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close this bookEmergency Vector Control after Natural Disaster (PAHO)
close this folderPart II: Control measures for specific vectors
close this folderChapter 6: Anopheline vectors of malaria
View the document(introductory text...)
View the documentSurveillance of Malaria
View the documentAnopheline Control
View the documentEvaluation of Control Measures

Anopheline Control

Mason and CavaliƩ reported (1965) on a malaria epidemic that followed Hurricane Flora in Haiti. They observed that the majority of the population was without shelter or lived in temporary shelters with the maximum exposure to mosquitoes. They also noted an almost complete removal of insecticide coverage in existing houses, and an increase in population movement. In Hurricane David in Dominica in 1979, approximately eighty percent of the roofs were blown off the dwellings, exposing interiors to heavy rainfall. Under such circumstances, there is little likelihood that much residual insecticide will be left on the structures. Factors such as these, as well as potential changes in vector densities, must be considered when control activities are planned.

The control approach to be taken should reflect the influence of such factors as the status of the routine spraying operations, the results of wall bioassays in treated houses, the predisaster malaria situation, and mosquito breeding sites and adult population densities. Housing conditions and human population movements should also be reflected in control measures. Antimalarial drugs will undoubtedly be part of any prevention or control campaign, but only vector control measures will be discussed here.

A basic anopheline control program may consist of the following elements:

(1) Source reduction, by filling and draining breeding areas

(2) Larval Control through these measures:

(a) Introduction of larvivorous fish
(b) Focal treatment of larval habitats with mosquito larvicides

(3) Adult Control through the following:

(a) Residual applications using hand compression sprayers
(b) Perifocal treatment in small or isolated areas with knapsack mist blowers
(c) Peridomestic treatment in large accessible areas with vehicle-mounted aerosol generators
(d) Aerial dispersal for the control of emergency outbreaks that can not be handled by ground equipment.

Larval Control

In malaria programs larval control measures have generally been taken secondarily to adult control measures. However, if the disaster has taken place in a country in which larval control is practiced, an attempt should be made to reinitiate these measures as soon as possible. The type of larval control measures that should be taken depends upon the breeding sites of the vector involved, and requires entomological guidance.

In areas in which environmental management is used, aerial reconnaissance should provide information about the condition of drainage facilities and standing water, and engineering observations regarding remedial actions. Some work can be started, either manually or with available equipment. However, environmental management is usually slow to initiate and it is too expensive at the start to play more than a superficial role in emergencies.

Biological agents, especially larvivorous fish, have been used in routine larval control. It is possible that these agents will be destroyed or widely dispersed during the disaster, and thus be of little value in control. If these agents have been used, however, their current status should be determined. High-risk areas should be restocked as soon as possible if fish breeding programs are present outside of the disaster area.

Although infectious disease outbreaks do not usually follow upon disaster, the potential for the transmission of vector-borne disease can increase through disruption of vector control efforts in some areas. During the five month period following the hurricane that struck Haiti in 1963, seventy-five thousand cases of falciparum malaria occurred.

Chemical control measures are more effective for epidemic conditions. Where larvicides have been applied routinely, their use can be continued if the biology and habitats of the vector warrant it. Insecticide susceptibility should be tested with field-collected larvae before larvicide is ordered or used.

There are various kinds of application equipment that can be used to treat water surfaces. These include hand compression sprayers, orchard and agriculture ground equipment, and aerial spray systems. A different technique and nozzle is used in larviciding with hand sprayers than in residual wall application. Products such as "Tossits," briquette, and granules can be dispersed by hand.

Besides the more routine organic chemicals, petroleum oil products, nonpetroleum monolayers, and insect growth regulators can be used for larval control. Perhaps in the future, bacterial and other biological agents can also be employed.

Adult Control

In emergency situations, adult control is the best approach for suppressing anopheline populations. Most malaria control programs in fact utilize adult control as the basic tool of their mosquito control effort. However, if it is to be effective, the vector must be susceptible to the insecticide used and must come into contact with it. A hand compression sprayer with an appropriate nozzle is the method of choice for residual application of insecticides on the walls of houses and on the other resting sites of anophelines.

The selection of the types of insecticide to use should depend upon entomological information such as the results of susceptibility tests and wall bioassays, and on the availability of the product. In an active malaria program, there is no reason to substitute insecticides or to change the routine approach. Spray teams should enter the area to be treated as soon as possible after a disaster, where they should treat temporary housing, and, when necessary, retreat permanent housing. They should keep abreast of the movement of people and of all new construction. Community involvement should be increased for several months following the disaster.

Wettable powder formulations are usually used in malaria programs. Emulsifiable concentrates can be used in houses with painted walls, where deposits left by wettable powders may be deemed objectionable. After natural disasters, however, the choices are quite often dictated by current availability or the speed of delivery.

Space spraying plays a role in control of disease outbreaks beyond that of residual spraying of insecticides on mosquito resting surfaces. In many malaria programs, thermal foggers are used in the consolidation and maintenance phase to spray around houses in which there are active malaria cases. This type of application is usually done at dusk, or immediately before the biting activity of the vectors. Ultra-low volume equipment can be used for the same task. When resting places and principal breeding sites of the vector have been identified, thermal fogging and ultra-low volume application can take place in and around the area. Aerial application of insecticides by ultra-low volume has been successful in use against anopheline mosquitoes in Haiti. (Am. J. Trop. Med. Hyg. 24 (1975): 183-205). In an emergency, these methods can be considered, especially if sufficient ground vector control personnel are otherwise lacking or ineffective.

Special concern should be given to the situation in emergency or refugee camps. The vector control staff should be consulted in the initial stages of planning camp locations, and entomological surveillance performed on a continuing basis. If at all possible, camps should be placed away from vector hazards, such as existing or potential mosquito breeding places. Once the camp sites have been established, attempts should be made to exclude the vector from the habitat of man. Whenever possible, screen windows and doors should be furnished and those individuals who are not protected should be provided mosquito nets and encouraged to use them. If this step cannot be taken, reliance must be placed upon personal prophylactic measures. When breeding places near the camp cannot be drained, they should be treated with oils or larvicides. Insect repellents, such as DEFT and pyrethrum coils, can be used on either an individual or a group basis. Regular use of antimalarial drugs should be recommended in malaria endemic areas.

Health education, combined with individual and community involvement, can minimize the effect of an epidemic. Thus, it also makes the work of the vector control staff easier.