Prevention And Control Measures
No vaccine is available yet for the prevention of dengue infection and there are no specific drugs for its treatment. Hence DF/DHF control is primarily dependent on the control of Ae. aegypti. Dengue control programmes in the Region have in general not been very successful, primarily because they have relied almost exclusively on space spraying of insecticides for adult mosquito control. However, space spraying requires specific operations which were often not adhered to, and most countries found it cost prohibitive. In order to achieve sustainability of a successful DF/DHF vector control programme, it is essential to focus on larval source reduction and to have complete cooperation with non-health sectors, such as nongovernmental organizations, civic organizations and community groups, to ensure community understanding and involvement in implementation. There is, therefore, a need to adopt an integrated approach to mosquito control by including all appropriate methods (environ-mental, biological and chemical) which are safe, cost-effective and environmentally acceptable. A successful, sustainable Ae. aegypti control programme must involve a partnership between government control agencies and the community. The approaches described below are considered necessary to achieve long-term, sustainable control of Ae. aegypti.
- Environmental management: Environmental management involves any change that prevents or minimizes vector breeding and hence reduces human-vector contact. The control of Ae. aegypti in Cuba and Panama in the early part of this century was based mainly on environmental management. Such measures remain applicable wherever dengue is endemic. The World Health Organization (1982) has defined three kinds of environmental management viz. Environmental modification, which involves long-lasting physical transformation of vector habitats, environmental manipulation, which includes temporary changes to vector habitats that involve the management of "essential" and "nonessential" containers; and management or removal of "natural" breeding sites and changes to human habitation or behaviour.
- Improved water supply: Whenever piped water supply is inadequate and available only at restricted hours or at low pressure, the storage of water in varied types of containers is encouraged, thus leading to increased Aedes breeding. The majority of such containers are large and heavy (e.g storage jars) and can neither be easily disposed of nor cleaned. In rural areas, unpolluted, disused wells become breeding grounds for Ae. aegypti. It is essential that potable water supplies be delivered in sufficient quantity, quality and consistency to reduce the necessity and use of water storage containers that serve as the most productive larval habitats.
- Mosquito-proofing of overhead tanks/cisterns or underground reservoirs: Where Ae. aegypti larval habitats include overhead tanks/cisterns and masonry chambers of piped waterlines, these structures should be mosquito-proofed. Similarly, mosquito-proofing of domestic wells and underground water storage tanks should be undertaken. Masonry chambers of sluice valves and water meters are required to be provided with soak pits as part of preventive maintenance.
Environmental manipulation including change in human habitat
- Draining of water supply installations: Water collection/leakages in masonry chambers, distribution pipes, valves, sluice valves, surface boxes for fire hydrants, water meters, etc. collect water and serve as important Ae.aegypti larval habitats in the absence of preventive maintenance.
- Domestic storage: The major sources of Ae. aegypti breeding in most urban areas of South-East Asia are containers storing water for household use including clay, ceramic and cement water jars of 200 litre size, 210 litre (50 gallon) metal drums, and smaller containers storing fresh water or rain water. Water storage containers should be covered with tight-fitting lids or screens; care being taken to replace them after water is used. An example of the efficacy of this approach has recently been demonstrated in Thailand.
Flower pots/vases and ant traps: Flower pots, flower vases and ant traps are common sources of Ae. aegypti breeding. They should be punctured to produce a drain hole. Alternatively, live flowers can be placed in a mixture of sand and water. Flowers should be removed and discarded weekly and vases scrubbed and cleaned before reuse. Brass flower pots, which make poor larval habitats, can be used in cemeteries in place of traditional glass containers. Ant traps to protect food storage cabinets can be treated with common salt or oil.
Aedes breeding in incidental water collections: Desert (evaporation) water coolers, condensation collection pans under refrigerators, and air conditioners should be regularly inspected, drained and cleaned. Desert water coolers generally employed in arid/semi-arid regions(44) of South-East Asia to cool houses during summer contain two manufacturing defects. These are as follows:
- The exit pipe at the bottom of the water-holding tray is generally fixed a few centimetres above the bottom. This exit pipe should be fitted at such a level that while emptying the tray, all the water should get drained off without any retention at the bottom.
