The Equestrian’s Guide to Roundworm in Horses

Roundworm in horses is the single most important parasite of foals and young stock, and the one most likely to do permanent damage before anybody notices it. Known to science as Parascaris equorum, these large, creamy-white worms are described by the UK worm-count laboratory Westgate Labs as reaching up to 40cm in length and turning up most often in youngsters under four years of age. Adult horses usually carry enough immunity to keep them in check. Foals have none, and a heavy burden can cost a young horse its growth, its lungs or its life. This guide covers how the parasite works, what it looks like on the ground, and — most importantly — why the old advice about which wormer to reach for no longer holds. Anyone caring for young stock should read the treatment section carefully.

What Is Roundworm in Horses?

Roundworm — also called ascarid — is a large nematode that lives as an adult in the small intestine. It is thick, firm and creamy-white to pale yellow, and horse owners who see one passed in a dropping almost always describe the same thing: something that looks far more like a piece of cooked spaghetti or a garden earthworm than anything they expected to come out of a horse. Westgate Labs puts the maximum length at around 40cm, and that size is the whole problem. A few are an inconvenience. Several hundred in a weanling's gut are a physical obstruction.

Roundworm is not a subtle parasite and it is not a rare one. The Scottish research institute Moredun notes that almost every grazing horse picks up parasitic worms of some kind, and where foals are being bred, ascarids are the species that matters most. Work by Zanet and colleagues in northern Italy (Veterinary Sciences 2021, PMID 33807857) opens with the same point: cyathostomins have become the dominant parasite of adult horses, but Parascaris remains the most important parasite infecting foals and weanlings.

Parascaris equorum or Parascaris univalens?

Almost every horse-care page you will read, this one included, uses the name Parascaris equorum. It is worth knowing that where researchers have actually gone to the trouble of karyotyping the eggs, they have often found a closely related species instead. When Martin and colleagues at the Swedish University of Agricultural Sciences examined ascarid eggs collected across Sweden (Veterinary Parasitology 2018, PMID 30503095), every sample was identified as Parascaris univalens, distinguished by having a single chromosomal pair.

For day-to-day management this changes nothing — the life cycle, the signs and the control measures are the same. It matters because it tells you how little of the routine advice on equine roundworm has been checked recently, which is a useful thing to bear in mind when you reach the section on wormers.

Which Horses Are Most at Risk?

Age is the dominant risk factor, and both of the UK sources most often quoted on this draw the line in the same place. Westgate Labs describes roundworm as most common in foals and youngsters under four years of age, and the Royal (Dick) School of Veterinary Studies likewise advises that youngsters less than four years old are more susceptible to worms and need more frequent testing and treatment than mature horses.

The reason is immunological rather than managerial. A foal is born with no acquired immunity to ascarids and has to build it through exposure. That immunity develops over the first years of life, which is why the heaviest burdens are found in sucklings, weanlings and yearlings, and why the same paddock that flattens a batch of foals will leave the mares grazing beside them looking perfectly well.

Older horses are not entirely exempt. Westgate Labs notes that ascarids occasionally affect adults too — typically those grazed on pasture previously occupied by wormy youngsters, or horses that had a poor start in life and never built normal resistance. The more common role of the adult horse, though, is as a reservoir: a mature animal can carry and shed eggs while looking and performing perfectly normally, quietly seeding the ground that this year's foals will graze.

How Horses Become Infected with Roundworm

Infection is by mouth. A horse swallows infective eggs from contaminated ground, from grass, from soiled hay or bedding, from feed and water buckets, or from a mare's udder and coat while nursing. There is no intermediate host and nothing has to bite the horse — the eggs simply have to be picked up and eaten.

Two features make that far harder to interrupt than it sounds.

The egg output is enormous. A mature female ascarid is a prodigious egg-layer, and the eggs pass out in the droppings in numbers that contaminate ground quickly. You will see very specific figures for daily egg production quoted across the equestrian web; we have not been able to trace them to a primary study, so we will not repeat one. The practical point stands without it — a small number of patent adults produces an enormous amount of contamination, and that contamination lands exactly where the next foal will graze.

