Genetic basis of benzimidazole resistance in Teladorsagia circumcincta in Ireland

Jason D Keegan,Theo De Waal,Barbara Good,Orla M Keane,June Fanning

doi:10.1186/s13620-017-0087-8

Abstract

Resistance to benzimidazole (BZ) anthelmintics is common in ovine nematodes of economic importance. Single nucleotide polymorphisms (SNP) at three positions in the isotype 1 β– tubulin gene have been associated with BZ resistance and molecular tests for the detection of BZ resistance have been developed. In order to determine if such tests are practicable in Ireland the polymorphisms associated with BZ resistance must be identified. To this end, BZ-resistant nematodes were recovered from four farms in Ireland. Resistant Teladorsagia circumcincta, Cooperia curticei and Trichostrongylus colubriformis were recovered, with resistant T. circumcincta the most common and the only species studied further. Sequencing of the isotype 1 β–tubulin gene from resistant T. circumcincta identified a T - A transition, resulting in an F200Y substitution known to be responsible for BZ-resistance, on three of the farms. However, on the fourth farm the frequency of the resistant A allele was only 0.33 indicating another BZ resistance mechanism may be present on this farm. An additional polymorphism resulting in a substitution of glutamate for leucine (E198L) was also found on this farm at low frequency (0.17). No polymorphisms at position 167 were identified on any farm. Therefore, molecular tests to detect BZ resistance in T. circumcincta in Ireland could prove useful; however, they may result in some instances of resistance remaining undetected.

Highlights

Anthelmintic drugs are heavily relied upon to control the effects of parasitic helminths in sheep, but their overuse has led to the development of anthelmintic resistance (AR)
Three non-synonymous single nucleotide polymorphisms (SNP) in the isotype 1 β-tubulin gene have been associated with BZ resistance in a variety of ovine gastrointestinal nematode (GIN) species
The most commonly reported indicator of BZ resistance in ovine nematodes is a T to A transition in the isotype 1 β-tubulin gene resulting in an F200Y substitution in the encoded protein

Summary

Introduction

Anthelmintic drugs are heavily relied upon to control the effects of parasitic helminths in sheep, but their overuse has led to the development of anthelmintic resistance (AR). Resistance to benzimidazole (BZ) on Irish sheep farms has been reported [1,2,3] but the prevalence may be underestimated, given the insensitivity of diagnosing AR based on faecal egg count [4]. In BZsusceptible nematodes, BZ binds to tubulin disrupting the formation of microtubules, resulting in a reduction in glucose uptake and protein secretion, which leads to starvation and death. An amino acid change in the tubulin protein prevents BZ from binding [5]. Three non-synonymous single nucleotide polymorphisms (SNP) in the isotype 1 β-tubulin gene have been associated with BZ resistance in a variety of ovine gastrointestinal nematode (GIN) species. The polymorphisms responsible for BZ resistance in Irish ovine nematodes remain unknown

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