Control of Aedes aegypti

Ideally, Aedes aegypti populations are controlled through rigorous environmental sanitation and the availability of a piped water supply that eliminates many of the man-made habitats of the species. In a postdisaster period, the disruption of existing water supplies causes people to store increased quantities of water in containers, thereby increasing the availability of man-made habitats.

The appropriateness of measures for the emergency control of Aedes aegypti that should be taken after a natural disaster depends upon the presence or absence of dengue or yellow fever in the affected area, and upon the population density of the vector. As stated previously, larval habitats may be Rushed out or destroyed during a disaster. Nonetheless, if rains occur during or after the disaster, new larval habitats may be created. However, it may take several weeks before the mosquito populations reach such a level that there is concern that disease will be transmitted. This lag in time should be sufficient for the initiation of routine control activities and for sanitation crews to haul away or empty many of the potential larval habitats. Through health education, the public should be asked to cooperate in a source reduction campaign especially since community involvement may be at a high level after a disaster. This is a key ingredient in a successful Aedes aegypti borne disease prevention program.

Larval Control

If the risk to health is immediate prior to the emergence of appreciable number of adults, source reduction will be recommended. The success of this type of campaign will depend upon the extent of organization, discipline and adequacy of number of staff members, and upon the completeness of the treatment of potential larval habitats. The Pan American Health Organization Manual of Operations for an Aedes aegypti Eradication Service can be consulted for basic organization of a campaign.

There are two insecticides that can be used for treating containers which hold potable water: one percent temephos (Abate^R) sand granules and methoprene (Altosid^R) miniquets. In many parts of the world temephos has been used as a larvicide for a number of years. Its effectiveness usually lasts from one to three months; an eight-week treatment cycle is recommended. The treatment dosage of temephos is 1 ppm. There may be objections made to the taste of the treated water, but these may be counteracted through public relations efforts.

Methoprene, an insect growth regulator, has been placed on the market only recently. Odorless and tasteless, methoprene is considered to be safe for use in potable water. It has been successfully used in Thailand, Indonesia, and Venezuela. The label must be consulted to determine proper dosage of the miniquets, which are available in a number of sizes. Since the period in which methoprene remains effective is considered to be shorter than that of temephos, a four-week treatment cycle may be necessary.

Both the temephos and methoprene insecticides can be used to treat watering containers used by animals. In many cases, however, it is unnecessary to treat containers used by animals with insecticides because frequent cleaning and changing of the water leads to effective control over the mosquito.

Other insecticides or formulations can be used for the treatment of larval habitats that are not in close association with man. As a temporary measure, tin cans and other containers can be treated before they are removed with emulsifiable concentrates and wettable powder insecticides applied with hand-operated compression sprayers and power spray equipment. Insecticides such as fenthion, temephos, pirimiphos-methyl, malathion, fenitrothion, chlorpyrifos, and methoxychlor may be used as well as diesel fuel, kerosene, and proprietary mosquito-control oils. The product label should be consulted about the rate of application and the safety recommendations. Since some of the products are highly toxic to mammals, operators should strictly observe the precautions. It is essential to prevent toxic or ecological accidents by very clearly defining the dosage to be used and the places to be treated.

Adulticiding

Efforts to control adult populations of Aedes aegypti in dwellings with residual sprays are not generally effective since as few as ten percent of the adults rest on the walls at any point in time: most rest on clothes, pictures, bedspreads, mosquito net poles and other objects. Residual spraying is also slow. As an effective means of Aedes aegypti control in urban areas it is, therefore, of doubtful value. However, it may be of greater usefulness in refugee camps. There, pirimiphosmethyl, malathion, resmethrin and synergized pyrethroids can be used. Again the manufacturer's instructions should be strictly followed.

Adulticiding, in conjunction with larviciding, will more rapidly cause decrease in the population. Use of modern application equipment can increase coverage and should be considered when (1) either dengue or yellow fever is endemic to the area, or at epidemic levels in the vicinity, (2) there is already an Aedes aegypti operational program in which this equipment is used and has effectively brought the Aedes aegypti populations under control, and, (3) the larviciding program is ineffective.

One problem that use of modern equipment poses is logistical. If the equipment is not readily available, considerable time may be lost while waiting for its arrival. Vehicles are usually taxed to the limit after a disaster, and unless vector-borne epidemic is imminent, they are usually put to other more urgent purposes than transporting modern equipment. Other problems posed by the use of modern equipment, especially in newly created programs, concern the lack of trained staff, inadequate organization and the tendency to attempt too much with limited equipment and resources. Use of modern equipment does not always entail simply negative aspects, however; it may create beneficial psychological effects, and the use ultra-low volume aerosols and thermal fogs is speedy and efficient.

A number of companies manufacture ground and portable space-spray equipment (see Annex IV). When these are utilized, the manufacturer's instructions for the operation, maintenance and calibration of' the equipment should be followed. Usually, aircraft equipment that is used in agricultural work is adapted to public health use.

Thermal fogging is the oldest of the two space spray methods. The equipment used for thermal fogging can be vehicle-mounted and portable. The portable equipment should not be used in indoor applications because it can create fire hazard. The outdoor machines are rather noisy and the fog can create a traffic hazard. There are also the disadvantages of the need to purchase and transport large quantities of nonactive oil carriers and the possible thermal decomposition of the insecticides. Despite these problems the machines are popular and provide an acceptable level of control. Thermal fog applications of chlorpyrifos, fenthion, fenitrothion, malathion, naled and pyrethroids have all shown promise in the control of Aedes aegypti. Concentrations, dosages and safety handling procedures should follow the recommendations on the label of the manufacturer.

