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MALARIA: Obstacles and Opportunities
ity to parasites and the probability it will survive long enough for the parasite to fully develop.
Vector biologists know very little about vector-related factors that affect sporozoite viability in nature. Epidemiologic studies indicate that, at most, between 1 and 20 percent of sporozoite inoculations produce infections in nature (Pull and Grab, 1974). Effective, direct assays for determining sporozoite viability for individual, field-collected mosquitoes do not exist. Human antibodies ingested by mosquitoes may play some role in regulating sporozoite infectivity. In one study, human immunoglobulin G antibody was found on sporozoites in over 80 percent of infected mosquitoes sampled in Kenya (Beier et al., 1989); the significance in terms of sporozoite infectivity is unknown.
Regulation of Vector Populations—Larval Ecology
The mechanisms that regulate vector populations are poorly understood but are of great importance for malaria control (Molineaux, 1988). For example, there is limited information on the biology of aquatic stages of malaria vectors. The factors affecting larval survival and the mechanisms controlling adult production are largely unknown for even the most important vector species. The basic concept of density-dependent regulation has never been studied for populations in nature. It is extremely important to know whether populations are regulated through competition (intra-and/or interspecific) and predation in the aquatic habitat. Furthermore, there is no baseline information on the foraging habits and strategies of larval-stage vector populations. The study of larval biology is complicated further by inadequate techniques for the identification of larvae belonging to species complexes. Consequently, few entomologists seek to tackle this important area of anopheline biology.
A basic understanding of the aquatic stages of vectors is extremely relevant to malaria control. Source reduction through the modification of larval habitats was the key to malaria eradication efforts in the United States, Israel, and Italy (Kitron and Spielman, 1989). In these countries, a variety of measures directed against the aquatic stages of important vectors reduced cases of malaria and eliminated parasite transmission.
The identification of anopheline mosquitoes responsible for malaria transmission is known as vector incrimination, and the approach is the same for any given area. Mosquitoes, preferably those coming to feed on humans, are collected, identified, and dissected to determine the presence of sporozoites in the salivary glands. Immunological techniques can be used to