Epidemiology ofPlasmodium falciparumin a rice field and a savanna area in Burkina Faso: seasonal fluctuations of gametocytaemia and malarial infectivity

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Abstract
For a better understanding of the epidemiology of Plasmodium falciparum in an African savanna area, the authors have: (a) defined the real gametocyte reservoir in the native population; (b) followed the fluctuations of gametocytaemia throughout the transmission period; and (c) measured the infectiousness of malarious individuals to mosquitoes. The transversal surveys, in different villages of this endemic area, have shown that gametocyte carrier rates decreased with age and malaria experience; 10·9% of the whole population were potentially infectious to mosquitoes, and of these 73% were children and only 27% were adults. The longitudinal studies have shown that the P. falciparum gametocyte rate depends on the equilibrium between the gametocyte conversion rates and the density of the asexual forms. When there are large numbers of children who become carriers of the sexual stage of the parasite and at the same time a small number who lose their gametocyte infection, the gametocyte rate increases in the population; and vice versa. The circumstances under which gametocytes are produced are not well-known. Two factors seem to be important: the level of the parasite density and immune mechanisms. The infectiousness of malarious individuals was estimated by the ‘mosquito infection probability’. The percentage of mosquitoes infected after feeding on gametocyte carriers (which may partly reflect the infectiousness of a human population to mosquitoes) was multiplied by the percentage of gametocyte carriers in the population. This indicated that, in this endemic area, 4% of biting mosquitoes would become infected; but this theoretical mosquito infection probability is over-estimated. Not all the gametocyte carriers are infectious, and the proportion of mosquitoes experimentally infected on heavy gametocyte carriers does not reflect the true percentage of mosquitoes found positive in natural conditions. It is concluded that the best method for estimating the infectiousness of a human population is (a) to feed laboratory-reared Anopheles on a representative population sample (without regard to the presence of gametocytes), or (b) to feed these Anopheles only on the gametocyte carriers detected in the population (without regard to gametocyte density). The second method allows estimation of the proportion of gametocyte carriers infectious to mosquitoes in each age-group. By multiplying this proportion by the mean percentage of infected mosquitoes, a more realistic mosquito infection probability can be evaluated.