Dynamics of blood stage and sporozoite-induced malarial infections in experimentally infected passerines

Complex experimental studies of vertebrate host, vector, and parasite interactions are essential in understanding virulence, but are difficult or impossible to conduct if vector species are unknown. Subinoculation of erythrocytic meronts of avian malarial parasites into susceptible hosts can avoid this problem, but this approach omits early exoerythrocytic stages, e.g. cryptozoites and metacryptozoites, that normally develop from sporozoites. A fundamental question that has remained unanswered is whether blood stage and sporozoite-induced malarial infections lead to differences in the dynamics of parasitemia in acute infections, patterns of parasite development, and host mortality. Here we demonstrate in a Carduelis spinus – Plasmodium relictum (genetic lineage pSGS1) system that experimental infections using inoculation of infected blood and using mosquito bite show similar peaks of parasitemias, but some measures of parasite development in the vertebrate host differ. Infected birds from all groups show decreased activity during the peak of parasitemia. There is no doubt that experimental infections using vectors provide the most precise information about the development of a parasite and its virulence in the host, but experimental infections using blood stages demonstrate similar parasitemias and effects on the host. These results are important for further experimental research of malarial parasites, especially studying avian Plasmodium parasites with unknown vectors.