Review Articles An update on cyathostomins: Anthelmintic resistance and diagnostic tools C. H. STRATFORD*, B. C. McGORUM, K. J. PICKLES † and J. B. MATTHEWS ‡ Department of Veterinary Clinical Studies, Royal (Dick) School of Veterinary Studies and Roslin Institute, University of Edinburgh, UK; † Department of Veterinary Medicine and Epidemiology, University of California, USA; and ‡ Division of Parasitology, Moredun Research Institute, UK. Keywords: horse; cyathostomin; anthelmintic resistance; diagnostic tools; sensitivity testing Summary Cyathostomins represent a potential cause of equine morbidity and have become the main focus of endoparasite control in managed horses. All grazing horses are at risk of infection with cyathostomins; therefore, the application of appropriate management measures is essential. Anthelmintics currently comprise the main method of control for equine nematodes and the ready availability of these products in some countries has resulted in their use becoming dissociated from veterinary involvement. This is concerning given the levels of anthelmintic resistance that have been recorded in cyathostomin populations. It is important that veterinarians re-establish control over the implementation of parasite control programmes, a major objective of which should be the preservation of anthelmintic efficacy. This article details the principles of cyathostomin control in horses with particular reference to anthelmintic resistance, and the use and interpretation of diagnostic tests for detecting cyathostomins and identifying anthelmintic resistance. Introduction Virtually all grazing horses are exposed to cyathostomins. Although over 50 species have been described, most horses are infected with 5–10 common species (Ogbourne 1976; Reinemeyer et al. 1984). Infection with cyathostomins may result in wide ranging clinical signs including vague malaise, colic, weight loss and anorexia. Furthermore, ‘larval cyathostominosis’, characterised by rapid weight loss, oedema and diarrhoea, may result in death in up to 50% of the cases that present with this syndrome (Uhlinger 1990; Hillyer and Mair 1997; Love et al. 1999; Lyons et al. 2000). It should be emphasised, however, that severe clinical disease resulting from cyathostomins is uncommon in the horse population at large and that the perceived incidence of larval cyathostominosis does appear to vary geographically. As most cyathostomin burdens in horses are low and rarely result in clinical disease, recommendations for targeted anthelmintic dosing should be instituted to reduce infection load. Cyathostomins have a direct, nonmigratory life cycle. Third stage larvae (L3) ingested from pasture penetrate the wall of the large intestine, where they undergo development before re-emerging and maturing to adults within the lumen. Female adult worms release eggs that are shed in faeces. These hatch to release first stage larvae, which undergo 2 moults to become infective L3. In some instances, L3 enter a state of inhibited development as early L3 (EL3) in the host for months to years (Murphy and Love 1997). It is not known why larvae become inhibited, although cold conditioning of L3 prior to ingestion, gradual accumulation of L3 infection, host immunity and population density of the parasites within the intestinal wall and lumen have been proposed as contributory factors (Love et al. 1999; Matthews 2008). Anthelmintic treatment that primarily targets luminal stages may serve as a stimulus for mucosal emergence by reducing the luminal nematode population. During autumn and winter, EL3 and other mucosal stages constitute the major cyathostomin burden. Control of cyathostomins is complicated because horses acquire variable, incomplete immunity, requiring some therapeutic intervention throughout their lives (Matthews 2008). Furthermore, faecal egg counts (FECs) underestimate the true parasite burden when larval populations predominate and luminal adult burdens are low (Dowdall et al. 2002). Finally, evidence obtained thus far indicates that EL3 stages are not particularly susceptible to most anthelmintics (Love and McKeand 1997). Anthelmintics Three classes of anthelmintic are licensed for the control and treatment of nematodes in horses; benzimidazoles (BZs), tetrahydropyrimidines (THPs) and macrocyclic lactones (avermectins and milbemycins, MLs). With no new anthelmintic classes expected to be available for use in horses in the short to medium term, current anthelmintics must be used appropriately to preserve their efficacy. Original controlled efficacy trials revealed noteworthy differences in the activities of the various anthelmintic classes against different cyathostomin life cycle stages in drug- sensitive populations. For example, a single dose of fenbendazole (FBZ) (5 mg/kg bwt) was shown to exhibit 100% efficacy against *Corresponding author email: claire.stratford@ed.ac.uk [Paper received for publication 14.01.11; Accepted 14.03.11] EQUINE VETERINARY JOURNAL 133 Equine vet. J. (2011) 43 (Suppl. 39) 133-139 doi: 10.1111/j.2042-3306.2011.00397.x © 2011 EVJ Ltd