Interactions among virulence, coinfection and drug resistance in a complex life-cycle parasite

Motivated by relatively recent empirical studies on Schistosoma mansoni, we use a mathematical model to investigate the impacts of drug treatment of the definitive human host and coinfection of the intermediate snail host by multiple parasite strains on the evolution of parasites' drug resistance. Through the examination of evolutionarily stable strategies (ESS) of parasites, our study suggests that higher levels of drug treatment rates (which usually tend to promote monomorphism as the evolutionary equilibrium) favor parasite strains that have a higher level of drug resistance. Our study also shows that whether coinfection of intermediate hosts affects the levels of drug resistance at ESS points and their stability depends on the assumptions on the cost of parasites paid for drug resistance, coinfection functions and parasites' reproduction within coinfected hosts. This calls for more empirical studies on the parasite.

► The snail–schistosome–human system is modeled.
► The evolutionarily stable strategy of the parasite's drug resistance is examined.
► Human treatments favor parasite strains with high levels of drug resistance.
► Coinfection of snail hosts may promote dimorphism or even polymorphism of the parasite.