Internal parasite control in horses has undergone a revolution in the past two decades. The shift from calendar-based anthelmintic dosing to targeted selective treatment (TST) based on faecal egg count monitoring has improved both horse welfare and responsible anthelmintic stewardship. Understanding current best practice is essential for horse owners and vets.
Key Parasites of Horses
Large strongyles (Strongylus vulgaris): Historically the most dangerous equine parasite — larvae migrate through intestinal arteries, causing arteritis and colic. Ivermectin control has made clinical large strongyle disease rare, though resistance monitoring is increasingly important.
Cyathostomins (small strongyles): The most clinically significant current challenge — over 50 species of cyathostomins inhabit the large intestine. Encysted larvae can emerge simultaneously causing cyathostominosis — acute, often fatal colitis with bloody diarrhoea and profound weight loss. Winter treatment with moxidectin is the primary approach for reducing larval burden.
Tapeworms (Anoplocephala perfoliata): Associated with spasmodic and ileocaecal colic; detected by specific ELISA blood test or faecal antigen test; treated with double-dose pyrantel or praziquantel-containing products.
Bots (Gasterophilus spp.): Larvae develop in the stomach; moxidectin or ivermectin treatment in late autumn when adult flies have died eliminates bot larvae before spring hatching.
Targeted Selective Treatment (TST)
TST recognises that most horses have low parasite burdens and do not require frequent anthelmintic treatment. Approximately 20% of horses contribute 80% of pasture contamination:
- Faecal egg count (FEC) testing every 3 months identifies high excretors (>500 eggs per gram requiring treatment)
- Low excretors (<200 eggs per gram) are monitored without treatment
- Post-treatment FEC at 14 days confirms anthelmintic efficacy — detecting resistance early
Anthelmintic Resistance
Resistance to ivermectin (cyathostomins) and to pyrantel is increasingly documented globally. Benzimidazole (fenbendazole) resistance is widespread — many practitioners consider it largely ineffective for cyathostomins without pre-testing. Moxidectin remains the most effective option but resistance monitoring is essential to preserve its efficacy. TST, strategic treatment timing, and faecal egg count reduction testing are the cornerstones of resistance management.
Pasture Management
Regular faecal removal (twice weekly) reduces pasture larval contamination substantially — particularly important in small paddocks. Resting pasture for 3 months (hot, dry conditions) allows larval die-off. Mixed grazing with cattle or sheep removes equine strongyle larvae while not establishing new equine parasites.