Importance of the mutation of amino acid 200 of the isotype 1 beta-tubulin gene in the benzimidazole resistance of the small-ruminant parasite Teladorsagia circumcincta.

In this work we demonstrated that the acquisition of benzimidazole (BZ) resistance in the small-ruminant parasite Teladorsagia circumcincta is linked to the selection of individuals that are characterized by a tyrosine (Tyr) at amino acid 200 of their isotype 1 beta-tubulin gene. This mutation appears to be recessive, since only homozygous mutant (Tyr/Tyr) individuals survived after BZ treatment of two resistant populations in which the three genotypes (rr, rs, ss) were initially present. In comparison with natural BZ-susceptible populations, a decrease in the restriction polymorphism (RFLP) of the isotype 1 beta-tubulin gene was observed in natural resistant populations. It seems that this decrease in beta-tubulin polymorphism results from the selection of homozygous mutant individuals.