is sometimes used. However, several factors must first be considered – including safety, timeliness, cost, meteorological conditions, vector behaviour, biological effectiveness and availability of equipment, operational sites, and highly skilled air and ground crews. The difficulties of ensuring penetration of insecticide droplets into the resting sites of the target species are similar to those for aerosols dispensed from road vehicles. For ground applications, maps of the areas to be sprayed showing all passable roads are helpful in planning routes. The development of Geographic Information Systems (GIS) may also be helpful. A communication plan should be prepared to inform the population, encouraging them to open their doors and windows in order to improve the effectiveness of the spraying programme.
Application rates vary with the susceptibility of the target species and environmental considerations. Wind speed has a strong effect on droplet distribution and contact with insects. In most situations, a wind speed of 1–4 metres per second (approximately 3.6–15 km/h) is needed to drift droplets downwind from the line of travel. Furthermore, space sprays should be applied when there are temperature inversions – i.e. colder air closer to the ground – which occur early in the morning or in the evening when the ground temperature begins to fall. Space spray applications should correspond to the activity of the target species. Ae. aegypti and Ae. albopictus are active during the day, with peak flight activity in the morning and afternoon. For these species, spraying outdoors is therefore usually carried out in the early morning or late afternoon. Indoor treatments with portable cold or thermal fog generators are particularly effective against Ae. aegypti because its resting behaviour is mainly indoors. Indoor treatments are the only choice where there is no access for vehicles.
For application from vehicle-mounted equipment in areas with narrow roads and houses close to the roadside, the spray should be directed backwards from the vehicle. In areas with wide roads and buildings far from the roadside, the vehicle should be driven close to the side of the road and the spray should be directed at a right angle (downwind) to the road rather than directly behind the vehicle. More detailed information on operational guidelines for space spraying is available in the WHO manual on this subject (5).
Cold fog applications from large fixed-wing aircraft are made at approximately 240 km/h and 60 m above the ground, with swath spacing of 180 m. Smaller, fixed-wing aircraft are flown at slower speeds and usually lower altitudes (approximately 160 km/h, 30 m above the ground, with a swath width of 50–100 m). In emergencies, agricultural spraying aircraft can be used so long as they are fitted with rotary atomizers or other suitable nozzles calibrated for the insecticide, its formulation and the desired application rate.
Treatment cycle
When a rapid reduction in vector density is essential, such as in emergencies, space treatment should ideally be carried out every 2–3 days for 10 days. Further applications should then be made once or twice a week to sustain suppression of the adult vector population. Continuous entomological and epidemiological surveillance should be conducted, however, to determine the appropriate application schedule and the effectiveness of the control strategy.
Precautions
Operators who carry out house-to-house space spraying using portable equipment should wear face masks in addition to normal protective clothing and should operate the equipment for short periods only. Fogging with vehicle-mounted equipment in urban areas can be a traffic hazard, and spotting of vehicle paintwork may result, particularly when large droplets are used. Ultra-low-volume aerial applications should be made only by highly skilled pilots trained to undertake spraying at the proper speeds and heights. Clearance from the local civil aviation authority must be sought. Ground reconnaissance should be made before treatment and the public advised to safeguard non-target animals and beehives.
3.2.4. Safe use of insecticides
All pesticides are toxic to some degree. Safety precautions for their use – including care in the handling of pesticides, safe work practices for those who apply them, and appropriate field application – should be followed. A safety plan for insecticide application can be organized along the following lines:
Instructions on pesticide labels should be followed carefully.
Spray operators should be provided with at least two uniforms to allow for frequent changes.
Safety gloves, goggles and masks should be used for high-exposure activities such as machine calibration.
Changing and washing facilities should be available.
All work clothes should be removed at the end of each day's operations and a shower or bath taken.
Work clothes should be washed regularly, preferably daily.
Particular attention should be given to washing gloves, as wearing contaminated gloves can be dangerous.
Spray operators should wash their hands and face before eating and should not smoke during work hours.
Spray operators should not be exposed to toxic material for periods that are longer than recommended.
Care must be taken in disposing of used insecticide containers.
After each day's operation, any unused liquid larvicide should be disposed of safely.
Blood cholinesterase levels should be monitored if organophosphate insecticides are used.
Operator supervision by a well-trained individual is essential.
During and immediately after indoor space spray operations, householders and pets must remain outside the dwelling.
