Mosquito mortality and the evolution of malaria virulence

Several laboratory studies of malaria parasites (Plasmodium sp.) and some field observations suggest that parasite virulence, defined as the harm a parasite causes to its vertebrate host, is positively correlated with transmission. Given this advantage, what limits the continual evolution of higher parasite virulence? One possibility is that while more virulent strains are more infectious, they are also more lethal to mosquitoes. In this study, we tested whether the virulence of the rodent malaria parasite P. chabaudi in the laboratory mouse was correlated with the fitness of mosquitoes it subsequently infected. Mice were infected with one of seven genetically distinct clones of P. chabaudi that differ in virulence. Weight loss and anemia in infected mice were monitored for 16-17 days before Anopheles stephensi mosquitoes were allowed to take a blood meal from them. Infection virulence in mice was positively correlated with transmission to mosquitoes (infection rate) and weakly associated with parasite burden (number of oocysts). Mosquito survival fell with increasing oocyst burden, but there was no overall statistically significant relationship between virulence in mice and mosquito mortality. Thus, there was no evidence that more virulent strains are more lethal to mosquitoes. Both vector survival and fecundity depended on parasite clone, and contrary to expectations, mosquitoes fed on infections more virulent to mice were more fecund. The strong parasite genetic effects associated with both fecundity and survival suggests that vector fitness could be an important selective agent shaping malaria population genetics and the evolution of phenotypes such as virulence in the vector.