In emergencies, one or two portable thermal foggers can be mounted on a vehicle, which serves as a mobile unit. The sizes of Aedes aegypti populations generally decline sharply within a few hours of fogging; the adults, however, reappear within a day or two. Treatment schedules should be adjusted accordingly.

The use of ultra-low volume equipment for the application of low dosages and volumes of undiluted, or partially diluted, insecticides has steadily increased. Ultra-low volume applications are rapid, and are effective against Aedes aegypti. They are also less expensive than thermal fogs because the cost of the solvent or carrier and of the transportation of thermal fogs is unnecessary.

Many control programs have had good results with vehicle-mounted, ultra-low volume cold aerosols, which are available from a number of companies (see Annex IV). Chlorpyrifos, fenthion, fenitrothion, malathion, naled, pirimiphos-methyl, and pyrethroids such as resmethrin have been used. Although initially expensive, these are relatively free of problems, and they can be operated for several years. The generator can be mounted on any one of a number of different types of vehicles. The type of vehicle that should be used will depend upon road conditions, which also determine whether or not a heavy-duty, four-wheel drive or a light, two-wheel drive vehicle should be used.

It should be noted that not all of the breeding and resting places of Aedes aegypti can be reached by road. Equipment should, therefore, include the portable or backpack ultra-low volume equipment. It is available from a number of manufacturers (see Annex IV). The performance of some of these approaches that of true ultra-low volume, while others merely have manufactured nozzle modifications for mist blowers. Since the latter equipment is used extensively in agriculture, this may provide a source of equipment during emergency situations. Although with portable equipment there is a tendency toward overdosing, it has been noted that overdosing produces a short-term residual effect that can be advantageous during disaster related emergencies. There should be two spraymen, working in shifts of thirty minutes, assigned to each piece of equipment. Workers should be provided protective gloves, respirators and clothing. Their uniforms should be changed daily and should be washed after each use, and, if possible, monthly routine cholinesterase determinations should be made on all spraymen.

Aerial ultra-low volume application is exceedingly rapid, and has been reported to be successful. Aerial applications have been successfully used in Puerto Rico, Mexico, Trinidad, the Bahamas, Honduras, and Jamaica for control of Aedes aegypti during dengue epidemics. Success of application, however, depends on the expertise with which it is performed. Aerial ultra-low volume application can be performed by specialized companies under contract. These companies generally use multi-engined aircraft that are capable of transporting insecticide over a great distance. The relatively large size of these airplanes makes it possible to treat a large area at one point in time. It is preferable to select companies that are experienced in public health spraying. Highly skilled pilots should be trained to carry out applications at the proper speeds and heights.

The single-engined aircraft and the helicopters that are used to apply insecticides and herbicides for agricultural purposes should also be considered for use of ultra-low volume application. Local and civil aeronautic regulations may restrict the use of such aircraft, but waivers can usually be obtained for some emergency usages. If aircraft used for agricultural purposes are employed, it should be noted that there is much greater coverage per acre/hectare of ultra-low volume application of pesticides than there is when agricultural pesticides are applied. Thus, the costs should not be the same. It should also be noted that the pilot of the aircraft may have to practice the application of the public health insecticide before it is actually carried out, because the method by which it is applied differs from that used for agricultural purposes.

The insecticides that can be used in aerial ultra-low volume application are malathion, fenitrothion, naled, pirimiphos-methyl and resmethrin. Unless there is an indication of resistance, or other insecticides are more readily available, an ultra-low volume formulation of malathion applied at 219 ml/ha to 440 ml/ha (3 to 6 oz.) is recommended. Multiple treatment is usually required for effective control, and entomological evaluation should be undertaken to decide upon frequency of treatment. If this is not possible, the insecticide should be applied weekly or twice-weekly, until the adult Aedes aegypti population is negligible.

When insecticides are applied, it is important to follow the instructions on both the equipment and the insecticide label. There are also a number of other factors which should be known if the equipment is to be safely used, and if it is to perform efficiently. One consideration concerns the droplet size of the insecticide. Droplets that are too small tend to drift out of the target area and may present a respiratory hazard, while allowing droplets to be too large wastes insecticide and may lead to damage to automobile paint. Nozzles for ultra-low volume ground equipment should be capable of producing droplets in the 5- to 27-micron range at the minimum. For malathion, the mass median diameter (MMD) should not exceed 17 microns. These limits change when the insecticide is applied from aircraft. When malathion is aerially applied, the nozzles should be capable of delivering droplets less than 50 microns (MMD); when naled is aerially applied they should be capable of delivering droplets of less than 30-80 microns (MMD).

The speed and time of application are important to consider when insecticide is applied by ground vehicle. The vehicle should not travel faster than 16 kilometers (10 miles) per hour, and when wind speed is greater than 16 kilometers (10 miles) per hour, or when the ambient air temperature is greater than 28°C (82° F), the insecticide should not be applied. The best time for applications is in the early morning approximately (0600-0830 hours) or late afternoon (approximately 1700-1930 hours). However, operations during the entire evening are applicable to Culex and pest mosquito control.

The information that should be known about aerial application of insecticides varies according to the types of aircraft, insecticide and equipment used. For application of malathion, the altitude of the aircraft should be between 30 to 65 meters (90 to 150 feet) and aircraft speeds should be between 160 to 260 kilometers (100 to 162 miles) per hour. Swath widths will vary according to the altitude. Early morning applications are preferable to application at other times of day. Temperatures should be less than 27°C (80°F) and wind velocity below 16 kilometers (10 miles) per hour. In addition, there should be a temperature inversion (when ground temperature is cooler than air temperature) when the insecticide is applied.

When insecticides are applied with small, portable equipment, care must be taken that the correct fuel mixture is used, that the insecticide does not leak and the engine does not overheat. Further details about the use of space spray equipment can be found in publications concerning vector control.