Factors affecting the inefectivity and early sporogonic development of plasmodium falciparum in the malaria vector, Anopheles gambiae s.s
Okech, Bernard Achero
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This study investigated the effects of abiotic factors particularly temperature and biotic factors (mosquito related) on early malaria parasite development and infectivity. Volunteer gametocyte carriers were recruited following malaria infection surveys in the schools and community and requested to donate blood (-3m1) for artificial infection of An. gambiae mosquitoes via the immediate membrane feeding assay using glass feeders and parafilm membrane. In studies related to the impact of temperature on malaria development, 2 experiments were carried out, (1) to determine if P. falciparum development can continue beyond 32°C temperature threshold and, (2) to determine the influence of indoor microhabitat temperature prevailing in the village houses on malaria development. Mosquitoes were exposed to these microclimatic conditions and later dissected to determine parasite densities in the mosquito midguts. Mosquito factors such as age and the previous dietary history was also studied for their influence on malaria infectivity to mosquitoes. Similarly, the impact of microclimatic and dietary factors on mosquito survival was studied both in the semi-natural and in the natural environment. The study showed that malaria parasites were able to develop at high temperatures above 30°C. Mean ookinete densities were decreased with increase in temperature and were significantly different (ANOVA: F = 3.705, d.f. = 2,49, P = 0.032). However, oocsyt densities were not statistically different (ANOVA: F=0.301, d.f=2, P=0.741) between 27°C, 30°C and 32°C temperature levels. Similarly, the indoor microclimates prevailing in the villages were suitable for malaria development as parasite densities in the mosquito midguts were not different (ANOVA: F=0.546, df=2, P=0.580) for experiments done in the semi-natural environments and also experiments done in the field (ANOVA: F=0.671, df=2, P = 0.523) conditions. The study also revealed that the infectivity of P. falciparum to mosquitoes was not influenced by the age of the mosquito (Chi-square, x2 =3.604, df = 2, P = 0.165) rather by the dietary factors the mosquito had previously fed on. Under similar microclimatic conditions, the survival potential of mosquitoes could enable malaria parasite transmission. In addition, the presence of sugar in the diet improved survival but increased feeding on blood in the absence of sugar and the presence of parasites in the blood meal lowered the survival probability. Although temperatures affects the rates pf parasite and vector survival, this study has shown that parasites can resist high temperatures and as such cannot stop transmission in hot areas. However, mosquito related biotic factors are more important determinants of malaria infectivity and development in the An. gambiae and influences malaria transmission in nature. This information is very useful in predicting how temperature changes can affect the distribution of malaria transmission in nature and also the role of dietary factors in transmission of malaria by mosquitoes.