Introduction
Grazing horses are constantly exposed to parasites, with the cyathostomins (“small redworms”) (Strongylida: Cyathostominae) being the commonest. These parasitic nematodes encompass >40 species in 14 genera, but coinfections with 15–25 species are most common (Bellaw and Nielsen 2020). Current recommendations for cyathostomin control are aimed at reducing the risk of pasture contamination and minimising the chances of disease caused by the parasites (Matthews 2000 and 2011) using strategic or targeted treatments based on faecal egg count (FEC) analysis, and assessment of the efficacy of routinely used anthelmintics (Saeed et al 2019). Three classes of anthelmintic are licensed for the treatment of cyathostomin infections in the UK: the benzimidazoles (fenbendazole), the tetrahydropyrimidines (pyrantel salts) and the macrocyclic lactones (ivermectin and moxidectin). Resistance among the cyathostomins to the benzimidazole and tetrahydropyrimidine drugs is now widespread across the world (Kaplan et al 2004; Lester et al 2013; Matthews 2014; Peregrine et al 2014), and there is recent evidence to suggest growing resistance to the macrocyclic lactones (Relf et al , 2014; Nielsen et al 2018; Nielsen et al 2020; Floreset al 2020; Abbas et al 2021; Nielsen et al 2022a), including in the UK (Bull et al 2023). However, reports suggest that in most regions, the macrocyclic lactones currently have considerably higher observed anthelmintic efficacy against strongyle infections than the other two drug classes (Nielsen et al 2018). Only two anthelmintics (fenbendazole administered daily for five consecutive days and moxidectin as a single dose) are licensed for the treatment of cyathostomin mucosal larval stages in the UK. In view of the widespread resistance to the benzimidazole and tetrahydropyrimidine drugs, coupled with the efficacy profiles of the different anthelmintic compounds against mucosal larval stages, routine treatment of all grazing horses with moxidectin (often combined with praziquantel to treat tapeworms) in the autumn/early winter is commonly practised in the UK, but this is likely adding to the selection pressure on the cyathostomins for developing resistance (Rendle 2017).
The egg reappearance period (ERP) is defined as the time between the administration of an effective anthelmintic and the recommencement of shedding of parasite eggs in faeces (Nielsen et al 2019). The determination of ERP following anthelmintic treatment has been suggested as a useful indicator for the early development of anthelmintic resistance in cyathostomins. A number of studies have been published that have assessed ERP following moxidectin treatment (reviewed by Macdonald et al 2023) and, in the past two decades, several of these have reported reduced ERP following treatment with moxidectin, suggesting that resistance to this compound is emerging. In view of the widespread routine use of moxidectin in competition and pleasure horses across all regions of the UK, information about how effective moxidectin is against cyathostomin populations in different regions of the country will be valuable in order to inform local guidelines for parasite control. The aims of this study were to evaluate the ERP following moxidectin in competition and pleasure horses in the southeast of the UK.