Virulence evolution: thinking outside of the host

Abstract

The main theory of the evolution of virulence relies on a trade-off between virulence and the rate of transmission. It has however proven difficult to measure the required trade-off. This lack of support may partly be due to the lack of information about the parasite behaviour in the environment between hosts, where the parasite finds itself during transmission. In this study, we used the parasiteVavraia culicisand its host, the mosquitoAnopheles gambiaeto investigate the interplay between the parasite virulence in its host and its longevity outside of its host. To do so, we selected parasite lines for early or late transmission and used a transmission decomposition framework to describe several traits of the parasite life cycle at two temperatures. Lines with greater virulence and that grew more rapidly within their host lived less long in the external environment, irrespective of temperature. These results underscore the importance of considering the full context of transmission and other parasite fitness traits in studying and predicting the evolution and spread of infectious diseases.