Other Names
Helminthiasis, intestinal parasites, internal parasites. The term “horse worms” encompasses numerous species including strongyles (redworms), roundworms (Parascaris equorum), tapeworms (Anoplocephala perfoliata), small redworms (cyathostomins), pinworms (Oxyuris equi), bots (Gasterophilus spp.), and lungworms (Dictyocaulus arnfieldi).
What are Equine Worm Infestations?
Intestinal parasites have co-existed with horses for millennia and remain one of the most significant health management challenges in equine practice today. All horses on pasture will acquire worm burdens throughout their lives — the question is not whether a horse has worms, but whether the worm burden is high enough to cause clinical disease and whether strategic treatment is needed.
Modern parasite control philosophy in the UK has shifted significantly from the old approach of routine, calendar-based worming every 6–8 weeks. This approach — driven largely by pharmaceutical marketing — contributed to widespread anthelmintic resistance, particularly in small redworms (cyathostomins), which are now resistant to benzimidazoles (e.g. fenbendazole, mebendazole) on many UK premises. The current evidence-based approach focuses on targeted selective treatment based on faecal worm egg counts (FWECs), with less frequent treatment of the majority of horses and more intensive focus on the minority that carry high burdens.
Understanding the different worm species, their life cycles, the damage they cause, and the tools available to monitor and treat them is essential for every horse owner in the UK.
Major Worm Species and Their Effects
Small redworms (cyathostomins): The most clinically significant parasites of adult horses in the UK. Over 50 species exist. They are ingested as larvae on pasture, penetrate the gut wall, and can encyst (become dormant) in the mucosa for months to years. Mass emergence of encysted larvae in late winter and spring causes cyathostominosis — a severe, sometimes fatal condition characterised by sudden weight loss, diarrhoea, ventral oedema, and colic.
Large redworms (Strongylus vulgaris): Historically the most dangerous equine parasite. Larvae migrate through the arteries supplying the gut, causing arterial damage and potentially fatal verminous arteritis. Widespread ivermectin use from the 1980s has made large redworm disease uncommon in the UK, but they must not be overlooked in unmanaged horses.
Roundworms (Parascaris equorum): Primarily a problem in foals and yearlings (adult horses develop immunity). Large worms (up to 30 cm) that can cause respiratory signs during larval migration, intestinal impaction, and rupture in heavily infected foals. Resistance to ivermectin and moxidectin is now documented in roundworm populations worldwide, and it is the most severe resistance problem in equine parasitology: Cai et al.’s meta-analysis of 60 studies covering 11,835 animals (Molecular and Biochemical Parasitology 2024;257:111600, PMID 38030084) found resistance in ascarids significantly more severe than in strongyles, with the macrocyclic lactones the worst-performing class against ascarids at 67.83% pooled efficacy. Pyrantel and fenbendazole resistance in foal ascarids has since been reported as well, so no class should be assumed effective without a reduction test.
Tapeworms (Anoplocephala perfoliata): Transmitted through ingestion of forage mites. Accumulate at the ileocaecal junction and have been strongly associated with spasmodic colic and, in high burdens, ileal impaction. Require praziquantel (or double-dose pyrantel) for treatment — most standard wormers are ineffective.
Pinworms (Oxyuris equi): Not dangerous to health but cause intense tail rubbing due to females depositing eggs around the anus. Frequently misdiagnosed as sweet itch. Eggs are detected by pressing sticky tape to the perineum rather than standard FWECs.
Bots (Gasterophilus spp.): Larvae of the bot fly. Yellow eggs laid on the horse’s coat are ingested during grooming; larvae attach to the stomach lining causing mild gastric irritation. Treated with ivermectin or moxidectin in autumn after the first frost.
Lungworms (Dictyocaulus arnfieldi): Donkeys are the primary host and often show no signs. Horses grazing with donkeys may develop a persistent cough from larval migration through the lungs. Ivermectin is effective.
Symptoms and Signs
Many horses with moderate worm burdens show no clinical signs — subclinical infection is common. When signs do occur, they vary by species and burden:
- Poor body condition, weight loss, and failure to maintain condition despite adequate feeding
- Dull, staring coat lacking bloom
- Pot-bellied appearance, particularly in foals and young horses with roundworm burdens
- Colic — recurring mild spasmodic colic can indicate tapeworm burden; severe colic in young horses may indicate roundworm impaction
- Diarrhoea — particularly in cyathostominosis; may be acute, profuse, and life-threatening
- Ventral oedema (fluid swelling under the belly) in severe cyathostominosis
- Tail rubbing — classic sign of pinworm; also consider sweet itch and other skin conditions
- Respiratory signs (cough, nasal discharge) in foals with migrating roundworm larvae or horses with lungworm
- Anaemia and lethargy in severe cases
Diagnosis
The cornerstone of modern parasite monitoring is the faecal worm egg count (FWEC). A fresh faecal sample is processed to count the number of strongyle eggs per gram (epg). Thresholds used by most UK vets:
- Under 200 epg: Low burden — treatment not recommended
- 200–500 epg: Moderate burden — treatment may be warranted depending on clinical signs and risk factors
- Over 500 epg: High burden — treatment recommended
Important caveats: FWECs detect adult egg-laying worms but do not detect encysted cyathostomin larvae — a horse can have a very high encysted larval burden yet show a low egg count. Tests for tapeworm require either the EquiSal Tapeworm Saliva Test or blood-based serology; standard FWECs are unreliable for tapeworm detection.
