The mosquito : public enemy no. 1 / Awash Teklehaimanot & Pushpa R. J. Herath

The mosquito: public. enemy Noi 1 by Awash Teklehaimanot & Pushpa ·R.J. Herath H uman malaria is normally trans-mitted from one person to another by the bite of a female anopheles mosquito infected with malaria parasites. lt is only the female anopheles mosquito that bites and feeds on people. The male mosquitos feed only on plant juices and thus do not play any role in the transmis- sion of the disease. The females need blood as a source of protein for the development and maturation of their eggs. Their mouth parts have developed so that they can pierce through the skin and blood vessels and suck blood. They also excrete anticoagulants to prevent the blood from clotting as they feed. There are some 400 species of ano- pheles mosquitos, but only about 70 species are known to be responsible for transmitting malaria. About 30 are of major importance, responsible for a signi- ficant amount of all malaria cases. In Africa, the most important vectors are members of the Anopheles gambiae group and A. funestus. lt is not clearly known why some species of mosquito are susceptible to malaria infection and others are not. or why some are more effective in trans- mitting the disease than others. These differences are partly explained by the way they feed, how long they live and how many there are. Some vectors feed exclusively on humans every 2 to 3 days, while others feed either on humans or other animals as opportunities arise. Mosquitos have four distinct stages in their life history-the egg, the larva, the pupa and the adult. The first three stages occur in water. while the adult is an active flying insect. Anopheles eggs are laid singly on the water surface and hatch into the larval stage within one to three days. The larva is an active feeder and obtains its food from the water in which it lives. After about ten days, the larva moults into a pupa, which also lives in water. lt does not feed, but has to come to the surface from time to time for air. After one to four days the pupal skin splits and the .adult mosquito slowly works its way out. lt rests briefly on the water surface and then flies away. The adult has an average flight range of one to two kilometres. Its life span can vary from a few hours to a few months. Anopheles mosquitos have success- fully adapted so that they can breed in almost any stagnant sheet of water from lakes down to temporary pools, cattle hoof prints and discarded containers. They can thrive in fresh or brackish waters. Malaria transmission can be prevented or reduced through different control methods directed against the aquatic stages and the adult mosquitos. The most direct and obvious precaution is to avoid being bitten by mosqu'itos using methods WORLD HEALTH. September-October 1991 explained on pages 22 and 23. Locating houses, villages and settlements away from major breeding sites will reduce the threat of mosquito bites. But this will call for a good water supply that is not dependent on water where the mosquitos breed. Improved housing design and construction to, pre- vent mosquitos from entering and resting indoors would also reduce the danger of bites. Anopheles gambiae eggs larva pupa " ~): .· .. Many mosquito breeding sites such as domestic water storage containers, aban- doned mines, pits resulting from housing and road building, wells, drains and irrigation systems are created by people themselves. People can take action to counter some of these. Chemical compounds such as teme- phos can kill the larvae in the water but they are only suitable when breeding sites are few and the areas involved are limited. Of course, toxic substances must not be applied to water used for drinking or washing. Standing pools of water near houses should be filled or drained off and irrigation canals should be properly main- tained· and cleared of vegetation to allow the free flow of water. Periodical drying and flooding of rice fields for short periods to disrupt the mosquito life stages have proved effective in some countries. Some types of fish eat mosquito larvae. Certain so-called "annual fish" (genus Notobranchus) can survive droughts and may be used in bodies of water that are not permanent. The . use of larvivorous fish is ideal for community involvement. lt is best if these fish can be bred along w ith types that can be eaten, as the breeding of such fish gives an additional financial motivation! Fish that feed on plants can also help to keep the edges of canals and drains free of vegetation, facilitating the free flow of water. Different types of insecticides are avail- able to kill adult mosquitos. The "knock- down" insecticides are effective for a matter of days while the "residual" · insecticides may last for months. DOT has been the most widely used residual insecticide since its introduction in the 1940s. lt is also the cheapest and safest for indoor use against adult mos- quitos which feed and rest inside houses. Use of residual insecticides is advocated only to control malaria outbreaks, and in selected situations. The use of insecticide spraying on a large scale or for long periods is not sustainable because of the high costs, problems with spraying, and mosquito resistance to insecticides. Spraying in the air with "knock-down" insecticides is useful when prompt action is called for, for example during epidem- ics and when large po'pulations congre- gate outdoors at night. The operation is extremely costly and must be carefully timed to coincide with times when mos- quitos fly around. Thus, various options exist for controll- ing vector mosquitos. These differ in efficacy and input requirements such as finance, technical expertise, manpower and community involvement. When appropriately used, each method can make a valuable contribution to the control of malaria. With a better under- standing of how the mosquito breeds and lives, it will be possible to target control efforts even more successfully against this Public Enemy No. 1 in many coun- tries. • Dr Awash Teklehaimanot and Dr Pushpa R. J. Herath are Scientists with the Malaria Con- trol Unit WHO. 1211 Geneva 27. Switzerland. 21