Malaria Site: Adult Mosquito Control

ontrol of adult mosquitoes is not an easy task, considering their ability to fly all over and to hide in nooks and corners. It is recommended in areas and seasons with high transmission of malaria. Control of adult mosquitoes involve the following measures:

Space sprays: These insecticides instantly kill the mosquitoes, but lack any residual effects. They are therefore sprayed into the air. By killing adult mosquitoes, not only bites are prevented, but breeding is also prevented, resulting in net reduction in the mosquito population. Space sprays must be repeated often, at least once every week. Pyrethroids are commonly used for this purpose.

Space spraying involves the application of small droplets of insecticide into the air, but recent studies have demonstrated that the method has little effect on the mosquito population. 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. (In fact, in Mangalore, the ward level committees formed in the year 1995-96, lost steam and became defunct after fogging operations were introduced in late 1996!) Although it is highly visible and conveys the message that the government is doing something about the disease, this can be only a poor justification for using space sprays. (Often, members of the City Corporation order fogging in their constituencies to 'satisfy' their voters!).

Space spraying 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. Fogging should be primarily reserved for emergency situations: halting epidemics or rapidly reducing adult mosquito populations. It must be timed to coincide with the peak adult activity, because resting mosquitoes are often found in areas that are difficult for the insecticide to reach (e.g., under leaves, in small crevices). Generally, there are two forms of space-sprays, namely thermal fogs and cold fogs and both can be dispensed by vehicle-mounted or hand-operated machines.

Thermal fogs

Thermal fogs are produced when an insecticide formulation condenses after being vaporized at a high temperature. These 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.

Ultra-low volume (ULV), aerosols (cold fogs) and mists

ULV involves the application of a small quantity (<4.6 litres/ha) of concentrated liquid insecticides. 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. Congested low-income housing areas, multistoried buildings, godowns and warehouses, covered drains, sewer tanks and residential or commercial premises are some examples.

Vehicle-mounted fogging 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. An educational effort may be required to persuade the residents to cooperate by opening doors and windows. The best time for application is in the early morning (6am-8.30am) or evening (5pm-7.30pm).

Low dosages of pyrethroid insecticides are usually more effective indoors than outdoors.

Indoor Residual Spraying: Residual spraying with DDT and dieldrin was the primary malaria control method used during the Global Malaria Eradication Campaign (1955-1969) and resulted in elimination of malaria from several areas and sharply reduced the burden of malaria disease in others. With the mosquitoes developing resistance to DDT and dieldrin on the one hand and reports of long term adverse environmental impact from these non-biodegradable chemicals on the other, the use of these insecticides declined over the years and malaria has now made a dramatic comeback.

Residual sprays are used for indoors, meticulously sprayed onto the walls and other surfaces in the interiors. The endophilic malaria vectors that rest inside the houses after taking a blood meal are killed when they come in contact with the insecticide and thus transmission of malaria is prevented. Killing of adult female mosquitoes also results in a net reduction in mosquito population. The residual insecticide is effective for several months. To be effective, the residual spraying should include a very high proportion of households in an area (usually >70%).

Malathion 25% and Deltamethrin are the two insecticides now routinely used for this purpose. But the cost of the operations is a major hindrance.

Novel Genetic Methods: Sterile male release has been successfully applied in several small-scale areas. However, the need for large numbers of mosquitoes for release makes this approach impractical for most areas. Genetic modification of malaria vectors aims to develop mosquitoes that are refractory to the parasite. This approach is still several years from application in field settings.