Large Strongyles

Large strongyles are parasitic worms, which are normally treated with horse wormers. There are three major species: Strongylus Vulgaris (up to 2.5cm in length.), Strongylus Edentatus (up to 4cm) and Strongylus Equinus (up to 5cm).

Large strongyles have a number of common names, including: blood worms, palisade worms, red worms, and sclerostomes. 


The life cycle for these parasite is:

  1. The adult form lives in the intestine, where they produce eggs which pass out of the horse with the dung.
  2. The eggs then hatch into larvae, which crawl into the grass.
  3. Horses eating the grass can consume these larvae, which is the means by which horses are infected with the parasite.
  4. The larvae leave the intestine, then migrate around the horse's body, before returning to the intestine.
  5. Upon returning to the intestine, the larvae mature into the adult form, where they start producing eggs to complete the life cycle.

The three species differ mainly in step 4, in terms of where they migrate:

  • Strongylus Vulgaris - The larvae migrates around the Cranial Mesenteric Artery (the artery which feeds blood to the intestines) and branches of this artery. There they feed on blood and mature for about 4 months. Upon returning to the intestine, they continue to mature for 6-8 months, after which they are adult and start producing eggs.
  • Strongylus Edentatus and Strongylus Equinus - The larvae of these two species do not may migrate in the Cranial Mesenteric Artery, instead migrating to the liver and then various other parts of the body (including pancreas, perirenal tissues,  or retroperitoneal tissues). They spend 6-9 weeks maturing in the liver, before eventually returning to the intestine, to mature to adults and start producing eggs.

Symptoms and Diagnosis

These parasites feed on blood and if the parasites are present in large numbers, the resulting blood loss and lead to a variety of issues: anemia, weakness and emaciation. Diarrhea is also common.

As strongylus vulgaris migrates through the cranial mesenteric artery, they damage this artery, which can lead to a variety of additional issues:

  • Inflammation of the artery (arteritis).
  • Blood clots (resulting from damage to the artery wall), which can reduce local blood flow (starving part of the intestine of blood) or can break off. Blood clots that break off can travel elsewhere in the body, causing reduced blood flood or blood blockage where they eventually end up.
  • As this artery is responsible for blood flow to most of the intestine, damage to it can result in a variety of intestinal problems (some of which can be fatal), including: colic, gangrene, intestinal statis/torsion/intussusception/rupture.
  • In some cases the damage to the artery is sufficiently severe that it can rupture, and the horse can internally bleed to death.

Diagnosis is normally done by examination of the dung (feces) for eggs.


Horses which are infected with Large Strongyles are usually also infected with Small Strongyles, so consideration should be given to treating both types of parasites if large strongyles are diagnosed.

Of the various medications, ivermectin is the most effective as it treats all three species and in all stages. Other common types of wormer address only some of the worms (which may be sufficient, depending on the species your horse is infected with. For details, see wormer medications