Molecular and phenotypic characterisation of fenbendazole resistance in a field-derived isolate of Ostertagia ostertagi.

The prevalence of anthelmintic resistance in the bovine nematode Cooperia oncophora has been well documented globally but lack of efficacy against the more pathogenic nematode species Ostertagia ostertagi is less common. The sensitivity of an O. ostertagi isolate to the benzimidazole class of anthelmintic was investigated using classical parasitological techniques following apparent clinical failure of controlled release fenbendazole capsule administration in first season grazers at pasture. A controlled efficacy test (CET) was conducted in conjunction with sequencing of the β-tubulin isotype 1 gene of larvae pre- and post-fenbendazole administration. Twelve helminth-naïve calves were infected experimentally with 20,000 third stage larvae; six received oral fenbendazole (7.5 mg/kg bodyweight) 28 days post infection. Total abomasal nematode burdens were compared between treatment and control groups to determine efficacy. Fenbendazole resistance in O. ostertagi was confirmed with a total treatment failure in reducing worm burden: efficacy of 0%. Sequence analysis of the β-tubulin isotype-1 gene from forty-five infective larvae from both control and treated groups was performed. The three commonest single nucleotide polymorphisms (SNPs) associated with benzimidazole resistance, namely F167Y, E198A and F200Y, were examined. The predominant resistance-associated SNPs were F200Y (78 % control and 79 % treated groups) and F167Y (remaining genotypes) and emphasises the importance of these SNPs in clinical disease in this isolate. The development of diagnostic molecular tools based on a characterised field-derived isolate of benzimidazole-resistant Ostertagia will enable future prevalence surveys to be undertaken to assess the possible risk posed by resistance in this economically important species.