Internal parasites cause significant welfare compromise in horses; equally, unnecessary anthelmintic treatment drives resistance development that will ultimately reduce the tools available for horse welfare protection. Targeted selective treatment balances these concerns.
Encysted small redworms (cyathostomins) are the most prevalent and welfare-significant internal parasites of horses. Mass emergence from encysted larvae in late winter/spring causes cyathostominosis—severe diarrhoea, weight loss, weakness, and death in extreme cases. Large redworms (Strongylus vulgaris) cause arterial damage; bots (Gasterophilus spp.) irritate the gastric mucosa; Parascaris equorum causes serious disease in foals; and tapeworms are associated with ileal impaction colic. Each parasite requires different treatment strategies.
Faecal egg count (FEC) monitoring provides objective assessment of individual horse worm burdens—replacing calendar-based blanket treatment protocols. Horses are categorised as low, medium, or high egg shedders; treatment is targeted to medium and high shedders while low shedders remain untreated. This "targeted selective treatment" (TST) approach dramatically reduces anthelmintic use, slowing resistance development, while maintaining welfare protection for genuinely high-burden horses.
Autumn treatment with moxidectin (the only drug with significant efficacy against encysted cyathostomins) addresses the main welfare-significant threat from encysted larvae. Tapeworm treatment (praziquantel or double-dose pyrantel) twice yearly manages tapeworm burden. Foal and youngstock treatment protocols differ from adult horses, reflecting higher parasite vulnerability and different parasite species prevalence. Integrated pasture management—removing droppings, avoiding overstocking—reduces pasture contamination alongside treatment programmes.
Anthelmintic resistance in cyathostomins is now widespread across all three main drug classes (benzimidazoles, pyrimidines, macrocyclic lactones). Preserving drug efficacy through TST, avoiding unnecessary treatments, and periodic efficacy testing (faecal egg count reduction tests) protects the tools needed for horse welfare management into the future. Farm-level resistance monitoring through veterinary parasitology services guides drug class rotation and efficacy assessment.