WHO has published specific guidelines on use of insecticides and safety procedures (3-7).
3.2.5. Monitoring of insecticide susceptibility
Insecticides have been used widely for dengue vector control since their development. As a result, insecticide-resistant populations of Ae. aegypti have been detected in a number of countries. Operationally significant levels of resistance to organophosphates, pyrethroids, carbamates and organochlorines have been documented.
Insecticide resistance must be considered as a potentially serious threat to effective dengue vector control. Routine monitoring of insecticide susceptibility should be integral to any programme.
In countries with a history of extensive DDT use, resistance may be widespread. Also, DDT resistance may predispose to pyrethroid resistance, since both insecticides have the same target site (the voltage gated sodium channel) and both have been associated with mutations in the kdr gene in Ae. aegypti. Consequently, in countries such as Thailand where pyrethroids – including deltamethrin, cypermethrin and permethrin – are increasingly being used in favour of organophosphates for space spraying, pyrethroid resistance is likely to occur sooner in mosquito populations that already have this mutation. This phenomenon reinforces the importance of carrying out routine susceptibility testing at regular intervals during any control programme.
WHO kits for testing the susceptibility of adult and larval mosquitoes remain the standard methods for determining the susceptibility status of Aedes populations. Instructions on testing and for purchasing kits, test papers and solutions are available to order from WHO1.
3.2.6. Individual and household protection
Clothing that minimizes skin exposure during daylight hours when mosquitoes are most active affords some protection from the bites of dengue vectors and is encouraged particularly during outbreaks. Repellents may be applied to exposed skin or to clothing. Repellents should contain DEET (N, N-diethyl- 3-methylbenzamide), IR3535 (3-[N-acetyl-N-butyl]-aminopropionic acid ethyl ester) or Icaridin (1-piperidinecarboxylic acid, 2-(2-hydroxyethyl)-1-methylpropylester). The use of repellents must be in strict accordance with label instructions. Insecticide-treated mosquito nets afford good protection for those who sleep during the day (e.g. infants, the bedridden and night-shift workers).
Where indoor biting occurs, household insecticide aerosol products, mosquito coils or other insecticide vaporizers may also reduce biting activity. Household fixtures such as window and door screens and air-conditioning can also reduce biting.
3.2.7. Biological control
Biological control is based on the introduction of organisms that prey upon, parasitize, compete with or otherwise reduce populations of the target species. Against Aedes vectors of dengue, only certain species of larvivorous fish and predatory copepods (Copepoda: Cyclopoidea) – small freshwater crustaceans – have proved effective in operational contexts in specific container habitats, and even then seldom on a large scale. While biological control avoids chemical contamination of the environment, there may be operational limitations – such as the expense and task of rearing the organisms on a large scale, difficulty in applying them and their limited utility in aquatic sites where temperature, pH and organic pollution may exceed the narrow requirements of the organism. Biological control methods are effective only against the immature stages of vector mosquitoes in the larval habitat where they are introduced. Importantly, the biological control organisms are not resistant to desiccation, so their utility is mainly restricted to container habitats that are seldom emptied or cleaned, such as large concrete or glazed clay water-storage containers or wells. The willingness of local communities to accept the introduction of organisms into water containers is essential; community involvement is desirable in distributing the fish or copepods, and monitoring and restocking containers when necessary.
3.2.7.1. Fish
A variety of fish species have been used to eliminate mosquitoes from larger containers used to store potable water in many countries, and in open freshwater wells, concrete irrigation ditches and industrial tanks. The viviparous species Poecilia reticulata adapts well to these types of confined water bodies and has been most commonly used. Only native larvivorous fish should be used because exotic species may escape into natural habitats and threaten the indigenous fauna. WHO has published further information on the use of fish for mosquito control (8).
3.2.7.2. Predatory copepods
Various predatory copepod species have also proved effective against dengue vectors in operational settings. However, although copepod populations can survive for long periods, as with fish, reintroductions may be necessary for sustained control. A vector control programme in northern Viet Nam using copepods in large water-storage tanks, combined with source reduction, successfully eliminated Ae. aegypti in many communes and has prevented dengue transmission for a number of years. To date, these successes have not been replicated in other countries.