A post-treatment FWEC (taken 2 weeks after treatment) assesses treatment efficacy. If the egg count has not reduced by 95%, resistance to the product used is likely — discuss alternatives with your vet.
For broader context on equine gut health, see our entry on equine asthma and related inflammatory conditions.
Treatment
Treatment should be guided by FWEC results and veterinary advice rather than calendar dates. Available drug classes in the UK:
Macrocyclic lactones (ivermectin, moxidectin): Broad-spectrum efficacy against most adult worm species. Moxidectin is active against late-stage encysted cyathostomin larvae at standard doses. Both are effective against bots. Moxidectin has a 13-week persistent activity; ivermectin has 6–8 weeks. Use in horses under 4 years with caution as safety margins differ.
Benzimidazoles (fenbendazole, mebendazole): Used for treatment of encysted larvae when administered at a 5-day elevated dose course (fenbendazole larvicidal dose protocol). Widespread resistance in adult small redworms makes single-dose treatment unreliable — confirm efficacy with post-treatment FWECs.
Pyrantel: Active against adult redworms and roundworms — but efficacy against roundworms can no longer be assumed. Martin et al. (Veterinary Parasitology 2018;264:69–73, PMID 30503095) found four of eleven pyrantel-treated foal groups at or below the 85% resistance threshold across nine Swedish stud farms, with 43% of pyrantel-treated foals still shedding eggs 10–16 days after treatment — the first report of pyrantel-resistant Parascaris in Europe. Double-dose pyrantel is used for tapeworm treatment. Some resistance reported in cyathostomins in the UK. Confirm efficacy with a post-treatment FWEC, particularly in foals and youngstock.
Praziquantel: Specifically targets tapeworms. Most often used in combination products (e.g. ivermectin + praziquantel) for autumn treatments.
Timing recommendations for UK horses (general guidance — always follow veterinary advice):
- Spring (March–April): FWEC-guided treatment of horses with counts above threshold
- Summer: FWECs every 8–12 weeks for monitoring; treat if above threshold
- Autumn (October–November): Tapeworm treatment (praziquantel-containing product) + bot treatment (ivermectin or moxidectin) after first frost
- Winter: Consider encysted larval treatment for high-risk horses using moxidectin or 5-day fenbendazole protocol
Prevention
- Adopt a targeted selective treatment programme guided by FWECs rather than routine calendar worming
- Pick up droppings from pasture regularly — at least twice weekly significantly reduces pasture larval contamination
- Avoid overgrazing — high stocking density increases pasture larval contamination
- Rotate pastures and ideally rest them for several months to reduce larval survival
- Co-graze with cattle or sheep where possible — they ingest equine parasites without becoming hosts
- Quarantine and FWEC test new arrivals before integrating into the resident group
- Keep accurate treatment records and rotate between drug classes strategically to preserve efficacy
The British Equine Veterinary Association and the ESCCAP UK (European Scientific Counsel Companion Animal Parasites) publish evidence-based parasite control guidelines for UK horse owners.
When to Call the Vet
- Sudden weight loss, profuse diarrhoea, or ventral oedema — may indicate cyathostominosis
- Young horse (foal or yearling) with pot belly, colic, or respiratory signs — roundworm burden
- Persistent colic episodes that do not resolve with standard treatment — tapeworm involvement possible
- Horse not responding to worming treatment — suspect resistance; discuss alternative drug choice with your vet
- You are unsure which wormers to use or want to set up an appropriate monitoring programme
- FWEC post-treatment shows inadequate reduction — evidence of anthelmintic resistance
Prognosis
The prognosis for most horses with worm burdens managed appropriately with targeted treatment and good pasture hygiene is excellent. Cyathostominosis carries a more guarded prognosis — mortality rates of 30–50% have been reported in severe cases. Early recognition and intensive veterinary care improve outcomes. Large redworm arteritis, though now uncommon, can cause fatal gut infarction if not treated before larvae cause irreversible arterial damage.
Long-term, the biggest risk is anthelmintic resistance rendering treatments ineffective. Responsible use of wormers — guided by FWECs rather than routine prophylactic dosing — is the most important thing UK horse owners can do to preserve the efficacy of the limited drug classes available.
Frequently Asked Questions
How often should I worm my horse?
The old recommendation of worming every 6–8 weeks regardless of clinical signs or worm burden is no longer considered best practice. Current UK guidelines recommend FWEC-based targeted treatment, which typically means most adult horses are treated 1–3 times per year. All horses should receive tapeworm and bot treatment in autumn. A foal and youngstock programme is more intensive due to higher susceptibility. Work with your vet or a BHS-qualified parasite adviser to set up a programme appropriate for your yard.
Are wormers safe to use in pregnant mares?
Most wormers can be used in pregnant mares with some important caveats. Ivermectin is considered safe at all stages of pregnancy. Moxidectin should be used with caution in the last month of pregnancy. Praziquantel is generally avoided in the first trimester. Always consult your vet before treating pregnant mares, particularly with combination products.
My horse has a low egg count — do I still need to treat?
Horses with FWECs under 200 epg consistently are typically low shedders and do not need treatment at that point. However, even low-count horses should receive the autumn tapeworm and bot treatments, as these are not detected by standard egg counts. Monitoring egg counts 2–3 times per year ensures you catch any change in burden.
Can horses pass worms to dogs, cats, or humans?
The major equine parasites are largely host-specific and do not infect dogs, cats, or humans under normal circumstances. Strongyloides westeri (threadworm) can occasionally cause skin irritation in humans handling heavily infected foals, but this is uncommon. Maintaining good personal hygiene when handling horses and their droppings remains sensible practice regardless.