The eggs are exceptionally tough. Ascarid eggs carry a thick, multi-layered shell that protects them from drying and from cold, and Westgate Labs describes them as surviving in the ground for many years. Koudela and Bodecek tested that resilience directly (Veterinary Parasitology 2006, PMID 16876952) by freezing egg suspensions at temperatures from −5°C down to −80°C: the eggs retained viability and infectivity after freezing, surviving longer at the higher freezing temperatures. The same study found eggs were killed when the water around them reached 60°C or more within a minute. The message for a UK or South African yard is blunt: a hard winter will not clean a contaminated paddock, and nor will a dry spell, because Westgate Labs notes that the same shell protects the eggs from dry conditions as well as from freezing.

Moredun makes the practical version of this point in its guidance on the roundworm species found in horses: the risk areas for ascarid infection are the paddocks and stables that housed foals the previous year. If you breed, your worst ground is the ground you used last season.

The Roundworm Life Cycle, Stage by Stage

Roundworm has a direct life cycle — no second host is involved — but it is not a simple one. The larvae do not stay in the gut. They take a long tour through the foal's internal organs first, and that migration, not the adult worm, causes much of the lasting damage.

From Egg to Migrating Larva

An infective egg is swallowed and its shell is broken down in the digestive tract. The larva hatches in the small intestine and immediately burrows through the intestinal wall, entering the bloodstream rather than staying where it landed.

Through the Liver and Lungs

The larvae are carried to the liver, where they feed and develop, and then on to the lungs. Moredun describes this migration through the liver and lungs as damaging in itself, with large numbers of migrating larvae causing respiratory signs such as coughing.

The liver copes comparatively well — it is a regenerative organ and the scarring left by a migration is not usually of lasting consequence. The lungs are a different matter. Lung tissue repairs by scarring rather than by regrowing, so damage done during migration is permanent, and a young horse has a finite amount of lung to lose. Westgate Labs notes that migrating larvae breaking into the lungs may cause haemorrhages. This is the strongest argument for keeping burdens low in foals rather than waiting for a problem: the respiratory cost is paid once and is not refundable.

Back to the Small Intestine

From the air spaces of the lungs the larvae crawl up the airways, irritate the throat, and are coughed up and swallowed for a second time. Back in the small intestine they mature into adults, and the females begin laying. The cycle then repeats.

How Long Before Eggs Appear?

This matters more than it looks, because it sets the gap between a foal becoming infected and a worm egg count being able to detect it. The classic experimental work is Clayton and Duncan's study of infection in worm-free pony foals (Research in Veterinary Science 1977;23(1):109–14, PMID 905641). Six foals aged two to four months were dosed with P. equorum eggs and the pre-patent periods recorded were 80 and 83 days — roughly twelve weeks from swallowing an egg to eggs appearing in the droppings.

That same study produced a finding worth holding on to, because it runs against intuition. Clayton and Duncan reported that coughing and a circulating eosinophilia were features of the large infections — from which only a small number of worms actually developed to maturity. In the small infections, by contrast, a high percentage of the parasites matured in the small intestine, and it was those foals that showed unthriftiness. In other words, a big dose of eggs can produce a coughing foal with a modest adult worm count, while a smaller dose can produce a poor-doing foal carrying plenty of adults. A single egg count never tells you the whole story.

Symptoms of Roundworm in Horses

The signs of worms in horses depend on the size of the burden and on which stage of the cycle is active at the time. Because migration and adult infection cause quite different problems, the same parasite can present as a chest problem or as a gut problem.

