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Anthelmintic resistance (AR) in gastrointestinal nematodes (GINs) has been reported worldwide in multiple nematode and livestock species (Kaplan and Vidyashankar 2012) and is a major constraint on production on affected farms (Sutherland and others 2010, Miller and others 2012). In the UK and Ireland, for example, AR in GINs and anthelmintic treatment failure is widespread in sheep (e.g. Bartley and others 2003, Keane and others 2014) and increasingly reported in cattle (e.g. O'Shaughnessy and others 2014). There is, therefore, a need to develop and adopt GIN control strategies that maintain the efficacy of anthelmintics and to identify risk factors for the development of AR.
Environmental constraints on farm management and the survival of nematodes in refugia appear to play an important role in the development of AR. In a random survey of sheep farms in Norway, AR was found only in coastal regions (Domke and others 2012a). Papadopoulos and others (2001) observed a higher incidence of AR on isolated Greek islands, suggesting that drought hastens the development of AR. In contrast, Rinaldi and others (2014) observed high anthelmintic efficacy in sheep in southern Italy despite the Mediterranean climate. This was attributed to the low number of anthelmintic treatments (usually two per year) and the absence of anthelmintic treatments during periods of drought, when environmental constraints on the free-living stages are highest. Calvete and others (2012) identified an association between AR, distance between farms with AR, management and bioclimatic …