Cyathostomine egg reappearance period following ivermectin treatment in a cohort of UK Thoroughbreds.

02 Mar 2018

Background: In spite of the emergence of populations of drug-resistant cyathostomins worldwide, little is known of parasite species responsible for 'early egg shedding' in cohorts of horses subjected to treatment with widely used anthelmintics (e.g. ivermectin [IVM]). In this study, we determined the cyathostomin egg reappearance period (ERP) after IVM treatment of a cohort of yearlings from a large Thoroughbred (TB) stud farm in the United Kingdom, and identified species of IVM-'resistant' cyathostomins using a combination of fundamental parasitology techniques coupled with advanced molecular tools. Methods: Individual faecal samples were collected from TB yearlings with cyathostomin infection prior to IVM treatment, as well as at 2, 14, 21, 28, 35, 42 and 49 days posttreatment. Faecal egg counts (FEC) were performed for each individual sample for determination of ERPs. In addition, individual larval cultures were performed and representative numbers of third stage larvae (L3s) harvested from each culture were subjected to molecular species identification via PCR-Reverse Line Blot (RLB). Results: Prior to IVM treatment, 11 cyathostomin species were detected in faecal samples from TB horses enrolled in this study, i.e. Cyathostomum (Cya.) catinatum, Cylicostephanus (Cys.) longibursatus, Cys. goldi, Cylicocyclus (Cyc.) nassatus, Cys. calicatus, Cya, pateratum, Cyc. radiatus, Paraposteriostomum mettami, Coronocyclus (Cor.) labratus, Cyc. insigne and Cyc. radiatus variant A. Of these, eggs of Cya. catinatum, Cys. longibursatus, Cyc. nassatus and Cyc. radiatus could be detected at 28 days post-treatment, while from day 42 onwards, cyathostomin species composition reflected data obtained pre-IVM treatment, with the exception of eggs of Cor. labratus and Cyc. insigne that could no longer be detected post-IVM administration. Conclusions: This study provides valuable data on the occurrence of IVM-resistance in cyathostomins in the UK. Nevertheless, further investigations are needed to shed light on the prevalence and incidence of drug-resistance in this country as well as other areas of the world where equine trade is substantial.