Phenotyping and Genotyping of Haemonchus Contortus Isolates Reveals a New Putative Candidate Mutation for Benzimidazole Resistance in Nematodes

In order to monitor and eventually control the spread of drug-resistant Haemonchus contortus, knowledge of the molecular mechanisms underlying resistance is essential. Here we phenotypically and genotypically characterize three multidrug-resistant H. contortus field isolates from Australia and South Africa. All were significantly less susceptible to thiabendazole than a sensitive reference strain in an in vitro egg-hatch assay. The sensitivity was further reduced in a surviving population after treatment of infected sheep with albendazole. The beta-tubulin genes were amplified from genomic DNA of the H. contortus isolates, cloned, and sequenced. There was a high degree of sequence variation. The known mutation phenylalanine-200 to tyrosine (F200Y) occurred in 60% of the sequences from resistant isolates, but not in the sensitive reference. Interestingly, 90% of the beta-tubulin sequences from resistant isolates lacking tyrosine-200 carried another mutation nearby, glutamate-198 to alanine (E198A). This mutation was not found in the sensitive isolate, nor in sequences from resistant isolates carrying the mutation F200Y. However, the mutation E198A is known from benomyl-resistant isolates of phytopathogenic fungi such as Monilinia fructicola. The finding that alanine-198 correlates with thiabendazole resistance in H. contortus isolates from South Africa and Australia suggests that also in nematodes, the mutation E198A plays a role in benzimidazole resistance. Alanine-198 alleles of beta-tubulin can be detected by PCR-RFLP and we suggest to include this test in future surveys of H. contortus field populations.