3.2.8. Towards improved tools for vector control
Some promising new dengue vector control tools are the subject of operational research but have not been sufficiently well field-tested under programmatic conditions for recommendations to be made for their use as public health interventions. In 2006, a WHO/TDR Scientific Working Group identified major streams of recommended research on dengue, including in the area of vector control (9).
3.2.8.1. Insecticide-treated materials
Insecticide-treated materials (ITMs), typically deployed as insecticide-treated bednets, have proved highly effective in preventing diseases transmitted by nocturnally active mosquitoes. Research on the efficacy of ITMs in controlling diurnally active Ae. aegypti is being encouraged. There is accumulating evidence that insecticide-treated window curtains (net curtains hung in windows, over any existing curtains if necessary) and long-lasting insecticidal fabric covers for domestic water-storage containers can reduce dengue vector densities to low levels in some communities – with prospects for reducing dengue transmission risk. Curtains also provide personal protection in the home. Although more studies are needed to confirm that transmission can be reduced by this type of intervention, ITMs appear to hold promise for dengue prevention and control. In studies in Mexico and Venezuela, ITMs (particularly curtains) were well accepted by the communities as their efficacy was reinforced by the reduction of other biting insects as well as cockroaches, houseflies and other pests (10).
The location or type of ITM need not be limited to those described or tested to date. Window curtains, screens, and doorway or wardrobe curtains, etc. all appear to warrant investigation in different settings. If the application of these interventions is shown to be efficacious and cost-effective, it may offer additional prospects for dengue vector control in the home, workplace, schools, hospitals and other locations, and allow for the selection of the most appropriate ITMs by the communities that will use them.
3.2.8.2. Lethal ovitraps
The ovitrap or oviposition trap used for surveillance of Aedes vectors can be modified to render it lethal to immature or adult populations of Ae. aegypti. Lethal ovitraps (which incorporate an insecticide on the oviposition substrate), autocidal ovitraps (which allow oviposition but prevent adult emergence), and sticky ovitraps (which trap the mosquito when it lands) have been used on a limited basis. Studies have shown that population densities can be reduced with sufficiently large numbers of frequently-serviced traps. Life expectancy of the vector may also potentially be shortened, thus reducing the number of vectors that become infective. In Singapore, ovitraps used as a control device reportedly eliminated Ae. aegypti from the international airport, but this level of success has not been repeated elsewhere (11). In Brazil, lethal ovitraps with deltamethrin-treated ovistrips substantially reduced adult densities of Ae. aegypti and produced almost 100% larval mortality during a one-month field trial (12). The potential advantages of lethal ovitraps for controlling Aedes vectors include their simplicity, their specificity for and effectiveness against container breeders such as Ae. aegypti, and the prospect of their integration with other chemical or biological control methodologies.
3.3. DELIVERY OF VECTOR CONTROL INTERVENTIONS
Whereas section 3.1 describes the main methods used in dengue vector control, section 3.2 focuses on the management approaches to their delivery, including intrasectoral as well as intersectoral collaboration. summarizes vector surveillance and control activities and their purposes. Further details of entomological surveillance and emergency vector control are described in Chapter 5.
Vector control services: activities and purpose.
3.3.1. Links to epidemiological services
Vector control services should be closely linked to epidemiological services that capture and analyse the occurrence of dengue cases (temporal and spatial information). The epidemiological surveillance system should be able to differentiate between transient and seasonal increases in disease incidence and increases observed at the beginning of a dengue outbreak. One such approach is to track the occurrence of current (probable) cases and compare them with the average number of cases by week (or month) of the preceding 5–7 years, with confidence intervals set at two standard deviations above and below the average (±2 SD). This is sometimes referred to as the “endemic channel”. If the number of cases reported exceeds 2 SDs above the “endemic channel” in weekly or monthly reporting, an outbreak alert is triggered. is an example from the surveillance system in Puerto Rico in 2007–2008. Such an approach is epidemiologically far more meaningful than year-to-year comparisons of cumulative totals of reported cases.
Surveillance data for dengue outbreak alerts, Puerto Rico, 2007–2008.
The spatial patterning of health events and disease outcomes has a long history. The development of GIS has facilitated the inclusion of a spatial component in epidemiological, entomological and environmental studies. A GIS is a collection of computer hardware, software and geographical data used for capturing, managing, analysing and displaying all forms of geographically referenced information. It allows users to choose different layers of information and to combine them according to what questions need to be answered or what data need to be analysed. describes the workings of a GIS.