- Desert coolers are normally fitted to windows with the exit pipe located on the exterior portion of the tray. These sites are usually difficult to access, and, therefore, there is a need to change the design so that both the filling and emptying of the water-holding trays can be manipulated from the room, thus eliminating the need of climbing to approach the exit pipe at the exterior of the building.
Building exteriors: The design of buildings is important to prevent Aedes breeding. Drainage pipes of rooftops sunshades/porticos often get blocked and become breeding sites for Aedes mosquitoes. There is a need for periodic inspection of buildings during the rainy season to locate potential breeding sites.
Mandatory water storage for fire fighting: Fire prevention regulations may require mandatory water storage. Such storage tanks need to be kept mosquito-proofed. In some municipalities in India(45), timber merchants are required to maintain two metal drums (50 gallons) full of water for fire fighting. These drums should be kept covered with tight lids. Also, metal drums used for water storage at construction sites should be mosquito-proofed.
Solid waste disposal: Solid wastes, namely tins, bottles, buckets or any other waste material scattered around houses, should be removed and buried in land fills. Scrap material in factories and warehouses should be stored appropriately until disposal. Household and garden utensils (buckets, bowls and watering devices) should be turned upside down to prevent the accumulation of rain water. Similarly, canoes and small boats should be emptied of water and turned upside down when not in use. Plant waste (coconut shells, cocoa husks) should be disposed of properly and without delay.
Tyre management: Used automobile tyres are of major importance as breeding sites for urban Aedes, and are therefore a significant public health problem. Imported used tyres are believed responsible for the introduction of Ae. albopictus into the United States, Europe and Africa. Tyre depots should always be kept under cover to prevent the collection of rain water. New technologies for tyre recycling and disposal are continually coming into use, but most of them have proved to be of limited application or cost-effectiveness. Used tyres can be filled with earth or concrete and used for planters or traffic/crash barriers. They may also be used as soil erosion barriers, or used to create artificial reefs and reduce beach erosion by wave action. Tyres can also be recycled for sandals, floor mats, industrial washers, gaskets, buckets, garbage pails and carpet backing, while truck tyres have been made into durable, low-cost refuse containers.
Filling of cavities of fences: Fences and fence posts made from hollow trees such as bamboo should be cut down to the node, and concrete blocks should be filled with packed sand, crushed glass, or concrete to eliminate potential Aedes larval habitats.
Glass bottles and cans: Glass bottles, cans and other small containers should be buried in land fills or crushed and recycled for industrial use.
Protective clothing: Clothing reduces the risk of mosquito biting if the cloth is sufficiently thick or loosely fitting. Long sleeves and trousers with stockings may protect the arms and legs, the preferred sites for mosquito bites. Schoolchildren should adhere to these practices whenever possible. Impregnating clothing with chemicals such as permethrin can be especially effective in preventing mosquito bites.
Mats, coils and aerosols: Household insecticidal products, namely mosquito coils, pyrethrum space spray and aerosols have been used extensively for personal protection against mosquitoes. Electric vaporizer mats and liquid vaporizers are more recent additions which are marketed in practically all urban areas.
Repellents: Repellents are a common means of personal protection against mosquitoes and other biting insects. These are broadly classified into two categories, natural repellents and chemical repellents. Essential oils from plant extracts are the main natural repellent ingredients, i.e. citronella oil, lemongrass oil and neem oil. Chemical repellents such as DEET (N, N-Diethyl-m-Toluamide) can provide protection against Ae. albopictus, Ae. aegypti and anopheline species for several hours. Permethrin is an effective repellant when impregnated in cloth.
Insecticide-treated mosquito nets and curtains: Insecticide-treated mosquito nets (ITMN) have limited utility in dengue control programmes, since the vector species bites during the day. However, treated nets can be effectively utilized to protect infants and night workers who sleep by day. They can also be effective for people who generally have an afternoon sleep. "Olyset net", a wide mesh net woven from polyethylene thread containing 2% permethrin, is yet another improvement in ITMN technology. This net has two advantages over traditional nets in that the wide mesh permits better ventilation and light, and the treated thread enables a slow release of permethrin to the fibre surface, ensuring a long residual effect (over a year). In studies carried out in Malaysia, four washings with soap and water did not diminish the efficacy and the mortality of Ae. aegypti was 86.7%. For control of DF/DHF in Vietnam, Olyset net curtains were hung on the inside against doors/windows; Ae. aegypti was adversely affected and dengue virus transmission was interrupted. Further studies on impregnated fabrics appear warranted.
