Abstract
Haemonchus populations were collected from cattle from mid-western and eastern Southern US (four and six populations, respectively) to determine the relative prevalence of Haemonchus contortus and Haemonchus placei and the frequency of the three isotype-1 β-tubulin polymorphisms associated with benzimidazole resistance. A minimum of 32 individual adult worms were genotyped at position 24 of the rDNA ITS-2 for each population to determine species identity (296 worms in total). One population from Georgia was identified as 100% H. contortus with the remaining nine populations identified as 100% H. placei. For the H. contortus population, 29 out of 32 worms carried the P200Y (TAC) isotype-1 β-tubulin and 2 out of 32 worms carried the P167Y (TAC) benzimidazole resistance associated polymorphisms respectively. For H. placei, six out of the nine populations contained the P200Y (TAC) isotype-1 β-tubulin benzimidazole resistance associated polymorphism at low frequency (between 1.6% and 9.4%) with no resistance associated polymorphisms being identified at the P198 and P167 codons. This is the first report of the P200Y (TAC) isotype-1 β-tubulin benzimidazole resistance associated polymorphism in H. placei. The presence of this mutation in multiple independent H. placei populations indicates the risk of resistance emerging in this parasite should benzimidazoles be intensively used for parasite control in US cattle.