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All mosquito vectors, capable to transmit malaria, belong to the genus Anopheles. Previous entomologic investigations in Armenia have revealed six anopheline species as potential vectors for malaria transmission in the country. Anopheles maculipennis is the most common malaria vector. An. sachavori, An. claviger, An. hyrcanus and An. plumbeus complete the list of anopheline fauna in the country, although they play secondary role in malaria transmission.
This section provides a general overview of various vector control methods (see Fig1).




Fig1. Vector control strategies

The basic rationale for malaria eradication comes from the mathematical equation refined by MacDonald. If Ro can be driven below zero, the disease will

disappear. The key elements of the equation are the man-biting rate (a), abundance of vector (m), vector competence (b), and expectation of infective mosquito life (p).
Chemical insecticides (e.g. DDT, pyrethroids, organophosphates etc.) affect directly two of these elements in a profound way. They reduce the man biting rate (a) by driving away mosquitoes through an irritant effect, as well as the expectation of infective life (p) by shortening the length of mosquito life. Chemical insecticides are classified according to their action on the life cycle of mosquitoes into larvicides and imagicides (adult form). They are either sprayed (imagicides) or applied to the water surfaces (larvicides).
Spraying with residual insecticides had a tremendous success worldwide. They retain their toxic action for a considerable period when applied to the surface with which adult Anopheles may come into contact. However, malaria has a very high reproductive rate, and complete eradication proved unattainable whenever mosquito resistance was emerging or when administrative inconsistency was involved.
In general, this method works very well, but most countries do not want to commit to the expense - in both money and personnel - of a residual spraying program. In Armenia, indoor spraying with Cyfluthrin (also Solfac, Pyrethroid chemical class) has been successfully utilized. Up today, all tests on major mosquito vector (An. maculipennis) in the country have demonstrated full susceptibility to Cyfluthrin.

Outside space spraying with insecticides can also be a valuable method to rapidly reduce the numbers of mosquitoes in outside breeding grounds. It is, however, expensive in terms of the insecticide itself, and the special vehicle for dispersion (e.g. airplane). Moreover, this method is less effective against Anopheles, than against Aedes and Culex.

Knowing mosquito biology is always very important in understanding various vector control methods. In particular, the larva and pupa always live in water. Therefore, in theory, at least, it is possible to reduce mosquito populations by reducing the number of water bodies with larval mosquitoes. This kind of larval habitat destruction is an environmental source reduction. The results depend upon details of biology of the mosquito in question, and the energy and persistence of the control workers.
Armenian experience once again has proved the significance of environmental management for mosquito control. Following the breakdown of the Soviet Union, more than 70% of water drainage system in Ararat Valley became inadequate due to the absence of technical maintenance. The resulting excessive stagnant water areas became ideal sites for mosquito breeding (see Fig2). Cleaning and restoration of the water drainage system were crucial in improving the overall irrigation system and in reducing mosquito breeding sites.



Fig2. Mosquito breeding site (Drainage way, Ararat Valley)

The use of larvivorous fish, such as Gambusia affini, has been a widely popular method of biological vector control. Gambusia is a natural enemy of mosquito larvae and it has been used with advantage for malaria control in many countries of the world including Armenia. However, this method is limited by the fact that not all aquatic systems are suitable for larvivorous fish re-population. Moreover, the mosquito bioantagonism can be only reached if fish is present in very large numbers.
Other biological agents considered for malaria control include bacteria (Bacillus thuringiensis, B. sphaericus), nematodes and predatory mosquitoes. In general, biological vector control looks very promising, although there is a place for skepticism around this issue. Almost all mosquito vectors have evolved locally, well adapted to their environment for a very long time. Their populations have been already affected by a diverse suite of pathogens and predators. Furthermore, according to MacDonald equation, larval control strategies, designed to reduce mosquito abundance (m) by disrupting reproduction, may actually result in selection for a stronger and healthier adult mosquito population, thereby increasing (p).

Genetic control of mosquito reproduction is a special type of biologic control. The method is again rooted in mosquito biology with the underlying assumption that female mosquitoes mate only once. Once mated, sperm is stored in a spherical sclerotized container (spermatheca) and it is doled out one by one as eggs are laid one by one. The idea here is to rear large numbers of irradiated sterile males, release them in large numbers, so that the first male to mate with females is a sterile one. The female will not mate again. Moreover, along with mating, the male transfers a kairomone which inhibits female's mate-seeking impulse, so that all eggs she lays are sterile. This sterile male technique has been tried for An. stephensi in India, and, famously, for Ae. aegypti in coastal Kenya without much success in both cases. It is difficult to produce enough males and ensure that they are sterile, and females are so fecund, that missing a few of them results in fairly large populations of adults quickly. Another problem is 'density dependence'. Larval mortality is normally very high in natural habitats due to competition. Reduction of the number of eggs hatching can thus lower larval mortality rate, with no reduction in the number of females emerging.

Another innovative approach, related to creating transgenic mosquitoes, is under development. The idea is to engineer mosquitoes which will be genetically refractory to infection with malaria, and then have such a refractory gene spread through natural mosquito population. However, driving genes through the natural populations is a very challenging problem.

There has been a great emphasis recently on insecticide treated materials (ITM) such as bednets, which effectively prevent human-vector contact. ITM reduce precisely same parameters in the MacDonald equation as residual spray. Bednets are most often treated with new synthetic pyrethroids. Their effect usually lasts for 3-4 months. Therefore, depending on the duration of malaria transmission season, bednets have to be repeatedly treated to maintain their effectiveness.
Other measures of personal protection against mosquito bites such as wearing protective clothing or using mosquito repellents are also very popular. Repellents are chemical substances applied to the skin or clothing to repel mosquitoes and prevent them fvrom biting. Several effective repellents are available on the market nowadays, among which N,N-diethyl-3-methylbenzamide (DEET) is the best against many blood-sucking insects. DEET is a major ingredient of almost 90% of commercial repellents.
Today it is difficult to deem one particular vector control method to be the most effective. The development of mosquito resistance towards insecticides as well the growing concern about environmental protection, have shifted the emphasis on integrated vector control strategy. Integrated vector control assumes using different methods synergistically. Thus, for example in Armenia, indoor residual spraying has been combined with various source-reduction methods and the use of larvivorous fish Gambusia.
In general, personal protection, skillful agriculture and water management, source reduction, availability of insecticides are the key components of successful vector control strategy.

 

© 2002. Malaria in Armenia.
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