The Classic Signs in a Foal

Westgate Labs lists the outward signs to watch for as lethargy, weight loss, coughing, a rough coat and a pot belly, and warns that infection can have a devastating effect in foals that accumulate massive burdens quickly — severe debilitation, poor growth, and even death. In practice the picture that should prompt action is:

• Failure to thrive or grow despite adequate milk and feed — often the earliest and most reliable sign
• A dull, staring, rough coat that will not shine up
• A distended barrel — the classic pot-bellied outline, often alongside poor muscle over the topline
• Lethargy and a general lack of the brightness a healthy foal should have
• Weight loss, or a body condition score that drifts downwards while stablemates hold theirs
• Constipation or diarrhoea, depending on whether the gut is obstructed or inflamed

The comparison that gives the game away is between foals on the same ground and the same feed. Two similarly bred foals running together should do similarly well. When one is lagging, the difference is worth investigating rather than explaining away.

Respiratory Signs and the Coughing Foal

While larvae are moving through the lungs and being coughed up, a foal may show a cough, a greyish nasal discharge, watery eyes or a mild fever. This is where roundworm is most often missed, because that combination looks exactly like a viral respiratory infection — the presentation covered under equine viral pneumonitis — and is usually treated as one.

The distinction is not academic. A foal being nursed through what everyone assumes is a cold can be actively accumulating permanent lung scarring at the same time. If you are dealing with coughing in a young horse on a stud where ascarids are known to be present, parasite migration belongs on the list of possibilities from the start, not after the antibiotics have failed.

Colic, Impaction and Rupture

This is the danger that kills foals. Adult ascarids are big, and enough of them in a young, narrow small intestine form a physical mass. Westgate Labs names intestinal obstruction or inflammation as a major risk of infection. A burden large enough to obstruct can cause impaction colic, and in the worst cases the intestine can rupture — which leads to peritonitis, a contamination of the abdominal cavity that a foal is unlikely to survive. Where obstruction is the problem rather than inflammation, colic surgery may be the only option, and it is a poor prognosis in a small foal.

Colic in a foal or weanling should always be taken seriously and always involve the vet early. It is worth knowing how it differs from the more common adult presentations such as spasmodic colic — in a young horse on wormy ground, an obstructing ascarid burden is a live possibility in a way it simply is not in a mature animal.

What You May See in the Droppings

Owners searching for small white worms in horse poo are very often looking at ascarids, particularly after a foal has been wormed. Passed roundworms are unmistakable once seen: pale, thick, firm and long. Seeing them confirms the parasite was there, but it tells you nothing useful about how many remain, and it is not a substitute for testing.

Loose droppings in a young foal have several possible explanations and worms are only one of them. Foal heat diarrhoea around the mare's first season after foaling is common and usually self-limiting, while scouring in foals has its own set of infectious causes. A foal that is scouring and failing to grow deserves a worm egg count as part of the work-up rather than in place of it.

Diagnosing Roundworm: Worm Egg Counts and Their Limits

The standard test is the faecal worm egg count (FWEC, or simply a worm egg count). A dung sample is examined under the microscope and the eggs present are identified and counted, giving a figure in eggs per gram. Ascarid eggs are large, round and thick-shelled, and are easy to tell apart from strongyle eggs.

What a Worm Egg Count Can and Cannot Tell You

A worm egg count detects adult worms that are actively laying. That is its strength and its limitation, and Westgate Labs states the limitation plainly: the test monitors adult worms that are laying eggs, and immature roundworm do not lay eggs.

Set that against Clayton and Duncan's pre-patent periods of 80 and 83 days and the practical consequence is stark. For something close to twelve weeks after infection, a foal can be carrying larvae migrating through its liver and lungs — doing the permanent damage — and return a clean egg count throughout. A zero worm egg count in a foal does not mean a parasite-free foal. It means no adults are laying yet.

This is why signs matter alongside tests. A coughing, pot-bellied, poor-doing foal with a clean count still warrants a conversation with your vet.

The Faecal Egg Count Reduction Test

The second test is the one that has become essential, and it is the one most yards skip. A faecal egg count reduction test (FECRT) means counting before treatment, treating, then counting again a set interval later to measure how well the wormer actually worked on your horses, on your yard.

The Royal (Dick) School of Veterinary Studies recommends that any horse requiring worming is tested for resistance at least once a year to confirm the wormer remains effective, by submitting a second sample two weeks after treatment, and that the same test is carried out for any horse with a significantly elevated count. Given what the research now shows about ascarid resistance, this is the single most valuable thing a breeding yard can do.