The application of biological control agents which are directed against the larval stages of dengue vectors in South-East Asia has been somewhat restricted to small-scale field operations.
Fish: Larvivorus fish (Gambusia affinis and Poecilia reticulata) have been extensively used for the control of An. stephensi and/or Ae. aegypti in large water bodies or large water containers in many countries in South-East Asia. The applicability and efficiency of this control measure depend on the type of containers.
Bacteria: Two species of endotoxin-producing bacteria, Bacillus thuringiensis serotype H-14 (Bt.H-14) and Bacillus sphaericus (Bs) are effective mosquito control agents. They do not affect non-target species. Bt.H-14 has been found to be most effective against An. stephensi and Ae. aegypti, while Bs is the most effective against Culex quinquefasciatus which breeds in polluted waters. There is a whole range of formulated Bti products produced by several major companies for control of vector mosquitoes. Such products include wettable powders and various slow-release formulations including briquettes, tablets and pellets. Further developments are expected in slow-release formulations. Bt.H-14 has an extremely low-level mammalian toxicity and has been accepted for the control of mosquitoes in containers storing water for household use.
Cyclopoids: The predatory role of copepod crustaceans was documented between 1930-50, but scientific evaluation was taken up only in 1980 in Tahiti, French Polynesia, where it was found that Mesocyclops aspericornis could effect a 99.3% mortality rate among Aedes (Stegomyia) larvae and 9.7% and 1.9%, respectively among Cx. quinquefasciatus and Toxorhynchities amboinensis larvae. Trials in crab burrows against Ae. polynesiensis and in water tanks, drums, and covered wells met with mixed results. In Queensland, Australia, out of seven species evaluated in the laboratory, all but M. notius were found to be effective predators of both Ae. aegypti and An. farauti but not against Cx. quinquifasciatus. Field releases in both northern and southern Queensland, however, showed mixed results. In Thailand, results were also mixed, but in Vietnam, results were more successful, contributing to the eradication of Ae. aegypti from one village. Although the lack of nutrients and frequent cleaning of some containers can prevent the sustainability of copepods, they could be suitable for large containers which cannot be cleaned regularly (wells, concrete tanks and tyres). They can also be used in conjunction with Bt.H-14. Copepods have a role in dengue vector control, but more research is required on the feasibility of operational use.
Autocidal ovitraps: Autocidal ovitraps were successfully used in Singapore as a control device in the eradication of Ae. aegypti from the Changgi international airport. In Thailand, this autocidal trap was further modified as an auto-larval trap using plastic material available locally. Unfortunately, under the local conditions of water storage practices in Thailand, the technique was not very efficient in reducing natural populations of Ae. aegypti. Better results can be expected if the number of existing potential larval habitats is reduced, or more autocidal traps are placed in the area under control, or both activities are carried out simultaneously. It is believed that, under certain conditions, this technique could be an economical and rapid means of reducing the natural density of adult females as well as serve as a device for monitoring infestations in areas where some reduction in population densities of the vector have already taken place. However, the successful application of autocidal ovitraps/larval traps depends on the number placed, the location of placement, and their attractiveness as Ae. aegypti female oviposition sites.
Chemicals have been used to control Ae. aegypti since the turn of the century. In the first campaigns against the yellow fever vector in Cuba and Panama, in conjunction with widespread clean-up campaigns, Aedes larval habitats were treated with oil and houses were fumigated with pyrethrins. When the insecticidal properties of DDT were discovered in the 1940s, this compound became a principal method of Ae. aegypti eradication programmes in the Americas. When resistance to DDT emerged in the early 1960s, organophosphate insecticides, including fenthion, malathion and fenitrothion were used for Ae. aegypti adult control and temephos as a larvicide. Current methods for applying insecticides include larvicide application and space spraying.
