WORMING CPD – SEPTEMBER 2025

Return of the redworm: rethinking large strongyle control in the age of resistance

Understanding the re-emergence of equine bloodworms - and how SQPs can help to protect horse health through risk-based screening and responsible control – with Claire Shand of Westgate Labs.

For decades, Strongylus vulgaris, dubbed the “colic worm”, one of the most feared equine parasites. Its reputation was well earned. Unlike small redworm (cyathostomins) that remain in the gut, large redworm larvae migrate through the horse’s organs and arteries, feeding on tissue as they go. The trail of damage left in their wake has potential to result in life-threatening colics, haemorrhage, and sudden deaths. Thanks to regular interval worming through the late 20th century, large strongyles were all but eradicated in the UK horses. For many prescribers, they’ve become little more than a footnote in parasitology textbooks, with current concern shifted to the control of small redworm, ascarid and tapeworm.

However, as we shift towards more targeted parasite control to tackle resistance, and as international horse movement increases, this is opening the door for a resurgence of this once-dominant parasite. No longer a tale from the past, we are turning the page for a new chapter of the large strongyle story as it begins to surface in UK horses.

What makes large strongyles so dangerous?

Large strongyles include several species that affect horses: Strongylus vulgaris, S. edentatus, and S. equinus are the most notable, with S. vulgaris long considered the most pathogenic.

What sets them apart is their complex and damaging larval migration. After being ingested as infective larvae on pasture, these parasites don’t simply stay in the digestive tract. Instead, they migrate through the blood vessels or organs, depending on the species, before returning to the gut to mature and reproduce.

• S. vulgaris travels via the cranial mesenteric artery, causing blood clots, arterial thickening and thromboembolic colic.
• S. edentatus migrates through the liver and peritoneum, causing fever, weight loss, and tissue damage.
• S. equinus targets the liver and pancreas, where inflammation may result in colic or loss of condition.

Because these effects occur internally during the larval stages, horses often show no outward signs of infection until something goes seriously wrong. Tragically, sudden death can sometimes be the first and only symptom, particularly in brood mares and competition horses, when weakened vessels come under heightened stress.

Why are we talking about them now?

The answer lies in a combination of progress and unintended consequence. Our over-reliance on blanket worming schedules, particularly from the 1960s onward, nearly eliminated large strongyles in the UK. But this approach came at a cost, leading to widespread resistance in cyathostomins, ascarids, and other species. To combat this, the industry has pushed for a vital shift towards evidence-based control treating only the horses that need it.

Current testing practices rely heavily on worm egg counts, which are invaluable for assessing cyathostomin (small redworm) egg excretion, but they can’t distinguish between small and large strongyle eggs, which look identical under the microscope. This hasn’t been an issue while large strongyles remained such a rare possibility, but its potential resurgence changes that picture.

In countries like Sweden, where prescription-only wormers have been in place since 2007 and treatment is guided strictly by test results, the prevalence of Strongylus vulgaris increased from 6% in 2008 to over 50% of yards by 2015 (Osterman-Lind et al 2023).

No comparable UK studies have been published, but we share the same risk factors: increased horse movement, tighter control protocols, and a reliance on diagnostic tools that don’t flag large strongyles. The Horserace Betting Levy Board identifies “there is significant concern about the re-emergence of large strongyle species, especially Strongylus vulgaris…” In its Codes of Practice for the 2025 equine breeding season.

While (Colgate and Floyd 2025) caution: “This serves as precautionary evidence that S. vulgaris could resume its role as a significant parasite of horses with the move to more targeted anthelmintic dosing regimens.”

Why worm egg counts aren’t enough

While worm egg counts remain one of the most valuable tools for strategic worm control they have a blind spot. For small redworm, low egg counts (under 200 eggs per gram) are often left untreated to help reduce selection pressure for resistance. But we would want to treat any indication of large strongyle infection.

Adding to the complexity, large strongyles have a long pre-patent period which means it takes six to seven months from ingestion before they begin to shed eggs. During that time, serious internal damage may already be underway, completely undetectable via worm egg count.

That’s why WECs alone are no longer enough in the fight against large redworm. Additional testing is needed to bring them back into focus.

