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.