The consequences of asymmetric parasite interactions for the evolution of virulence

Background/Question/Methods

Parasite fitness typically peaks at an intermediate level of virulence which balances the need to increase both transmission and exposure to new hosts. This balance can be shifted when multiple strains or species co-infect a host, thereby altering the evolution of virulence. While early studies have shown that resource competition between co-infecting strains of a parasite can lead to evolution of increase virulence, recent studies suggest that the evolution of virulence should depend on the specific type of interaction between co-infecting parasites. However, the majority of these studies have focused of coinfection by different strains of a single parasite species with symmetrical interactions, such as two highly related parasites which both engage in spiteful chemical warfare. However, when different parasite species co-infect hosts, interactions between parasites are frequently asymmetrical, i.e. the pathway through which parasites affect each other can differ between species (e.g. species A affects B directly through resource competition or toxins while B affects A indirectly through cross immunity). Here we take a modeling approach to ask how different types of pairwise parasite interactions drive the evolution of virulence.  

Results/Conclusions

We considered parasites which engaged in either spiteful behavior, immune suppression, cross-immunity, immunopathology, or resource competition, producing 25 possible pairwise interactions. For each of these 25 parasite interactions, we looked at the virulence evolution of both parasite species, first in individual-based evolutionary simulations, and then using the theory of adaptive dynamics to look at evolutionary stable strategies. We found large variation in virulence levels from these pairwise interactions, both between and within pairings. Ultimately, certain parasite interactions, such as those involving resource competition, acted as a driver towards high virulence, while others, such as Immunosuppression, facilitated evolution towards lower virulence relative to other interactions. These results emphasize the importance of co-infections for virulence evolution and highlight the need to consider the type of interactions among co-infecting parasites to predict virulence in natural populations.