The influence of drug treatment on the maintenance of schistosome genetic diversity

Drug treatment of patients with schistosomiasis may select for drug-resista nt parasites. In this article, we formulate a deterministic model with mult iple strains of schistosomes (helminth parasites with a two-host life cycle s) in order to explore the role of drug treatment in the maintenance of a p olymorphism of parasite strains that differ in their resistance levels. The basic reproductive numbers for all strains are computed, and are shown to determine the stabilities of equilibria of the model and consequently the d istribution of parasite phenotypes with different levels of drug tolerance. Analysis of our model shows that the likelihood that resistant strains wil l increase in frequency depends on the interplay between their relative fit ness, the cost of resistance, and the degree of selection pressure exerted by the drug treatments.