Large Redworm In Horses

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 strongyles have a migratory larval stage through the horse’s body. They travel through the blood vessels (primarily the cranial mesenteric artery which supplies blood to the intestines) and other organs leading to colic, weight loss, and even fatal complications.

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	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 increase the risk of infection.

Worm egg counts cannot differentiate between large and small strongyle. Because of this, large yards, studs and other high-risk establishments are advised to begin screening for large strongyle infection to monitor for incidences of this parasite on their properties.

 

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 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.

 

Detecting Large Strongyles in Horses

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 a good starting point; 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.

More information on the application of both methods is included below:

Larval Culture

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

PCR

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
  
  

What does this mean for horse owners?

Small redworm remain the most common parasitic threat to horses, but large redworm—Strongylus vulgaris—should not be overlooked. Their ability to cause sudden, severe, and often fatal colic makes it critical to include specific testing for large strongyles as part of a responsible, targeted worming programme.

Annual testing is strongly recommended for larger yards, studs, competition horses, and any premises where horses frequently move on and off the property. 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.

Ready to begin? Purchase a larval culture test and take the next step in protecting your horses from this hidden threat.

SHOP TESTS

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. 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