Veterinary Research Communications, 31 (2007) 53–65 DOI: 10.1007/s11259-006-3402-5 C  Springer 2007 A Field Study on the Effect of Some Anthelmintics on Cyathostomins of Horses in Sweden E. Osterman Lind 1,∗ , T. Kuzmina 2 , A. Uggla 1 , P.J. Waller 1 and J. H ¨ oglund 1 1 Department of Parasitology (SWEPAR), Swedish University of Agricultural Sciences and National Veterinary Institute, Uppsala, Sweden; 2 Department of Parasitology I.I., Schmalhausen Institute of Zoology, Kiev, Ukraine ∗ Correspondance: E-mail: eva.osterman-lind@sva.se ABSTRACT The objective of the study was to investigate different aspects on the efficacy of three anthelmintics on cyathostomin nematodes of Swedish horses. A faecal egg count reduction (FECR) test was performed on 26 farms. Horses were treated orally with recommended doses of ivermectin, pyrantel pamoate or fenbendazole. Faecal samples were collected on the day of deworming and 7, 14 and 21 days later. No resistance was shown against ivermectin; the FECR was constantly >99%. The effect of pyrantel was assessed as equivocal in 6 farms 14 days after treatment; the mean FECR was 99%. As many as 72% of the fenbendazole-treated groups met the criteria for resistance; the mean FECR was 86%, ranging from 56% to 100%. A re-investigation of two farms where pyrantel resistance had been suspected clearly revealed unsatisfactory efficacy of pyrantel on one of these farms; the FECR varied from 72% to 89%. Twenty-six of the horses previously dosed with pyrantel or fenbendazole, and which still excreted ≥150 eggs per gram of faeces 14 days after treatment, were dewormed with ivermectin and fenbendazole or pyrantel in order to eliminate the remaining cyathostomins. A total of 13 cyathostomin species were identified from horses that initially received fenbendazole and seven species were identified from pyrantel-treated individuals. The egg reappearance period (ERP) following treatment with ivermectin and pyrantel was investigated on two farms. The shortest ERP after ivermectin treatment was 8 weeks and after pyrantel was 5 weeks. We conclude that no substantial reversion to benzimidazole susceptibility had taken place, although these drugs have scarcely been used (<5%) in horses for the last 10 years. Pyrantel-resistant populations of cyathostomins are present on Swedish horse farms, but the overall efficacy of pyrantel is still acceptable. Keywords: anthelmintic resistance, cyathostomins, egg reappearance period (ERP), faecal egg count reduction test (FECRT), horse Abbreviations: ECR, egg count ratio; FECRT, faecal egg count reduction test; FEC, faecal egg count; epg, eggs per gram of faeces; ERP, egg reappearance period; LC, the lower 95% confidence limit INTRODUCTION Strongyle nematodes of the tribe Cyathostominea, commonly called cyathostomins, par- asitize the large intestine of virtually all horses. Although more than 50 species of cy- athostomins have been described in equids, only six were found to represent more than 90% of the cyathostomin population in Swedish horses (Osterman Lind et al., 2003). Clinically, cyathostomins are considered the principal helminth pathogen of the horse (Love et al., 1999) and accordingly extensive resources are directed towards their con- trol. On Swedish horse farms, endoparasite control is usually based on drug treatment alone, but in some establishments such treatment is combined with some form of grazing management. 53