The effect of partial host immunity on the transmission of malaria parasites.

Experiments were carried out to determine the effect of partial host immunity against the rodent malaria parasite Plasmodium chabaudi on the transmission success of the parasite. There was a fourfold reduction in both the blood-stage, asexually replicating parasite density and the gametocyte (transmissable stage) density in immunized hosts. Some of the reduction in asexual parasite densities was due to strain-specific immunity, but there was no evidence that strain-specific immunity affected gametocyte densities. However, immunity did affect transmission in a strain-specific manner, with a fivefold reduction in gametocyte infectivity to mosquitoes in homologous challenges compared with heterologous challenges or non-immunized controls. This implies the existence of a mechanism of strain-specific infectivity-reducing immunity that does not affect the density of gametocytes circulating in peripheral blood. The proportion of asexual parasites that produced gametocytes increased during the course of infection in both non-immunized and in immunized hosts, but immunity increased gametocyte production early in the infection.