For routine monitoring of mature horses, the Dick Vet's current targeted-worming guidance is to submit samples every 10–12 weeks through the grazing season from early March to early October, with at least three samples in that window, and to treat adult horses returning more than 400 eggs per gram. They note they raised that threshold from 250 to 400 EPG to slow the development of resistance without increasing the risk of clinical disease. Two cautions on borrowing those numbers: that guidance is written for adult horses and is aimed principally at strongyles, and the Dick Vet is explicit that youngsters under four need a plan tailored by your own vet rather than the adult protocol.

Treatment of Roundworm in Horses

All broad-spectrum equine wormers are licensed to treat roundworm. Whether they still work on your yard is an entirely separate question, and it is the most important thing in this article.

Wormer Resistance Has Changed the Picture

The standard advice repeated across the equestrian web — including by Westgate Labs, whose guidance is otherwise excellent — is that there is documented resistance to ivermectin and moxidectin, so you should treat roundworm with fenbendazole or pyrantel instead. The first half of that is well supported. The second half has been overtaken by the evidence, and a breeder who follows it without testing may be dosing foals with a product that no longer works.

Macrocyclic lactones (ivermectin, moxidectin) are the worst affected. Cai and colleagues' systematic review and meta-analysis of anthelmintic resistance in equine nematodes (Molecular and Biochemical Parasitology 2024;257:111600, PMID 38030084) pooled 60 studies published between 1994 and 2022 covering 11,835 animals. It found resistance in ascarids significantly more severe than in strongyles, with the macrocyclic lactone class showing the lowest efficacy against ascarids at a pooled 67.83%.

But pyrantel resistance in ascarids is now documented in Europe too. Martin and colleagues ran reduction tests on 142 foals across nine Swedish stud farms (Veterinary Parasitology 2018;264:69–73, PMID 30503095). Using the American Association of Equine Practitioners' thresholds — resistant to pyrantel at a reduction of 85% or less, and to fenbendazole at 90% or less — four of eleven pyrantel-treated groups fell at or below 85% efficacy, and 43% of the foals treated with pyrantel were still shedding eggs 10 to 16 days afterwards. Fenbendazole performed better in the same study: one of six groups fell at or below 90%, and only 6% of foals were still shedding. That paper was the first report of pyrantel-resistant Parascaris in Europe.

And multi-drug resistance occurs on individual farms. Armstrong and colleagues tested ivermectin, pyrantel embonate and fenbendazole against P. equorum on five farms in New South Wales (Veterinary Parasitology 2014;205(3–4):575–80, PMID 25224788). Patent infection was present in 58.3% of the foals sampled (147 of 252). Resistance to at least one anthelmintic was found on all five farms. Ivermectin was effective on two farms and ineffective on three; fenbendazole was effective on two, equivocal on one and ineffective on one; pyrantel was effective on three and ineffective on one. Two farms showed evidence of multiple drug resistance.

Read those three together and the conclusion is not "use fenbendazole instead of ivermectin". It is that no class can be assumed effective against roundworm, and the only way to know what works on your yard is to measure it with a reduction test. Note too that the Dick Vet has withdrawn its recommendation of the five-day fenbendazole course it previously advised for encysted redworm, citing concerns about efficacy due to resistance — fenbendazole is not the safe harbour it was once treated as.

The other consequence is that worming less, but better, is genuinely in your foals' interest. Every unnecessary dose selects for the resistant worms that survive it, and no new class of equine anthelmintic has reached the market in around forty years. There is no replacement coming to bail anyone out.

The Risk of Killing a Heavy Burden Too Fast

There is a second hazard specific to roundworm, and it catches people out precisely because they have done the right thing. When a large burden of adult ascarids is killed rapidly, the dead and dying worms can detach from the gut wall together and form a bolus that obstructs the intestine. The resulting colic is caused by the dead worms, not by the drug.

