Analyses of emerging macrocyclic lactone resistance: Speed and signature of ivermectin and moxidectin selection and evidence of a shared genetic locus.

Subtherapeutic treatment or 'underdosing' is considered a common problem in the control of parasitic helminths of animals and people and can hasten the emergence of anthelmintic resistance. Increasing reliance on the long-acting macrocyclic lactone, moxidectin, in both veterinary and medical settings may increase exposure of incoming helminth populations to subtherapeutic drug concentrations due to its extended half-life. However, we lack genetic markers to monitor emerging resistance as the mechanism(s) underlying resistance to the macrocyclic lactones are unresolved in parasitic helminths. Furthermore, the impact of prior ivermectin exposure on the evolution of moxidectin resistance is unclear. To test the impact of subtherapeutic selection on the emergence of macrocyclic lactone resistance, we exposed a fully drug susceptible isolate of an economically important parasitic helminth of livestock, Haemonchus contortus, to low but increasing doses of ivermectin or moxidectin in vivo for phenotypic, genomic, and transcriptomic analyses. After a single subtherapeutic dose of ivermectin or moxidectin, we find evidence of selection at a shared genetic locus on Chromosome V, with the signal of selection increasing with subsequent doses. After only three subtherapeutic treatments, ivermectin-selected lines were resistant to a full standard (label) dose of ivermectin. However, moxidectin selected lines remained susceptible to a half dose of moxidectin. This was despite showing higher resistance to ivermectin in vitro and a stronger signal of selection at the Chromosome V locus than the equivalent ivermectin-selected lines. Our findings highlight the rapid selection for anthelmintic resistance with subtherapeutic treatment and implicate the pre-existence of ivermectin and moxidectin resistance haplotypes in a drug-naïve population. We demonstrate that ivermectin selected lines show emerging moxidectin resistance, underpinned by a shared genetic locus of resistance. Finally, we speculate that key differences in the resistance phenotype between ivermectin and moxidectin selected lines relate to differences in the inheritance of resistance within this shared locus, with ivermectin resistance manifesting as dominant trait while moxidectin resistance appears to be recessive.