The impact of multiple infections on wild animal hosts: a review

Bordes, Frédéric; Morand, Serge

Predictions related to multiparasitism impacts on host fitness benefit from both theoretical and experimental studies investigating the expression and evolution of parasite virulence during coinfections (28Á31). The main result emerging from this active field of research suggests that competitive interactions among strains select parasites that are able to exploit their host more rapidly than their parasite competitors, thereby causing increased host exploitation (30) . In multiple infections, natural selection is then expected to favour higher levels of virulence. This prediction was supported by experimental studies concerning coinfections by strains/ genotypes in rodent-malaria, snail-schistosome or Daphnia-microparasite models (29, 31, 32) . Unfortunately, few laboratory studies have considered interspecific interactions from a virulence perspective. In multi-strain infection, we may expect cross immunity, kin selection and potential recombination between strains to occur, which should not happen in interspecific coinfection. Consequently the related outcomes could be different. Indeed, the few models dealing with interspecific interactions stressed that sub-lethal pathogens like helminths may lead to the evolution of lower virulence in multiple infections (30) .