Geographical information system (GIS), Singapore. GIS allows the layering of health, demographic and environmental data sources to be analysed by their location on earth's surface.
In setting up a GIS to support vector control services, data are organized in different layers to describe features such as streets, residences, buildings, train stations, schools, construction sites, shopping centres, medical clinics and electoral divisions. Above these base layers can be added entomological data, case data, virus serotype, enforcement data, demographics, weather data and so on.
shows a snapshot of a GIS map with eight layers of data used to support vector control operations in Singapore.
Snapshot of geographical information system (GIS) mapping, Singapore.
GIS are widely used in public health to map diseases with different pathologies, to analyse the distribution of disease data in space and space-time, to identify risk factors and to map areas of risk. Typically, each case is located at either the residential or work address, and these locations are integrated into a GIS for mapping and analysis. Because a GIS allows epidemiologists to map environmental risk factors associated with disease vectors – such as construction sites, derelict or uninhabited premises and areas of congregation – it is especially relevant for the surveillance of vector-borne diseases such as dengue and malaria. For dengue, such mapping (epidemiological, entomological and environmental stratification) can serve to identify areas where transmission repeatedly occurs and which may warrant intensified or targeted control activities, or to stratify areas based on characteristics of larval habitats. The availability of such information in a timely manner could determine the outcome of vector control operations and even help to reduce the intensity of outbreaks. GIS technology has been found particularly useful for planning vector control operations, for managing and deploying resources for dengue control, and for presenting the dengue situation of any locality.
Free-access computer software programs are available; some software packages and maps can be downloaded from the Internet. Some dengue control programmes use hand-held global positioning system (GPS) devices and other hand-operated computer equipment to record data that are uploaded to a central database.
Where such resources are unavailable, commercial or hand-drawn maps may be used, with pins or labels indicating reported data.
3.3.2. Advocacy, social mobilization and legislation
Advocacy is a process through which groups of stakeholders can be influenced to gain support for and reduce barriers to specific initiatives or programmes. Multiple strategies, often used simultaneously, are key to the success of any advocacy effort. Strategies may include social mobilization and administrative, legislative, regulatory, legal and media advocacy (13). While there may be different target audiences and even different objectives for the strategies, it is the coordination of actions that leads to the achievement of the overarching goal of the strategic advocacy effort. Some actions may have a longer time frame – such as legislative and regulatory advocacy to address tyre disposal at national level – while others such as mobilizing local authorities and residents to carry out specific actions before the start of the rainy season may be time-limited.
A strategic advocacy plan usually includes one or more of the following types of advocacy ( shows examples of advocacy efforts in Asia and Latin America):
Examples of advocacy efforts in Asia and Latin America.
Social mobilization – brings people together to achieve a common goal with a shared interpretation and direction.
Administrative advocacy – informs authorities, decision-makers and opinion leaders of the importance of the programme, the costs and benefits of its activities and programme needs in order to enlist their support and cooperation.
Legislative advocacy – uses federal/state/provincial/departmental/local legislative processes to promote legislation that addresses issues beyond the responsibility of any one governmental entity (e.g. legislation to address tyre disposal at national or regional level, the establishment of sanitary landfills, or the modification of housing designs or water catchment and storage systems).
Regulatory advocacy – creates rules through which legislation is implemented (e.g. efforts to implement or modernize existing sanitary laws).
Legal advocacy – uses the judicial system to enforce sanitary legislation and regulations (e.g. fines on contractors whose building construction sites persistently harbour aquatic foci of dengue vectors, or on householders and estates that fail to prevent mosquito breeding on their premises).
Media advocacy – systematically engages the media to place issues of community interest on the social agenda, with a view to influencing public agendas. One such example, although not dengue-specific, is the PAHO Caribbean Media Awards for Health Journalism, an annual prize-giving event involving all mass media communication channels in the subregion.
3.3.3. Social mobilization and communication
Unlike chronic or sexually transmitted infections, vector-borne diseases require more than individual behaviour change in order to influence disease transmission. Behavioural change is required at both individual and community levels in order to reduce vector larval habitats successfully, and in turn to reduce the number of adult mosquitoes available to transmit disease. This has led to greater emphasis on social mobilization and communication activities which are fully integrated into dengue prevention and control efforts (14).
