Eco-evolutionary dynamics of anthelmintic resistance in soil-transmitted helminths.

Anthelmintic resistance (AR) of soil-transmitted helminth parasites against the most widely available drugs is an ongoing concern for both human-infecting and livestock-infecting species. There has been substantial evidence of AR in livestock but less in humans, which may be due to a variety of reasons. In this paper, we develop an eco-evolutionary model that couples the life cycle of these parasites with their underlying evolution in a single biallelic genetic locus that confers resistance to treatment drugs. We determine the critical treatment frequency needed to effectively eliminate the population, for a fixed drug efficacy (without evolution) and use this to classify three qualitative distinct behaviors of the eco-evolutionary model. Then, we describe how aspects of the life cycle influence which qualitative outcome is achieved and the rate of spread of the resistance allele, comparing across parameterized models of human-infecting and livestock-infecting species. For all but one species, we find that lower fecundity rates and lower contact rates speed the spread of resistance, while lower larval death slows it down. The life cycle parameters of Ancylostoma duodenale and Ostertagia circumcincta are associated with the fastest and slowest spread of resistance, respectively. We discuss the mechanistic reason for these results.