Larval development assay for detection of anthelmintic resistance in cyathostomins of Swedish horses Eva Osterman Lind * , Arvid Uggla, Peter Waller, Johan Ho ¨glund Department of Parasitology (SWEPAR), Swedish University of Agricultural Sciences and National Veterinary Institute, SE-751 89 Uppsala, Sweden Received 10 June 2004; received in revised form 4 November 2004; accepted 23 November 2004 Abstract The aim of this study was to investigate the suitability of a larval development assay (LDA) for the determination of anthelmintic resistance in cyathostomin nematode populations of the horse. In addition, comparison of results between geographic regions, types of horse establishment, and the use of anthelmintics in Sweden, was established. Seventy horse herds from different parts of Sweden were sampled, and strongyle eggs from the faeces of 54 of those were investigated by an LDA (DrenchRite TM ). The following anthelmintics were tested: thiabendazole (TBZ), levamisole (LEV), ivermectin monosaccharide (IVM-MS), ivermectin aglycone (IVM-AG) and pyrantel (PYR). The LC50 values for TBZ and LEV were generally lower than those previously reported in other LDA studies on horse nematodes. This could be related to the infrequent use of these compounds for the past 20 years in Sweden. In this study, there was a great variation within and between assay plates that could not be explained. Still the LC50 values differed significantly between the regions for all anthelmintics, except for pyrantel. The highest LC50s were observed in parasite populations from the south of Sweden. There were no significant differences between riding schools and studs. Limitations of this technique exist, namely the lack of established cut-off values for susceptible and resistant populations and interpretation problems related to multi-species infections. Although there are advantages with LDA such as the possibility of testing several compounds simultaneously without interference with the deworming programmes on the farms, we conclude that LDA currently is not a reliable alternative to the faecal egg count reduction test (FECRT). # 2004 Elsevier B.V. All rights reserved. Keywords: Horse; Cyathostomins; Anthelmintic Resistance; Larval development assay; Sweden 1. Introduction Anthelmintic resistance in populations of cyathos- tomin nematodes of the horse is a well-known and widespread problem (Kelly et al., 1981; Uhlinger and Johnstone, 1985; Bauer et al., 1986; Nilsson et al., 1989; MacG. King et al., 1990; Bjørn et al., 1991; Boersema et al., 1991; Fisher et al., 1992; Ihler and Bjørn, 1996; Borgsteede et al., 1997; Craven et al., 1998). Until the end of the 1990s, resistance in horse parasites was restricted to benzimidazole anthelmintic compounds. However, reports from the USA and www.elsevier.com/locate/vetpar Veterinary Parasitology 128 (2005) 261–269 * Corresponding author. Tel.: +46 18 674037; fax: +46 18 309162. E-mail address: eva.osterman.lind@bvf.slu.se (E.O. Lind). 0304-4017/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.vetpar.2004.11.029