Strongylus vulgaris-associated Disease in Horses

Last Updated on March 29, 2022 by Allison Price

The equine GI helminth Strongylus vulgaris is considered the most dangerous. Although the prevalence was estimated at 80%-100%, decades of routine deworming have reduced its incidence to very low levels. Acute peritonitis is the most common finding in horses. In such cases, fecal egg counts are not diagnostic. A coproculture will only detect adult parasites within the intestinal lumen and not the migrating larvae. The only treatment for S vulgaris-associated infarction is resection.

Strongylus vulgaris is a member of the strongylidae group and one of three weakylus species that infect horses. The S. edentatus or S. equinus have not been linked to distinct clinical syndromes so they are not covered here. This is likely why this parasite is so rare in horses managed with anthelmintic drugs. This parasite can be found in horses that have been treated with very few, or none, anthelmintic products. It has also been reported to be reemerging in countries like Sweden and Denmark where prescription-only laws limit anthelmintic use.

Strongylus vulgaris

In the normal life cycle, third-stage larvae (L3) are subendothelially transported in intestinal arterioles to the root of celiac and cranial mesenteric arteries. They then molt to L4 and enter the arterial lumen where they molt into the L5 stage. They are then carried down the circulatory trees to eventually form abscesses, measuring 1 cm in size, in the walls of their cecum, ventral colon and dorsal colonies. The larvae are released into the intestinal lumen when these abscesses burst. Most adult S vulgaris are located in the cecum. However, a few may be found in the ventral colon. The entire life cycle lasts approximately 6 months. There are 4 months in the mesenteric veins.

In certain climates S vulgaris occur in seasonality. Infections are acquired during the grazing season, while the migration phase takes place over the winter months. As they move on to their extraintestinal migration, egg-shedding adults become more common in spring.

Clinical Signs

Strongylus veridis-associated illness is typically a nonstrangulating infarction of the intestinal wall. It presents as peritonitis.

  • Fièvre
  • Borborygmus can be absent or reduced.
  • Hyperemic mucous membranes
  • Normal to slightly elevated heart beat
  • Mild or no pain
  • negative gastric reflux
  • A sore mass visible on rectal examination

Common clinical laboratory findings include:

  • Increased serum amyloid A, fibrinogen levels
  • Lower plasma iron concentration
  • Leukopenia
  • Lactate levels in plasma and peritoneal fluids are higher

Abdominocentesis shows a fulminant case of peritonitis, characterized by high levels of white blood cells and higher protein and lactate contents.Verminous endarteritis


An infection with S vulgaris can invariably lead to a chronic-active, verminous end-arteritis of the mesenteric vessels. However, this condition has not been linked with any clinical syndromes. No clinical signs have been found to be linked with the formation or rupture of intestinal abscesses. Although adult S vulgaris Parasites can sometimes be found attached to the intestinal walls, and they are known to ingest blood, this is rarely if ever the main cause of clinical anemia.


exploratory laparotomy can diagnose a non-strangulating infarction. Diagnostic value is not provided by egg counts or flotation. Coproculture can be used to identify third-stage larvae. It also helps diagnose the presence adult parasites in your intestinal lumen. This is however of limited use as the condition is not caused either by adult parasites or by migrating larvae. Although a serum ELISA has been developed that detects antibodies against migrating S vulgaris larvae, it is not yet commercially available.

Peritonitis can be diagnosed by abdominalocentesis. Peritonitis should not be diagnosed in areas where S vulgaris is common.


After diagnosing an intestinal infarction, surgical correction must be performed immediately. This should include:

  • Resection of infarcted intestinal tissue
  • If adherences are present, they should be removed

The majority of medical treatment fails. The extent of intestinal lesions can be determined by exploratory laparotomy. This allows for the determination of whether surgery is possible. Patients who are eligible for corrective surgery have a good to excellent prognosis. If surgery is performed more than 24 hours after the onset of clinical signs, the prognosis for corrective surgery patients is good to excellent.

Standard post-surgical care includes the following:

  • Fluid therapy
  • Anti-inflammatory Therapy
  • Antibiotics

In the acute phase of nonstrangulating infarction, anthelmintic medications are not recommended. Once the patient has stabilized and peritonitis is gone, it is recommended to deworm with an anti-helmintic larvicidal. You can choose from ivermectin (0.25 mg/kg), Moxidectin (0.40 mg/kg) or a 5-day course (10 mg/kg) of fenbendazole (10mg/kg).


The current S vulgaris control guidelines include a foundation for anthelmintic treatment to all horses with the specific goal of decreasing the incidence of S vulgaris. This should include one to two yearly, strategically administered treatments with an effective antihelmintic.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!