Chemical Larviciding: Larviciding or "focal" control of Ae. aegypti is usually limited to domestic-use containers that cannot be destroyed, eliminated, or otherwise managed. It is difficult and expensive to apply chemical larvicides on a long-term basis. Therefore chemical larvicides are best used in situations where the disease and vector surveillance indicate the existence of certain periods of high risk and in localities where outbreaks might occur. Establishing the precise timing and location are essential for maximum effectiveness. Control personnel distributing the larvicide should always encourage house occupants to control larvae by environmental sanitation. There are three insecticides that can be used for treating containers that hold drinking water.
Temephos 1% sand granules: One per cent temephos sand granules are applied to containers using a calibrated plastic spoon to administer a dosage of 1 ppm. This dosage has been found to be effective for 8-12 weeks, especially in porous earthen jars, under normal water use patterns. Although resistance to temephos in Ae. aegypti and Ae. albopictus populations has not been reported from the South-East Asia Region, the susceptibility level of Aedes mosquitoes should be monitored regularly in order to ensure the effective use of the insecticide.
Insect growth regulators: Insect growth regulators (IGRs) interfere with the development of the immature stages of the mosquito by interference of chitin synthesis during the molting process in larvae or disruption of pupal and adult transformation processes. Most IGRs have extremely low mammalian toxicity (LD50 value of acute oral toxicity for methoprene (Altosid) is 34 600 mg/kg). In general, IGRs may provide long-term residual effects (three to six months) at relatively low dosages when used in porous earthen jars. Because IGRs do not cause immediate mortality of the immature mosquitoes, countries with legislation stipulating that the breeding of Aedes larvae is an offense, will require some alteration of the law, so as not to penalize home owners who use these compounds.
Space sprays: Space spraying involves the application of small droplets of insecticide into the air in an attempt to kill adult mosquitoes. It has been the principal method of DF/DHF control used by most countries in the Region for 25 years. Unfortunately, it has not been effective, as illustrated by the dramatic increase in DHF incidence in these countries during the same period of time. Recent studies have demonstrated that the method has little effect on the mosquito population, and thus on dengue transmission. Moreover, when space spraying is conducted in a community, it creates a false sense of security among residents, which has a detrimental effect on community-based source reduction programmes. From a political point of view, however, it is a desirable approach because it is highly visible and conveys the message that the government is doing something about the disease. This, however, is poor justification for using space sprays. The current recommendations are that space spraying of insecticides (fogging) should not be used except in the most extreme conditions during a major DHF epidemic. However, the operations should be carried out at the right time, at the right place, and according to the prescribed instructions with maximum coverage, so that the fog penetration effect is complete enough to achieve the desired results.
When space sprays are employed, it is important to follow the instructions on both the application equipment and the insecticide label and to make sure the application equipment is well maintained and properly calibrated. Droplets that are too small tend to drift beyond the target area, while large droplets fall out rapidly. Nozzles for ultra-low volume ground equipment should be capable of producing droplets in the 5 to 27 micron range and the mass median diameter should not exceed the droplet size recommended by the manufacturer. Desirable spray characteristics include a sufficient period of suspension in the air with suitable drift and penetration into target areas with the ultimate aim of impacting adult mosquitoes. Generally, there are two forms of space-spray that have been used for Ae. aegypti control, namely "thermal fogs" and "cold fogs". Both can be dispensed by vehicle-mounted or hand-operated machines.
Thermal fogs: Thermal fogs containing insecticides are normally produced when a suitable formulation condenses after being vaporized at a high temperature. Generally, a thermal fogging machine employs the resonant pulse principle to generate hot gas (over 200oC) at high velocity. These gases atomize the insecticide formulation instantly so that it is vaporized and condensed rapidly with only negligible formulation breakdown. Thermal fogging formulations can be oil-based or water-based. The oil(diesel)-based formulations produce dense clouds of white smoke, whereas water-based formulations produce a colorless fine mist. The droplet (particle) size of a thermal fog is usually less than 15 microns in diameter. The exact droplet size depends on the type of machine and operational conditions. However, uniform droplet size is difficult to achieve in normal fogging operations.
