Similar patterns of benzimidazole resistance alleles in ovine gastrointestinal nematodes from Western Canada and Eastern United States supports their shared origins and subsequent spread.

Livestock movement facilitates translocation of anthelmintic resistant parasites, but the extent to which resistance emergence depends on animal movement is still poorly understood. Benzimidazole resistance is widespread in ovine trichostrongylid nematodes, and our understanding of its molecular basis now allows for molecular epidemiology investigations. This study applies deep amplicon sequencing of the isotype-1 β-tubulin locus to compare the prevalence and frequency of benzimidazole resistance Single Nucleotide Polymorphisms (SNPs), and their alleles, for trichostrongylid populations from 102 Western Canadian and 28 Eastern USA sheep flocks. For H. contortus, benzimidazole resistance SNPs were at fixation tin almost all flocks from both regions; that is, present at, or close to, 100 % frequency. For T. circumcincta and T. colubriformis, although at fixation in most Eastern USA flocks, resistance SNPs they were at a much lower prevalence in Western Canada, consistent with the lower anthelmintic use and selection pressure. The benzimidazole resistance SNP profiles were identical across these regions: F200Y (TTC > TAC) predominated for all three species in both regions, but there were differences between the species at codons 167 and 198. For H. contortus, F167Y (TTC > TAC) was at moderate prevalence but no codon 198 resistance SNPs occurred in either region. For T. circumcincta, E198A (GAA > GCA) was at low prevalence and for T. colubriformis, F200Y (TTC > TAC) was the only resistance SNP detected in both regions. Analysis of diversity and distribution of Amplicon Sequence Variants (ASVs) carrying resistance SNPs revealed that, in all three species, the same major resistance alleles were present in both regions at very similar relative frequencies. These results are consistent with a model of benzimidazole resistant ovine gastrointestinal nematodes (GIN) spreading across North America from common origins facilitated by animal movement. This model emphasizes the importance of biosecurity in limiting the emergence and spread of anthelmintic resistance in ruminant GIN. Keywords: molecular epidemiology, deep amplicon sequencing, anthelmintic resistance, nemabiome, benzimidazoles.