ASSOCIATION BETWEEN MACROCYCLIC LACTONE AND BENZIMIDAZOLE RESISTANCE IN THE DOG HOOKWORM ANCYLOSTOMA CANINUM

Anthelmintic resistance is a growing threat to the treatment and control of parasitic nematode infections in both humans and animals. In the canine hookworm, Ancylostoma caninum, multi-anthelmintic drug-resistant (MADR) isolates have been identified, but the molecular mechanisms underlying resistance and the potential for these mechanisms to promote cross-resistance between drug classes remain unclear. In this study, we report evidence for a previously uncharacterized association between resistance to macrocyclic lactones (MLs) and benzimidazoles (BZs) in A. caninum. In a wild-type isolate with no history of drug exposure, serial passaging without treatment led to an unexpected rise in F167Y allele frequency, accompanied by increased resistance to ivermectin (ML) but not to thiabendazole (BZ). Separately, 3 rounds of combination treatment of a triple-resistant isolate with moxidectin (ML) and pyrantel (tetrahydropyrimidine) led to a substantial increase in the frequency of the β-tubulin F167Y mutant allele, despite the absence of BZ selection pressure. These findings suggest that ML resistance in A. caninum may be associated with canonical BZ resistance mutations. Furthermore, the F167Y mutation does not appear to confer any negative fitness effects; instead, the mutation correlated with increased egg output and prolonged infection. This surprising observation raises concerns about the appearance, persistence, and spread of resistance alleles in populations not exposed to drugs. These results have critical implications for resistance management strategies and highlight the urgent need for surveillance tools capable of detecting early cross-resistance in hookworm populations.