Evolution of parasite virulence to vectors

Vectorborne parasites are commonly predicted to be less virulent to the vector than to the definitive host as the parasite gains little by harming its main route of transmission. Here we assess the empirical evidence from systems where insects vector vertebrate parasites. The body of evidence supports lower (but non-zero) parasite virulence to vectors than to plant or invertebrate hosts but not to vertebrate hosts. We consider why this might be by assessing evolutionarily stable strategies for an insect parasite that can infect both vector and definitive host and can have distinct virulences in these two potential hosts. In a homogeneous environment the parasite is predicted to be equally virulent to vector and host. However, in a patchy environment it is predicted to become benign towards the more mobile of the two potential hosts, provided competitive displacement among strains in a patch is weak. This prediction may not meet reality for two different reasons. First, relative mobility of vector to host depends on the spatial scale under consideration: malaria mosquitoes are the more mobile hosts from house to house within a human settlement, but human hosts may be more mobile from one settlement to the other. Second, as in malaria, the main host and therefore probably also the vector may be multiply infected and this is likely to increase virulence and to level off differences between vector and definitive hos