In order to develop an appropriate communication strategy, it is necessary to understand that c through various channels such as interpersonal communication or the mass media. Communication is a two-way interactive process through which two or more participants (individuals or groups) create and share information in order to reach a common understanding and to identify areas of mutual agreement. This in turn allows for collective actions such as advocacy and social mobilization to be implemented. Once collective actions have been identified, social mobilization can be used to engage people's participation in achieving a specific goal through their own efforts (15). Social mobilization is not just a single activity; it involves all relevant segments of society (i.e. decision-makers and policy-makers, opinion leaders, bureaucrats and technocrats, professional groups, religious groups, commerce and industry, communities and individuals). It also takes into account the perceived needs of the people, embraces the critical principle of community involvement, and seeks to empower individuals and groups for action.
To date, most dengue-related communication (including more traditional information, education and communication (or IEC) efforts) and social mobilization activities have targeted individuals and communities generally defined by geographical boundaries – such as neighbourhoods, schools, and houses that fall within the radius of a confirmed dengue case. Little attention has been given to creating and sustaining a dialogue at policy level in order to address the underlying causes of increasing availability of vector larval habitats, such as ineffective refuse disposal services or an inconsistent or poor-quality water supply. Policy efforts require different communication strategies in order to engage the diverse target audience, which may include representatives from ministries of natural resources and the environment (water and sanitation), urban planning, finance and tourism, as well as municipal authorities. While many countries have a national dengue committee that may be activated during outbreaks or epidemics, these committees generally do not address the broader issues that lead to the ongoing propagation of the mosquito vectors of dengue fever.
Communication plans and strategies are often lacking, resulting in short-term information campaigns and ad hoc activities in reaction to outbreaks. In 2004, WHO published Planning social mobilization and communication for dengue prevention and control: a step-by-step guide (16) to assist programme managers in developing effective mobilization and communication strategies to promote behavioural change as part of routine vector control programming. The guide uses the COMBI (communication-for-behavioural-impact) planning methodology to focus communication and mobilization efforts on promoting and measuring changes in behaviour, and not just changes in knowledge and attitudes. This focus on behaviour rather than knowledge builds on many years of IEC efforts to increase community knowledge of dengue, the mosquito vector(s) and their larval habitats. Understanding the precise steps needed to carry out a recommended behaviour will help programmes to shift from the use of general messages that are often ignored by the target audience to messages that promote and encourage the process of behavioural change (17).
3.3.4. Collaboration within the health sector and with other sectors
Intersectoral collaboration among partners is a key strategy of IVM. Networking facilitates a more coordinated approach than the individual and independent efforts of different sectors, and provides a platform for partners to resolve cross-agency issues and to share best practices while reducing duplication of efforts. Networking for dengue control also helps to leverage the strengths of partners and to synergize their efforts, thereby enhancing the effectiveness and efficiency of actions for dengue prevention and control.
3.3.4.1. Collaboration within the health sector
Control measures can be integrated with strong local health systems by transferring responsibility, authority, resources and knowledge from central to local level. However, it is critically important for the transfer of responsibility to be accompanied by the transfer of financial and technical resources. Transfer can be accomplished by offering, for instance, capacity-strengthening workshops or training courses in vector biology and control, epidemiology, and communication among other topics at the local level. At all administrative levels of government (state, provincial, departmental and local), the dengue control programme is usually part of the local health system, wherein lies the responsibility for planning, implementing, monitoring and evaluating the local programme.
Contacts, liaison and cooperative activities should be promoted within the different divisions of the health sector. This cooperation with the dengue programme is necessary since the prevention and control of dengue is not the responsibility of a single department. Regardless of whether the programme is led by the Ministry of Health, collaboration within this ministry is essential among those departments responsible for vector control and surveillance, epidemiological surveillance, clinical diagnosis and management, maternal and child health (e.g. the programme on integrated management of childhood illnesses), health education, community participation and environmental health. Entities such as national health institutes and schools of public health and medicine can also contribute by carrying out activities for which the Ministry of Health may not have resources, such as training and research projects.
3.3.4.2. Collaboration with other sectors and with the community
Dengue prevention and control necessitates an effective intersectoral approach, requiring coordination between the lead ministry, usually the Ministry of Health, and other ministries and government agencies, the private sector (including private health providers), nongovernmental organizations (NGOs) and local communities. Resource-sharing is an important aspect of this (). Such cooperation is critical in emergency situations when scarce or widely dispersed human and material resources must be mobilized rapidly and their use coordinated to mitigate the effects of an epidemic. The process can be facilitated by policy adjustment.