This is why a heavily infected foal is a case for veterinary supervision rather than a tube of wormer bought off the shelf. Where a big burden is suspected, the choice of product matters: Chapter 11 of the UK CANTER Guidelines notes that impactions triggered by treatment have been seen especially with wormers that paralyse the worms, such as ivermectin and pyrantel salts, and the foal should be watched closely afterwards for any sign of colic. Do not treat a suspected heavy burden in a young foal on your own initiative — phone your vet first and let them plan it.

Worming Foals and Youngstock

Because foals have no meaningful immunity and because egg counts stay negative through the damaging migratory phase, youngstock are the one group still generally treated proactively rather than purely on test results.

The current UK schedule is set out in Chapter 11 of the CANTER Guidelines (Foals, Weanlings, Yearlings and Youngstock, May 2026): every foal is treated at 2–2.5 months and again at 4–5 months, because egg counts before about three months miss the immature worms, with a reduction test at 4–5 months to check the product worked, then faecal egg counts from 6–7 months to guide what comes next. Westgate Labs' foal protocol was updated to the same two treatments in summer 2026. You will still see "worm foals every six to eight weeks until six months" quoted, including on Westgate's older roundworm factsheet and in an earlier version of this page. That is the older interval schedule, and it is not what the UK guideline now recommends.

Two things should sit alongside it. First, the Royal (Dick) School of Veterinary Studies' position that under-fours need a plan tailored by your own vet — your foaling dates, stocking density and paddock history all change the answer. Second, and given the resistance data above, whatever product that plan lands on should be checked with a reduction test rather than trusted. For the wider picture across all the species a horse can carry, our guide to equine worming and parasite control covers the full annual programme.

Alongside the CANTER Guidelines, a second freely available UK reference is the ESCCAP UK & Ireland guideline GL8, A guide to the treatment and control of equine gastrointestinal parasite infections, from the ESCCAP UK & Ireland guidelines page, and both are documents to hand your vet if you want to build a programme properly.

Preventing Roundworm in Horses

With resistance closing off the chemical options, management is no longer the boring part of worm control. It is the part that still reliably works.

Pasture and Dung Management

Removing droppings takes eggs out of the environment before they can be eaten, and it is the intervention with the best return for the effort. The two UK sources quoted through this article give slightly different frequencies, and both are worth knowing: the Royal (Dick) School of Veterinary Studies advises regular poo picking at least twice weekly, while Westgate Labs recommends removing droppings from the pasture three to four times per week. More often is better, and for foal paddocks the UK CANTER Guidelines set the floor at three times a week, ideally daily, so twice a week is not enough on ground where foals graze.

• Poo pick consistently. At least twice weekly per the Dick Vet, three to four times weekly per Westgate Labs, and on foal paddocks at least three times a week, ideally daily, per CANTER.
• Avoid overstocking. Both sources name this. Too many horses on too little ground forces them to graze close to droppings they would otherwise refuse.
• Rest and rotate paddocks. The Dick Vet advises rotating paddocks frequently to avoid overgrazing and to let eggs die off before horses graze the area again, and says a 9–12 month gap is ideal. Given how long ascarid eggs persist, treat that as a minimum on foaling paddocks rather than a guarantee.
• Alternate the ground used for mares and foals year on year where the acreage allows, as Westgate Labs advises — this is the single most targeted measure against ascarids specifically, because last year's foal paddock is this year's highest-risk ground.
• Co-graze with cattle or sheep where you can. The Dick Vet notes that other livestock species ingest the eggs and larvae, preventing horses from eating them.
• Keep muck heaps away from grazing so larvae cannot migrate back into the fields.
• Do not rely on frost. Koudela and Bodecek's freezing work is the evidence for this: a cold winter is not a paddock hygiene measure.

A note of caution on harrowing, which is often recommended in the same breath as the measures above. Spreading droppings to expose larvae to sun and drying only helps in genuinely hot, dry conditions; done in mild damp British weather it simply distributes contamination across ground the horses will then graze. If you are considering harrowing your pasture, time it for a dry spell and keep horses off afterwards — and remember that ascarid eggs are far more resistant to drying than strongyle larvae, so it does less for roundworm than for redworm either way.

