Experimental studies on the infectibility of anopheles gambiae with malaria parasites

The feasibility of introducing genes for refractoriness to malaria into populations of Anopheles gambiae s.s.. is studied. Morphologically wild-type strains of A. grunbiae s.s. refractory and susceptible to Plasmodimo yoelii nigeriensis were selected, although complete refractoriness was not achieved. In crosses between the strains refractoriness showed intermediate dominance. In a cage replacement experiment, the susceptibility of a population of A. gambiae s.s. to P. y. nigeriensis was reduced from 100% to 600% by 8 generations of releases of refractory males. The susceptibility of the population showed no reversion towards the original high level after 15 more generations of breeding without releases or artificial selection. This indicated that the low fitness which had been observed in the refractory strains was not conferred by the refractoriness genes, since these were able to recombine into a mere well-adapted genome. The population cage results were compared with those expected on a simple theoretical model. The reliability of a membrane-feeding technique for producing infections of P. falciuarum in A. gambiae s.s. was studied in The Gambia. It was observed that the infeotivity of P. falciparum gametooyte carriers depended largely on the gametocytaemia. Selection to produce a strain of A. gambiae s.s, completely refractory to P. falciparum did not succeed, and there were indications that the local strain had a low or zero frequency of genes for refractoriness. It was shown that strains refractory to P. y. nigeriensis were not refractory to P. falciparum. Oocyst counts of P. falciparum were found to be distributed according to the negative binomial distribution. The possible means of introducing refractoriness genes into mosquito populations are discussed, taking into account the fitness of refractory types both in the presence and absence of malaria infection.