Ultra-low volume (ULV), aerosols (cold fogs) and mists: ULV involves the application of a small quantity of concentrated liquid insecticides. The use of less than 4.6 litres/ha of an insecticide concentrate is usually considered as an ULV application. ULV is directly related to the application volume and not to the droplet size. Nevertheless, droplet size is important and the equipment used should be capable of producing droplets in the 10 to 15 micron range, although the effectiveness changes little when the droplet size range is extended to 5-25 microns. The droplet size should be monitored by exposure on teflon or silocone-coated slides and examined under a microscope. Aerosols, mists and fogs may be applied by portable machines, vehicle-mounted generators or aircraft equipment.
House-to-house application using portable equipment: Portable spray units can be used when the area to be treated is not very large or in areas where vehicle-mounted equipment cannot be used effectively. This equipment is meant for restricted outdoor use and for enclosed spaces (buildings) of not less than 14m3.Portable application can be made in congested low-income housing areas, multistoried buildings, godowns and warehouses, covered drains, sewer tanks and residential or commercial premises. Operators can treat an average of 80 houses per day, but the weight of the machine and the vibrations caused by the engine make it necessary to allow the operators to rest, so that two or three operators are required per machine.
Vehicle-mounted fogging: Vehicle-mounted aerosol generators can be used in urban or suburban areas with a good road system. One machine can cover up to 1500-2000 houses (or approximately 80 ha) per day. It is necessary to calibrate the equipment, vehicle speed, and swath width (60-90m) to determine the coverage obtained by a single pass. A good map of the area showing all roads is of great help in undertaking the application. An educational effort may be required to persuade the residents to cooperate by opening doors and windows. The speed of the vehicle and the time of day of application are important factors to consider when insecticides are applied by ground vehicles. The vehicle should not travel faster than 16 kph (10 mph). When the wind speed is greater than 16 kph or when the ambient air temperature is greater than 28oC (82oF), the insecticide should not be applied(25). The best time for application is in the early morning (approximately 0600-0830 hours) or late afternoon (approximately 1700-1930 hours).
Insecticide formulations for space sprays: Organophosphate insecticides, such as malathion, fenitrothion and pirimiphos methyl have been used for the control of adult Aedes vectors. Undiluted technical grade malathion (active ingredient 95%+) or one part technical grade diluted with 24 parts of diesel have been used for ULV spraying and thermal fogging respectively. For undiluted technical grade ULV malathion applications from vehicles, the dosage on an area basis is 0.5 liters per hectare. Apart from the above-mentioned formulations, a number of companies produce pyrethroid formulations containing either permethrin, deltamethrin, lambda-cyhalothin or other compounds which can be used for space spray applications. It is important not to under-dose during operational conditions. Low dosages of pyrethroid insecticides are usually more effective indoors than outdoors.
Integrated Control Approach: The use of insecticides for the prevention and control of dengue vectors should be integrated into environmental methods wherever possible. During periods of little or no dengue virus activity, the routine source reduction measures described earlier can be integrated into larvicide application in containers that cannot be eliminated, covered, filled or otherwise managed. For emergency control to suppress a dengue virus epidemic or to prevent an imminent outbreak, a programme of rapid and massive destruction of the Ae. aegypti population should be undertaken with both insecticides and source reduction, using the techniques described in these guidelines in an integrated manner.
Insecticide susceptibility monitoring: During the past 40 years, chemicals have been widely used to control mosquitoes and other insects from spreading diseases of public health importance. As a result, Ae. aegypti and other dengue vectors in several countries have developed resistance to commonly-used insecticides, including temephos, malathion, fenthion, permethrin, propoxur and fenitrothion. It is therefore advisable to obtain baseline data on insecticide susceptibility before insecticidal control operations are started, and to continue monitoring susceptibility levels periodically. WHO kits are available for testing the susceptibility of adult and larval mosquitoes and other arthropod vectors to commonly-used insecticides. These can be obtained from the Communicable Diseases Cluster, World Health Organization, 1211 Geneva 27, Switzerland, or through WHO Regional Offices or WHO Representatives in the countries.