Strongylus spp and Triodontophorus spp are common intestinal parasites of horses, found worldwide. These worms, located in the large intestine and cecum, are stout-bodied. Strongylus vulgaris is the smallest at 2 cm long. S. equinus reaches 5 cm and is the largest of the three. When freshly collected, all are reddish brown due to the ingestion of blood.
The eggs of the 3 species develop to infective third stage larvae on pasture. Moisture is critical for larval survival, and warmth speeds development to as little as 3 days. Third- stage ingested larvae drop their protective sheath in the small intestine. After this point, the Strongylus species have different developmental patterns:
- Strongylus vulgaris - the third-stage larvae penetrate the intestinal mucosa, where they molt to fourth-stage larvae in several days. Penetrating nearby blood vessels, the larvae wander through the arteries for 2 weeks before reaching the anterior mesenteric artery, where they remain for 4 months. After moulting to immature adults, S. vulgaris returns via the arteries to the large intestine and burrows into the lumen. Six to 8 months after the original infection eggs begin to pass in the feces.
- Strongylus equinus - after burrowing into the submucosa and moulting to fourth-stage larvae, these parasites migrate to the liver where they wander for 6-7 weeks. Emerging from the liver, they molt to to immature adults in various abdominal organs, then return to the large intestine. About 9 months after infection mature adults lay eggs.
- Strongylus edentatus - larvae penetrate the intestine and migrate via the portal vein to the liver, where moulting occurs. After 9 weeks, the fourth-stage larvae wander in the peritoneum, causing the formation of nodules. Migrating to the large intestine, the strongyle larvae form nodules in the gut wall, which they rupture to enter the colon lumen.
The large strongyles are the most important parasites of horses. Of the three species, S. vulgaris is the most pathogenic.
Larvae irritate the walls of the small intestine and the arteries. Strongylus vulgaris larvae cause roughening of the arterial walls, providing sites for clotting. These clots may break off and lodge in other vessels, cutting off the blood supply to various body parts. The weakened arteries may develop aneurysms, or more correctly, verminous arteritis. These aneurysms may burst, causing death. Adult large strongyles are "plug feeders," which feed by ingesting plugs of mucosa and capillaries. Heavy feeding of this type produces intestinal damage, anaemia, fluid loss into the intestine, and blood protein loss. Intestinal damage causes diarrhea, fever, edema, anaemia, anorexia, depression, weight loss, and dehydration. Lameness may result from hind leg circulation being blocked by blood clots. Strongylus equinus and S. edentatus cause liver damage and peritonitis. The majority of colic in horses has been attributed to the lesions caused by the migratory stages of S. vulgaris.
Affected horses appear normal at rest; however, graded exercise results in an increasing severity of weakness of the hindlimbs with unilateral or bilateral lameness, muscle tremor, and sweating. Severely affected animals may show signs of exercise intolerance, weakness, and atypical lameness that resolves after a short rest. Subnormal temperature of the affected limbs may be detectable, along with decreased or absent arterial pulsations and delayed and diminished venous filling. Rectal palpation may show variation in pulse amplitude of the internal or external iliac arteries (or both) and asymmetric vasculature. In severe cases, the hindquarter muscles atrophy, and lameness may become evident with only mild exercise. Complete embolic or thrombotic occlusion of the distal aorta may produce acute bilateral hindlimb paralysis and recumbency in horses. Affected animals are anxious, appear painful, and rapidly go into shock. The hindlimbs are cold, and rectal palpation reveals an absence of pulsation in either iliac artery. Transrectal ultrasound can be helpful in determining bloodflow in the aorta and iliac arteries.
After adult worms are present, eggs may be found in the feces. Larvae are needed for species identification. Diagnosis is difficult during the prepatent, migratory phase.
Treatment with an effective anthelmintic is necessary. In particular, since foals are very susceptible, brood mares should be treated and moved to clean pastures. Overcrowding must be avoided. Removal of feces, plowing to break up dense ground cover, and frequent stool checks will aid in control of Strongyles.