Genotypic variation in host response to infection affects parasite reproductive rate

Parasite fitness is largely influenced by a variation in host response due to the host's genetic background. Here we investigated the impact of host genotype on pathogen success in the snail vector of its castrating parasite, Schistosoma mansoni. We infected five inbred lines of Biomphalaria glabrata with two infection doses and followed their growth, reproductive output and parasite production throughout the course of infection. There was no difference in resistance to infection among inbred lines, but lines varied in their responses to infection and the numbers of parasites produced. Snails did not compensate for castration by increasing their fecundity during the early phase of infection (fecundity compensation). However, some lines were able to delay parasite shedding for up to 30 weeks, thus prolonging reproduction before the onset of castration. Here we propose this strategy as a novel defense against castrating pathogens in snails. Gigantism, a predicted outcome of castration due to energy reallocation, occurred early in infection (