Managing Mares, Foals and New Arrivals

Because adult horses shed eggs without showing signs, the group matters as much as the individual. The Dick Vet's guidance is that all horses kept together should be tested, and treated if necessary, at the same time — treating one horse in a field achieves very little if the others are reseeding the ground.

• Quarantine new arrivals. The Dick Vet advises isolating any horse coming onto the yard and testing it with a worm egg count before worming, then stabling it for at least three days after treatment so eggs are not shed onto your pasture.
• Include the mares. A quiet, healthy broodmare can be the source of the contamination her own foal picks up.
• Think about weaning. Grouping weanlings concentrates susceptible young horses together, so paddock choice at weaning matters as much as the weaning method itself.
• Start early. Hygiene in the foaling box and the first paddocks is part of newborn foal care, not a separate job that starts at two months old.

Roundworm Compared With Other Horse Worms

"Roundworm" is used in two different senses, which causes endless confusion. Strictly, roundworm means any nematode, and Moredun uses it that way — its list of the types of roundworms in horses includes redworms, hairworm and pinworm. In everyday yard usage, and throughout this article, roundworm means the large ascarid Parascaris.

• Roundworm (ascarid). Large, creamy-white, up to around 40cm. Primarily foals and youngstock. Migrates through liver and lungs. Main risks are obstruction and permanent lung damage.
• Small redworm (cyathostomins). The main parasite of adult horses. Moredun notes their quick life cycle of about five to six weeks and their ability to encyst in the gut wall for months or years, undetectable by egg counting, then emerge en masse.
• Large redworm (Strongylus vulgaris). Much less common now. Moredun gives a length of up to 5cm; the larvae migrate through blood vessels.
• Tapeworm. Not detected well by egg counts — saliva testing is more accurate, as both Moredun and the Dick Vet note.
• Pinworm (Oxyuris equi). Causes itching around the tail head rather than systemic illness.
• Threadworm (Strongyloides westeri). Another foal parasite, causing diarrhoea and poor condition.

The practical significance is that a single wormer, a single test and a single season's plan do not cover all of these, which is why an annual programme built with your vet beats reacting to whatever you last read. Our overview of equine worm infestations sets the species side by side.

When to Call Your Vet

Roundworm is a condition where the sensible threshold for picking up the phone is low, because the foal deteriorates faster than an adult horse and the worst outcomes are sudden.

• Any colic in a foal or weanling. Not "if it does not settle" — any colic, straight away.
• A foal that is not growing or thriving alongside others on the same ground and feed.
• A cough or nasal discharge in a foal on a stud with known ascarid history, so migration is considered rather than assumed away.
• Any fever in a foal. A temperature above what is normal for that individual warrants advice rather than waiting on a number you have read somewhere.
• A suspected heavy burden, before you worm. This is the call that prevents a treatment-induced obstruction.
• Egg counts that do not fall after treatment, which points to resistance and changes what you use next.

World Horse Welfare's general advice on controlling worms in horses is a good plain-English companion to the clinical detail here, and the MSD Veterinary Manual's section on gastrointestinal parasites of horses goes deeper for anyone who wants the veterinary reference version.

Final Thoughts

Roundworm in horses is a young horse's disease with an old horse's reputation for being easily dealt with, and that mismatch is what makes it dangerous. The eggs outlast winters and paddock rests. The larvae do their worst damage during a twelve-week window when no egg count will find them. And the wormer that the internet still recommends for ascarids may already have stopped working on your yard without anyone checking.

None of that makes it unmanageable. It does mean the work has shifted from the dosing gun to the muck heap and the laboratory: pick up the droppings, rest and rotate the foaling ground, test rather than guess, treat the group rather than the individual, and — the part almost nobody does — run a reduction test so you know whether the product in your hand is doing anything at all. Do those things and your foals get to grow up with the lungs and the gut they were born with, which is the entire point.

Frequently Asked Questions