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[dagger]. These authors contributed equally to this work.
INTRODUCTION
Parasitic nematodes of the group Cyathostominae are a potential cause of serious disease in equids. These parasites have a high prevalence and most horses that graze encounter them (Matthews, 2008). Cyathostomins exist as a group of around 50 species (Lichtenfels et al. 2008); however, most infections comprise 5-10 common species, with low numbers of rarer species present (Chapman et al. 2002a ). Species compositions are similar across regions, with the same group appearing as the most prevalent globally (Ogbourne, 1976; Reinemeyer et al. 1984; Mfitilodze and Hutchinson, 1990; Bucknell et al. 1995; Gawor, 1995; Lichtenfels et al. 2001; Collobert-Laugier et al. 2002). For all species, the life cycle involves larval encystment in the wall of the large intestine and larvae can persist for up to 2 years (Gibson, 1953; Smith, 1976; Murphy and Love, 1997). In some horses, encysted larvae can constitute up to 90% of the total burden, with some individuals harbouring millions of worms (Dowdall et al. 2002). Encysted larvae are important in the pathogenesis of cyathostomin infections, because these stages can emerge in great numbers to cause larval cyathostominosis, a colitis syndrome that can be fatal in up to 50% of cases (Giles et al. 1985; Love et al. 1992). Larval cyathostominosis is most commonly observed in horses of 2-5 years of age (Reid et al. 1995); however, animals can have a lifelong susceptibility to infection and disease has been observed in horses of all ages (Mair, 1993). The potential clinical effects of cyathostomins are confounded by high levels of anthelmintic resistance in these nematodes, with resistance to benzimidazole and pyrantel compounds almost ubiquitous in some regions (Kaplan, 2002; Matthews, 2014). The macrocyclic lactones are the most commonly used anthelmintics in horses and reduced efficacy against cyathostomins has been reported for ivermectin in Brazil (Canever et al. 2013), with several studies reporting shortened strongyle egg reappearances period following ivermectin (Geurden et al. 2014; Relf et al. 2014) and moxidectin (Rossano et al. 2010; Relf et al. 2014) treatment. A shortened helminth egg reappearance period is regarded as an early indicator of resistance (Sangster, 1999), highlighting the threat...