Abstract
Cyathostomins are ubiquitous in horses and high strongyle burdens are associated with colic. Parasite control, often using anthelmintic drugs, is an important element of equine healthcare. Anthelmintic dosing is also a risk factor for colic, which may be due to disruption of the mutualistic microbiota within the hindgut. This thesis describes studies to evaluate the current efficacy of equine anthelmintics and interactions with intestinal bacterial populations which may underlie the reported association with colic. Faecal egg count data from horses throughout Great Britain was used to calculate strongyle egg reappearance periods (ERP) for ivermectin and moxidectin. For both drugs reduced ERP was detected compared to earlier studies. The ovicidal efficacy of fenbendazole was evaluated in resistant cyathostomins. Using an in vitro egg hatch viability assay, an 80% reduction of egg hatch rate was detected but this only persisted for three days following treatment. These studies further characterise the growing problem of anthelmintic resistance in equine parasites. Microbiota-anthelmintic interaction was evaluated over three studies: i) In vitro feed fermentation was used to detect a decrease in fermentation rate and gas production following moxidectin administration; ii) 16S rRNA sequencing of faecal microbiota before and after moxidectin treatment revealed no change in bacterial community profile; and iii) 1H NMR metabolomics identified no significant differences in urinary and faecal metabolite profiles pre- and post-moxidectin treatment but in-vitro fermentations did show differences in alanine, ethanol, 5-HT, formate and maltose following moxidectin treatment. These studies further characterise reduced efficacy of anthelmintics against equine cyathostomins and suggest that although moxidectin treatment has no measurable effect on bacterial community profile, it may be associated with functional changes in the gut microbiome.