A quantitative analysis of the distribution of Plasmodium falciparum oocysts in Anopheles gambiae

It is now admitted that antimalarial immunity is, at least in part, strain specific. This finding implies hat qualitative differences exist among sporozoites and might exit among oocysts. Our study concerned two distributions of Plasmodium falciparum oocysts. The first distribution was largely exposed in Tchuinkam et al. (1993) and has been obtained from An. gambiae s.s. infected through experimental devise with blood of 154 gametocytes carriers. The other distribution was exposed in Pringle (1966) and has been obtained from indoor resting freshly fed An, gambiae s.l. collected in an hyperendemic area. The totals of mosquitoes were 5,704 and 1,540, and the totals of oocysts were 2,440 and 1,308, respectively Each oocyst population in each experimental infection of Tchuinkam et al. was negative binomially distributed. They had variable means and a constant degree of aggregation. This constant, common for each infection (k(c) = 0.267 +/- 0.020) was calculated by maximum likelihood method. This value was very low comparing to k = 1 observed for bancroftian microfilariae. That suggest a non-proportional (i.e. density-dependent) relation between the numbers of ingested gametocytes and oocysts which were produced. in the field, the system << P. falciparum oocysts in An. gambiae >> was also well adjusted to a binomial negative distribution with a very high degree of heterogeneity (k < 0.05, depending on the situations). As a consequence, in laboratory as in the field, the calculation of the degree of oocyst infection appears to be possible directly from the oocyst index. The Pringle's data analysis permit to conclude that results of epidemiological studies must be presented without mixing different vector species. The authors highlight the notion of << non-parasitable >> vector (composed of refractory mosquitoes to the ingested plasmodial strain and mosquitoes fed on human host without gametocyte) and propose a method to estimate the relative part of each of these two components.