Abstract
It is important to understand how anthelmintic drug resistance mutations arise and spread in order to determine appropriate mitigation strategies. We hypothesised that a molecular genetic study of Haemonchus contortus in southern India, a region where resistance may be less advanced than in western Europe and North America, might provide some important insights into the origin and spread of anthelmintic resistance. The F200Y (TAC) isotype-1 β-tubulin benzimidazole resistance mutation is common in H. contortus throughout the world and the F167Y (TAC) and E198A (GCA) mutations, although less common, have been reported in a number of different countries. We have investigated the haplotypic diversity and phylogenetic relationship of isotype-1 β-tubulin benzimidazole resistance alleles for 23 H. contortus populations from small ruminants across southern India. The F200Y (TAC) mutation was most common, being detected in 18/23 populations at frequencies between 9% and 84% and the E198A (GCA) mutation was also detected in 8/23 populations at frequencies between 8% and 18%. The F167Y (TAC) mutation was not detected in any of the 23 populations. Phylogenetic haplotype network analysis suggested that the F200Y (TAC) mutation has arisen multiple independent times in the region with at least three independent origins of resistance alleles across the populations surveyed. In contrast, the E198A (GCA) mutation was present on a single haplotype which, given the high level of haplotypic diversity of the susceptible alleles in the region, suggests this particular mutation has spread from a single origin, likely by anthropogenic animal movement. Population genetic analysis of 12 of the H. contortus populations, using a panel of eight microsatellite markers, revealed extremely low genetic differentiation between populations, consistent with the hypothesis of high gene flow among sites. Additionally, there was no significant genetic differentiation between H. contortus taken from sheep and goats which is consistent with H. contortus populations being freely shared between these two different hosts. Overall, we believe these results provide the first clear genetic evidence for the spread of an anthelmintic resistance mutation to multiple different locations from a single origin.