When and how to test; a risk-based approach

Given the renewed concern around large strongyle infections, annual testing is advised, especially for horses at higher risk including:

• Studs and yards with foals or youngstock
• Sales or competition yards, especially with frequent overseas travel
• Properties with limited worming or minimal recent anthelmintic use
• New arrivals from abroad or other high-risk yards

Two main tests are available to detect large strongyles: larval culture and PCR.

For routine monitoring, larval culturing is recommended; it’s cost-effective and suitable for regular checks. You can submit individual or composite samples (up to four horses), ideally grouped by how the horses live or graze together. This screening should be carried out alongside routine worm egg counts and tapeworm testing.

Testing methods:

• Larval culture: A faecal sample is incubated for 10–14 days to hatch any strongyle eggs. Resulting larvae are then identified under the microscope.
• ✅ Cost-effective for regular screening
• ✅ Differentiates small and large strongyles
• ❎ Time consuming (allow 2 weeks)
• ❎ Requires skilled identification

• PCR testing: Detects parasite DNA directly in faeces.
• ✅ Fast (1-2 days)
• ✅ Highly sensitive
• ✅ Species-specific
• ❎ More expensive than larval culture
• ❎ Not currently commercially available in the UK

When to test:

• Horses one year old and over: Research shows little seasonal variation in large strongyle egg shedding. Testing can be done year-round, guided by risk factors rather than calendar dates (Steuer, Duncan, Ogbourne et al)
• Foals: Test between 7 and 9 months old, especially following summer turnout. Egg shedding tends to begin in late autumn.
• After potential exposure: If a horse has travelled or been exposed to others from high-risk areas, wait 6–7months before testing to allow time for larvae to mature and become detectable.

What to do if large strongyle are identified?

The large strongyle larval culture test involves incubating faecal samples under controlled conditions to allow strongyle eggs present to hatch and develop into third-stage larvae. These larvae are then identified under a microscope, enabling accurate differentiation between the large strongyle species, Strongylus vulgaris, Strongylus edentatus, and Strongylus equinus as well as the less pathogenic non-migratory large strongyle species, Triodontophorus brevicauda, Triodontophorus serratus and Craterostomum acuticaudatum. The more common small redworm species are not reported due to their ubiquity.

If any large strongyle species are identified, immediate treatment is advised. Several licensed wormers in the UK are effective, but not all cover all species or life stages, so care is needed in selection. Check both the chemical and the brand data sheets.

• Ivermectin: Generally has the most wide-ranging efficacy claims for large strongyle species with most brands covering larval and adult stages of each of large strongyle species listed above with the exception of larval stages of Strongylus equinus.
• Moxidectin: Effective against adults and larval stages of S. vulgaris, S. edentates, Triodontophorus brevicauda, T. serratus
• Fenbendazole: Targets adult and migrating larval stages of S. vulgaris and S. edentates.
• Pyrantel: Licenced for S. vulgaris, S. edentates, S. equinus but differing brand claims over larval stages of S. vulgaris and S. edentates.

NB. there are no UK-licensed equine anthelmintics indicated for the treatment of immature or larval stages of Strongylus equinus.

Your choice of product should also consider time of year, age of horse, worm egg count and other test results, as well as previous worming history. It’s good practice to repeat a worm egg count alongside larval culture to monitor for concurrent cyathostomin burdens. There are currently no known resistances to large redworm treatment but equally we are not aware of any studies that have been done. This truly is an emerging area of research in equine parasitology.

Responsible action, not regression

As awareness grows around the resurgence of large strongyles, it’s tempting to reach for broad-spectrum wormers “just in case.” But that’s a step backwards.

Instead, screening is our best defence; it allows us to respond proportionately, treat only when needed, and continue to protect anthelmintic efficacy for all parasites, not just large strongyles.

Prescribers and SQPs are uniquely positioned to guide this balance. By advising on risk-based testing, interpreting results, and recommending suitable treatments, you help ensure both horse welfare and long-term sustainability.

The return of large strongyles is a timely reminder that parasites adapt to our actions. With careful diagnostics, informed decisions, and smart treatment, we can stay one step ahead. New tools like large strongyle larval culture testing make this possible.

Let’s use them wisely.