Reservoir & Transmission

Diagram shows Anopheles mosquito taking its blood meal

Malaria is transmitted from man to man by the female Anopheles mosquito, one of the most capable vectors of human disease. Various species have been found to be the vectors in different parts of the world. However, out of the 380 species of Anopheline mosquito, only 60 can transmit malaria. A.gambiae complex is the chief vector in Africa and A.freeborni in N. America. Nearly 45 species of the mosquito have been found in India and A.culicifacies, A.fluviatilis, A.leucosphyrushave, A.philippinensis, A.stephensi, A.sundaicus, and A.minimus been implicated in the transmission of malaria. The areas of distribution are different for these mosquitoes: A.fluviatilis and A.minimus are found in the foot-hill regions; A.stephensi and A.sundaicus are found in the coastal regions; A.culicifacies and A.philippinensis are found in the plains. Species like A.stephensi are highly adaptable and are found to be very potent vectors of human malaria.

Transmission of Plasmodium falciparum can be reduced by immune factors present in the mosquito blood meal. Specific antibodies and white blood cells (WBCs) can interact with the sexual stages of the parasite inside the mosquito midgut. The relative contribution of serum factors and WBCs on transmission reduction in gametocyte carriers from an endemic area in Cameroon and in travelers with a first malaria experience was studied. Blood from these gametocyte carriers was fed to mosquitoes through membrane feeders after serum replacement, WBC depletion, or both. In most imported malaria cases, serum factors, WBCs, or both showed a significant effect on transmission reduction, while infectiousness of gametocyte carriers from Cameroon was reduced by humoral plasma factors only. In addition, the infectivity of gametocytes from semi immune carriers was significantly lower compared with that of nonimmune carriers, and infectivity was independent of gametocyte density and the presence of WBCs or plasma factors (or both) in the blood meal.Understanding the biology and behavior of Anopheles mosquitoes can help understand how malaria is transmitted and can aid in designing appropriate control strategies. Factors that affect a mosquito's ability to transmit malaria include its innate susceptibility to Plasmodium, its host choice and its longevity. Factors that should be taken into consideration when designing a control program include the susceptibility of malaria vectors to insecticides and the preferred feeding and resting location of adult mosquitoes.

The reemergence of epidemic malaria in the East African highlands (elevation >1,500 m above sea level) is a public health problem. Research indicates that the mechanisms leading to epidemic malaria in the highlands are complex and are probably due to the concerted effects of factors such as topography, hydrology, climate variability, land-use/land-cover change, and drug resistance. Effective disease control calls for a clear understanding of the interaction between these epidemiologic factors.

Perennial malaria transmission in the lowlands has been attributed to high vector densities throughout the year. For example inhabitants of the basin region of Lake Victoria, western Kenya, experienced up to 300 infective bites per year. Vector density and transmission intensity in the highlands are much lower than in the lowlands. For example, a transect study from lowland (300 m elevation) to highland (1,700 m elevation) in the Usambara Mountains in Tanzania found a >1,000-fold reduction in transmission intensity between the holoendemic lowland and the hypoendemic highland plateau. At high altitudes in the highlands and on hilltops, where malaria transmission intensity is low, human populations have poorly developed immunity to malaria because exposures are infrequent, and persons are vulnerable to severe clinical illness and complications from Plasmodium infection. High risk for severe malaria is seen in persons living in areas with low-to-moderate transmission intensities . In such areas, the proportion of asymptomatic persons is usually lower than in high-transmission areas, where P.falciparum prevalence and parasite density varies little between seasons.