Selected examples of potential intersectoral actions.
Policy adjustment
The Ministry of Health and the programme manager should seek mutual agreement with other ministries, sectors or municipal governments – and even the adjustment of existing policies and practices – to place public health centrally among the goals of those bodies (administrative advocacy). For instance, the public works sector could be encouraged to give priority to improvements in water supply for those communities at highest risk of dengue.
Potential roles of government ministries
Public works. The ministry responsible for public works and its municipal counterparts are responsible for providing dependable water supply, sanitation and solid waste management services to all planned communities. The dimensions and quality of those services have a direct bearing on the availability of larval habitats. Additionally, through the adoption and enforcement of housing and building codes (legislative and regulatory advocacy), a municipality may mandate the provision of utilities such as piped water or sewerage connections for individual households and rainwater run-off control for new housing developments, or it may prohibit the construction of open groundwater wells. Such opportunities are prescient when planning urban redevelopment schemes and because of the benefits of reduced risk of dengue and of mosquitoes and other pests.
Education. The Ministry of Education should be a key partner as it is responsible not only for educating children and young people but also for inculcating social norms which include appropriate hygiene behaviours. Where dengue prevention and control involve a health communication component targeted at schoolchildren, the Ministry of Health can work closely with the Ministry of Education to develop, communicate and impart appropriate messages and skills for behaviour change. Such messages and skills should ideally be integrated into existing curricula to ensure long-term continuity (18).
Health education models can be jointly developed, tested and evaluated for different age groups. Research programmes in universities and colleges can be encouraged to include components that generate information of direct importance (e.g. vector biology and control, case management) or of indirect importance (e.g. improved water supplies, promotion of community sanitation, waste characterization studies, analysis of cost and cost-effectiveness of interventions) to aid evidence-based decision-making.
Tourism. Coordination with the Ministry of Tourism can facilitate the timely communication of outbreak or epidemic alert messages to tourists and to the hotel industry so that actions can be taken to reduce the risk of exposure to infection.
Environment. The ministry responsible for the environment can help the Ministry of Health to gather information on ecosystems and habitats in and around cities and smaller communities at high risk of dengue so as to aid in programme planning. In at least one country (Singapore), the Ministry of the Environment has direct responsibility for dengue vector control and promotes healthy public policies that include sound management of public health pesticides.
Collaboration with nongovernmental organizations
NGOs can play important roles in promoting and implementing environmental management for dengue vector control, most often involving health communication on reduction of sources and improvement of housing. Community NGOs – which may be informal neighbourhood groups such as private volunteer organizations, religious groups and environmental and social action groups – should be identified as potential partners.
With appropriate orientation and guidance, particularly on source reduction methods, NGOs can collect discarded containers (e.g. tyres, food and drink containers), or can clean drains and culverts, remove abandoned vehicles and roadside litter, and fill tree and rock holes. During outbreaks NGOs may be influential in mobilizing householders and other community members to eliminate important larval habitats of the vector or to manage the containers in ways that do not allow mosquito emergence (e.g. by emptying and cleaning water-storage containers at weekly intervals). NGOs may also encourage public cooperation and acceptance of space spraying and larvicide application measures.
Communities organize themselves in many ways, so there is no prescribed formula for interaction. However, social mobilization initiatives must be socially and culturally sensitive and should be developed between partners in a spirit of mutual respect. There are many examples of voluntary organizations and women's associations taking the lead in providing money, advertising and political support to successful community-based campaigns for source reduction or in organizing regular household activities to reduce mosquito populations.
Collaboration with industry and the private sector
Collaboration with industry and the private sector can advance the manufacture and utilization of mosquito-proof designs of water-storage containers and room-coolers, and can promote the collection and recycling of used tyres, plastic, aluminium, glass and other containers. In the construction industry, architects' associations can help to promote the design and building of mosquito-proof and otherwise healthy houses and workplaces.
