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Professional Version

Strongylus vulgaris-Associated Disease in Horses


Martin K. Nielsen

, DVM, PhD, DACVM, DEVPC, Maxwell H. Gluck Equine Research Center, Department of Veterinary Science, College of Agriculture, Food and Environment, University of Kentucky

Reviewed/Revised Dec 2019 | Modified Nov 2022
Topic Resources

The equine bloodworm Strongylus vulgaris is regarded as the most pathogenic equine GI helminth. The prevalence was reported to be 80%–100%, but decades of routine deworming has reduced its occurrence to very low levels. Most horses present with an acute peritonitis as the primary finding. Fecal egg counts have no diagnostic value in such cases, and a coproculture will only allow detection of adult parasites in the intestinal lumen, not the migrating larvae that cause the condition. The diagnosis of S vulgaris-associated intestinal infarction is made by exploratory laparotomy, and the only curative treatment is resection of the infarcted tissue, where possible.

Strongylus vulgaris belongs to the group of large strongyles (strongylidae) and is one of three Strongylus species infecting horses. The other two, S edentatus and S equinus, have not been associated with distinct clinical syndromes and are not be covered here. Resistance has not yet been established to any of the available anthelmintic products, which is probably the main reason this parasite has become rare in managed horses. The parasite can occur in equine operations managed with few or no anthelmintic treatments and has been documented to be re-emerging in countries such as Denmark and Sweden, where anthelmintic usage is restricted by prescription-only legislation.

As part of the normal life cycle, ingested third-stage larvae (L3) migrate subendothelially in intestinal arterioles toward the root of the cranial mesenteric and celiac arteries. Here, they molt to the fourth stage (L4) and enter the arterial lumen, where they molt to the L5 stage. Subsequently, they are carried down the circulatory tree to eventually form abscesses (~1 cm in diameter) in the walls of the cecum, ventral colon, and dorsal colon. These abscesses eventually rupture, releasing the larvae into the intestinal lumen. The large majority of adult S vulgaris specimens are found in the cecum, but a few can sometimes be located in the ventral colon. The complete life cycle takes ~6 months, with ~4 months spent in the mesenteric arteries.

In some climates S vulgaris occurrence and abundance are seasonal, with infections being acquired over the course of the grazing season and the migratory phase happening over the winter months. This causes egg-shedding adults to be more abundant in the spring as they complete their extraintestinal migration.

Clinical Signs of Strongylus vulgaris-associated Disease in Horses

Typically, Strongylus vulgaris-associated disease is a nonstrangulating intestinal infarction presenting as peritonitis and accompanied by these clinical signs:

  • fever

  • decreased or absent borborygmus

  • hyperemic mucous membranes

  • normal to slightly elevated heart rate

  • no or mild pain

  • negative gastric reflux

  • a sore mass palpable on rectal examination

Clinical laboratory findings typically include:

  • increased serum amyloid A and fibrinogen concentrations

  • decreased plasma iron concentration

  • leukopenia

  • increased lactate in plasma and peritoneal fluid

Abdominocentesis reveals a fulminant peritonitis with high numbers of white blood cells and increased protein and lactate content.

Infection with S vulgaris will invariably cause a chronic-active verminous end-arteritis in the mesenteric arteries, but this has not been associated with a distinct clinical syndrome. Similarly, no clinical signs have been linked to the formation and subsequent rupture of the intestinal abscesses. Finally, although adult S vulgaris parasites can be found attached to the intestinal walls and are described to ingest blood, they rarely, if ever, are the primary cause of clinical anemia.

Diagnosis of Strongylus vulgaris-associated Disease in Horses

A nonstrangulating intestinal infarction can only be diagnosed by exploratory laparotomy. Fecal flotation and egg counts have no diagnostic value. A coproculture allows identification of third-stage larvae and can help diagnose the presence of adult parasites in the intestinal lumen. However, this has limited value because the condition is not caused by adult parasites but by migrating larvae. A serum ELISA that measures antibodies against migrating S vulgaris larvae has been developed but is currently not commercially available.

Abdominocentesis is required to diagnose the peritonitis. In areas where S vulgaris is endemic, a nonstrangulating intestinal infarction should be considered in cases presenting with peritonitis.

Treatment of Strongylus vulgaris-associated Disease in Horses

Once intestinal infarction is diagnosed, surgical correction should follow immediately. This should include:

  • resection of infarcted intestinal tissue

  • removal of adherences, if present

Medical treatment is generally not successful. Exploratory laparotomy reveals the extent of intestinal lesions and allows determination of whether surgical correction is feasible. The prognosis for patients found eligible for corrective surgery is good to excellent. In comparison, the prognosis is guarded if surgery is attempted >24 hours after onset of clinical signs.

Standard postsurgical care includes:

  • fluid therapy

  • anti-inflammatory therapy

  • antibiotics

There is little or no value in administering anthelmintic medication in the acute phase of a nonstrangulating intestinal infarction. However, deworming with a larvicidal anthelmintic is recommended once the patient is stabilized and peritonitis has been resolved. This can be either ivermectin (0.2 mg/kg), moxidectin (0.4 mg/kg), or a 5-day course of fenbendazole (10 mg/kg).

Prevention of Strongylus vulgaris-associated Disease in Horses

Current S vulgaris control recommendations include a foundation of anthelmintic treatments administered to all horses, with a specific goal of reducing the occurrence of S vulgaris. In most locations, this should consist of one or two strategically administered yearly treatments with an effective anthelmintic.

A detailed presentation of current recommendations for equine parasite control can be found in the American Association for Equine Practitioners (AAEP) Guidelines (see below).

Zoonotic Risk of Strongylus vulgaris-associated Disease in Horses


Key Points

  • Strongylus vulgaris-associated disease presents as a nonstrangulating intestinal infarction.

  • The condition may manifest as colic in the initial phases.

  • Most patients present with peritonitis as the primary finding.

  • The only method of diagnosis is exploratory laparotomy.

  • Surgery as early as possible is necessary to successfully resolve the condition.

  • Current recommendations for equine parasite control include a baseline of anthelmintic treatments administered to all horses to minimize the risk of S vulgaris infection.

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