Large redworm (STRONGYLUS SPP.)
Large strongyles (Strongylus species), also known as bloodworms, are dangerous equine parasites that can cause severe damage within the horse's intestines and blood vessels. Unlike small strongyles (cyathostomins) which remain in the gut for their full lifecycle, large strongyle larvae spend a part of their life cycle in the horse’s blood vessels. Here, they can cause life-threatening damage, including colic, haemorrhage and intestinal infarction, often before any signs appear. That’s why detection of their presence within populations is so important.
The adult large redworm is a darker red colour and bigger than the small redworm at up to 5cm long. There are three main species, each with characteristic migration patterns and effects in the body. While Strongylus vulgaris is considered the most dangerous, all have capacity to cause considerable disease.
Common Large Strongyle Species
|
Species |
Migration Pathway & Effects |
|
Strongylus vulgaris |
The most pathogenic species, migrating through the arteries supplying the intestines, causing blood clots (thrombosis) and colic. |
|
Strongylus edentatus |
Migrates via the liver and peritoneum, causing weight loss, fever, and organ damage. |
|
Strongylus equinus |
Thought the least common, migrating through the liver and pancreas, potentially leading to inflammation and colic. |
The emerging threat of Large Strongyles
Regular blanket worming strategies in the UK had largely eradicated the large strongyle as a parasite of concern in our horses. However, moving to more targeted control measures to slow resistance, combined with increasing movements of horses from countries where this parasite is more prevalent increases the risk of infection. There is evidence the parasite is making a comeback.
In Sweden, where wormers have been prescription only since 2007 and administered only on evidence of positive test results, more than half of the equestrian yards studied in 2015 had at least one infected horse. An increase from just 6% in 2008. Osterman-Lind et al (2023). To our knowledge no similar studies have been conducted in the UK , however research cautions: “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.” (Colgate and Floyd 2025). 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.
Symptoms; WHY LARGE REDWORM ARE OF SUCH CONCERN
Because of the severe effects of infection associated with larval migration of large redworm in the horse, early detection is critical to avoid life-threatening complications. Large Strongyles feed on blood and tissue as they move through the intestine. This migration can cause inflammation, blockages, and potential rupture of the artery, leading to severe colic, haemorrhage and sudden death. Anaemia can also result from this blood loss.
- Sudden or recurring colic
- Weight loss despite proper feeding
- Lethargy and poor condition
- Diarrhoea or abnormal droppings
- Fever and anaemia
- Blood clot formation (Strongylus vulgaris)
- Liver damage (Strongylus edentatus & Strongylus equinus)
Strongylus vulgaris was once the most dangerous gut parasite in horses and a leading cause of serious colic. Thanks to decades of regular interval worming, the severe artery damage and blood clot-related colic it causes—known as verminous arteritis—has become rare. However, with the shift toward more targeted worming programmes that involve fewer treatments, there's a risk this deadly parasite could make a comeback. (Colgate and Floyd 2025)
As with all parasites, horses can have a severe infection without symptoms or can die soon after symptoms appearing which makes regular screening all the more important.
Large and small redworm are indistinguishable on a worm egg count.
Detecting Large Strongyles in Horses
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.
Given the renewed concern around large strongyle infections, additional testing is advised, especially for horses at higher risk. This includes studs, breeding and competition yards, and any premises with frequent horse movements, particularly from overseas.
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.
Westgate Labs now offers larval culturing as a testing service for all equines.
Large Strongyle Larvae (Strongylus spp.)
More information on the application of diagnostic methods is included below:
LARGE STRONGYLE Larval Culture TEST
A faecal sample is incubated for 10-14 days to allow eggs to hatch into larvae. The larvae are then identified under a microscope based on morphological features.
- Pros:
✅ Differentiates large and small strongyles
✅ Cost-effective compared to molecular testing - Cons:
❌ Time-consuming (10-14 days)
❌ Requires skilled identification
LARGE STRONGYLE PCR TEST
A PCR (polymerase chain reaction) test is a way to look for genetic material (DNA or RNA) of the parasite. PCR detects parasite DNA in faecal samples, identifying species with high precision.
- Pros:
✅ Fast (1-2 days)
✅ Highly sensitive – detects low-level infections
✅ Differentiates large and small strongyle species without incubation - Cons:
❌ More expensive than larval culture
❌ Not currently commercially available in the UK
What does this mean for horse owners?
Small redworm remain the most common parasitic threat to horses, but large redworm, Strongylus spp., should not be overlooked. Their ability to cause sudden, severe, and often fatal disease makes it critical to include specific testing for large strongyles as part of a responsible, targeted worming programme.
Annual testing is especially recommended for:
- Studs and breeding operations with youngstock
- Competition and sales yards with frequent international movement
- Properties with high stocking density or recent horse introductions
- Horses on minimal or selective worming schedules
This screening should be carried out alongside routine worm egg counts and tapeworm testing.
For routine monitoring, larval culture is a practical and cost-effective method. It's especially useful in the spring and autumn, when moderate temperatures and moist conditions support the development of infective larvae. In the UK, March-July is considered optimal as this is when larvae that have migrated through the body are most likely to return to the gut and begin producing eggs.
You can submit samples from individual horses or send composite samples made from up to four horses grouped by grazing or stable arrangements. Please allow two weeks from the sample’s arrival at the lab for culture and results.
Taking the sample is very similar to performing a worm egg count test. An easy-to-use test kit to take the sample, including a postpaid return envelope in Westgate Labs’ trademark plastic free packaging, is included in the price, along with follow up help and support from their friendly, qualified team.
Ready to begin? Purchase a larval culture test and take the next step in protecting your horses from this hidden threat.
Treatment
Wormers licenced to treat large redworm in the UK include fenbendazole, pyrantel, ivermectin and moxidectin. There are no known/recorded instances of resistance that we are aware of, but also no studies done that we can find. Their slower lifecycle (9–11 months compared to 4–5 weeks for small redworm) may explain why large strongyles haven’t developed resistance to wormers as quickly.
References
Victoria A Colgate, Emily F Floyd. Common equine parasites: past, present and future. UK Vet Equine.
2025;9(2):66. https://doi.org/10.12968/ukve.2024.0015
HBLB Code of Practice for the 2025 equine breeding season https://codes.hblb.org.uk/index.php/page/197
Osterman-Lind E, Holmberg M, Grandi G. Selective anthelmintic treatment in horses in Sweden based on coprological analyses: ten-year results. Animals (Basel). 2023;13(17):2741. https://doi.org/10.3390/ani13172741