Abstract
Anthelmintic resistance has been reported in most sheep producing countries. Prior to the mid 1990s, reports of anthelmintic resistance in Ireland were sparse and focused on benzimidazole, one of the three classes of anthelmintic available during this period. This evidence for efficacy issues on Irish farms combined with awareness that anthelmintic resistance was increasingly being reported in other countries prompted the need for more comprehensive investigations on Irish farms. Faecal egg count reduction and micro-agar larval development tests were employed to investigate resistance to benzimidazole, levamisole and macrocyclic lactone. There is compelling evidence for resistance to both benzimidazole (>88% of flocks) and levamisole (>39% of flocks). Resistance of nematode populations to macrocyclic lactone was suspected on a small number of farms (11%) but needs to be confirmed. The recent introduction of two new classes of anthelmintics, after over a 25 year interval, together with the evidence that anthelmintic resistance is reported within a relatively short time following the introduction of a new anthelmintic compound means that the challenge to the industry is immediate. Actions are urgently required to manage anthelmintic resistance so as to prolong the lifespan of anthelmintics.
Highlights
The effective management of the effects of gastrointestinal nematode parasites on grazing livestock relies heavily on the use of highly efficacious chemotherapy
Three and 4 farms were excluded from the LM and BZ faecal egg count reduction test (FECRT) studies, respectively, because the average pretreatment faecal egg count (FEC) was below 250 eggs per gram [14]
Eighteen farms were suitable for inclusion in the DrenchRiteW Assay; the results from this test (Figure 2) indicated that susceptibility to BZ and LM in nematode populations was observed on 39% and 72% of farms
Summary
The effective management of the effects of gastrointestinal nematode parasites on grazing livestock relies heavily on the use of highly efficacious chemotherapy. With the recent introduction of two new classes of anthelmintic (amino-acetonitrile derivatives (AADs) in 2010 and spiroindole (SI) in 2012) to the Irish market, there are five anthelmintic classes available for the control of gastrointestinal nematodes of sheep. Anthelmintic resistance (AR) has been identified in most sheep producing countries worldwide [1]. The evidence for nematode resistance to anthelmintics, in particular to the benzimidazoles, is overwhelming. There are increasing reports of multi-drug resistant nematodes [2]. The first evidence for AR in nematode populations on Irish sheep farms was reported in the early 1990s [3,4,5]
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