3.3.5. Integrated approaches to control
Instead of targeting only the vector or vectors of dengue, there may be opportunities to integrate Aedes control with control of pests or vectors of other diseases. Addressing two or more public health problems simultaneously may improve cost-effectiveness and may help promote public acceptance and involvement in the programme. For example, control of Ae. aegypti in some urban areas can be combined with control of Culex quinquefasciatus, the latter species usually being a much greater nuisance to the public and an important vector of lymphatic filariasis in many tropical environments. Collection of solid waste as an environmental management component of Aedes control programmes need not be restricted just to the container sources of Aedes production but can also include items that are associated with filth flies and rodents. Moreover, given that urban yellow fever and chikungunya viruses are also transmitted by Ae. aegypti, and chikungunya by Ae. albopictus as well, their control is an effective way of reducing the risk of outbreaks of these diseases in addition to dengue. In many cities in India, Ae. aegypti and the malaria vector Anopheles stephensi share common larval habitats and can be targeted simultaneously.
3.3.6. Strengthening capacity
In vector control, as in other areas of public health, staffing levels and capacity-strengthening are important. In particular, public health entomologists, vector control personnel, environmental specialists, social scientists and communication specialists play pivotal roles.
3.3.6.1. Social scientists and communication specialists
Because social mobilization and communication are often the least planned and most under-funded elements of the dengue prevention and control programme, it is even more important to use resources in a targeted and cost-effective manner. This can be accomplished by working with social scientists who have expertise in using behavioural change theories in programme development, and with communication specialists who have a background preferably in health communication (i.e. the use of communication strategies to inform and influence individual and community decisions that enhance health). This will require that the dengue programme includes in its annual budget an allocation for social mobilization and communication activities in order to integrate these specialists into routine programme planning.
Most ministries of health have health promotion or health education departments, and it is increasingly common to find communication included in them. However, communication is frequently viewed as use of the mass media, and therefore the communication specialists may be individuals with backgrounds in public relations, journalism or mass media who have limited knowledge of the principles of health promotion or behavioural change. In this case it is even more important to involve a health promotion specialist or a social scientist to ensure that the messages focus on appropriate and feasible behaviours that target the principal vector larval habitats, and that the impact of the social mobilization and communication activities are evaluated for changes in behaviour and not just for changes in knowledge or attitudes. Health promotion and health education personnel can generally be found at the central and state or provincial levels, while at the local level this role may be filled by a nurse or social worker if a person qualified in communication is not part of the health clinic staffing.
Ongoing training and practice in communication at all levels is critical for ensuring appropriate and effective interpersonal communication between vector control staff and householders, between the dengue programme manager and vector control staff, and between the dengue programme manager and partners within and outside the health sector. When and how to use the mass media at national, regional and local levels can be determined in collaboration with the communications specialist. Using the media may entail training sessions in public speaking for key spokespersons within the programme as well as for epidemiologists and medical personnel who may also be required to speak to the media. Training in how to work with the media, particularly during an outbreak or epidemic, is vital to ensure that accurate and useful information is shared with the broader community through the media, to avoid sending mixed messages about what is expected of the community during the outbreak and to reduce the chance of misinterpretation or sensationalization of information (particularly the number of cases of dengue).
3.3.6.2. Public health entomologists and vector control and environmental management personnel
The skills for managing, implementing, monitoring and evaluating the programme must be determined in accordance with availability of resources, programme objectives and intervention strategies. Training activities, including in-service training, should be tailored to the needs of the various groups of personnel. WHO has published guidance on needs assessment for all components of the dengue control programme (19).
Whether or not vector control activities are aligned with centralized or decentralized health systems, and whether they are distinct from or integrated with other health sector activities, the available skills at any given level of administrative responsibility should be commensurate with those responsibilities. This is equally important for the purposes of financial and operational planning – including workforce planning and strategic, technical guidance – as well as for essential physical infrastructure.
3.3.7. Operational research
Operational research should be oriented to the priority needs of the programme in order to generate the evidence base for adaptation of strategies and interventions. This may include, for example, studies on the ecology of the vector, the efficacy, effectiveness and cost-effectiveness of existing and promising new vector control methods, formative research on relevant cultural practices, and guidance for engaging communities in programme activities.
3.3.8. Monitoring and evaluation
Regular monitoring of the delivery of dengue prevention and control services and evaluation of the impact of interventions are important activities for effective programme management. Suitable indicators should be identified to measure the progress of implementation, as well as output and outcome indicators. gives examples of good and bad practice in dengue prevention and control.
Dengue prevention and control: examples of good and bad practice.
