Virulence in rodent malaria: host genotype by parasite genotype interactions

In an effort to understand what limits the virulence of malaria parasites, we infected inbred mice of three genotypes (C57Bl/6J, CBA/Ca and DBA/2) with one of two parasite lines of the rodent malaria Plasmodium chabaudi. One of these parasite lines had been serially passaged through C57Bl/6J mice and had evolved higher asexual growth rate, virulence and transmission in the process. The other parasite line was the unadapted ancestral line which had low virulence. In all three host genotypes, the C57Bl/6J-adapted parasite line was more virulent than the ancestral line thus indicating that trade-offs in virulence between alternative host genotypes had not placed strong constraints on the evolution of high virulence in this system. By examining the infection dynamics for fitness-related components-asexual parasite population growth, transmission and virulence-we revealed alternative possible explanations for what sets the upper limit to virulence in nature. The total number of transmission forms (gametocytes) produced during the infection, a measure of parasite Darwinian fitness, was four-fold higher in mice that survived the infection than those which died. Among mice that survived, total gametocyte production was greatest in the host genotype that suffered intermediate levels of morbidity (anaemia and weight loss). Thus, there were transmission costs of high virulence that were partly due to host death (as most theoretical models of virulence evolution assume), but perhaps partly due to some factor related to high morbidity. Both mortality and morbidity-related factors might therefore influence the upper limit on virulence of malaria parasites.