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What are Large Strongyles?

Large strongyles are a subsection of a type of parasite known as a nematode, or roundworm. Horses acquire these worms, also known as bloodworms or redworms, by ingesting the larvae while grazing. There are three varieties of long strongyles that can infect horses; Strongylus edentatus, Strongylus equinus, and Strongylus vulgaris. S. edentatus and S. equinus exit the intestines and travel to the liver or pancreas, but S. vulgaris migrates to the blood vessels instead, which can induce anemia, weight loss, and even aneurysms.

Large strongyles, also known as bloodworms, are parasites that infest horses and then migrate to either the liver or the blood vessels. Blockages caused by large strongyles can become critical quickly.

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Symptoms of Large Strongyles in Horses

The symptoms that are experienced by horses infected with large strongyles can be varied, depending on the variety of strongyles and the intensity of the infestation. Symptoms that your horse has been infested by strongyles parasites may include: 

  • Anemia
  • Aneurysm
  • Colic
  • Dehydration 
  • Depression
  • Diarrhea
  • Emaciation
  • Fever
  • Loss of appetite
  • Weakness
  • Weight loss
  • Sudden death (from burst aneurysms)


Close relatives of large strongyles, small strongyles also live in the equine gut, but they do not penetrate the intestinal wall. Instead, they dig into the intestinal lining and remain there protected by a cyst-like structure in a dormant state known as encysted. Although most horse infested with small strongyles remain asymptomatic, scores of small strongyles can cause serious damage to the lining of the intestine. Critical infestations by small strongyles can lead to colic, diarrhea, weight loss, dull coat, and poor growth. There are only two types of dewormer available for small strongyles that will also help to eliminate the worms in their protected dormant state.

Causes of Large Strongyles in Horses

Strongylus edentatus

Once this toothless nematode has been ingested it burrows its way out of the wall of the gut, and into the bloodstream. It takes about nine months to mature and spends that nine months traveling through the organs of the abdomen, like the liver and the pancreas. Once it has matured its size can reach four to five centimeters long, at which time it returns to a part of the intestine known as a caecum.

Strongylus equinus

This parasite has three teeth and reaches about five centimeters in length. When they have been consumed, they migrate to the veins that lead towards the liver and follow them to their destination. Once in the liver, they develop for approximately eight weeks before again traveling to the intestine and producing more eggs. 

Strongylus vulgaris

This is the most divergent of the strongylus species that affect equines, and the most dangerous. It is equipped with two teeth and is smaller than the other large strongyles, only reaching two centimeters in length. Instead of migrating to the abdomen or the liver it settles in the blood vessels where it can cause its host hazardous complications like blood clots and blood loss.

Diagnosis of Large Strongyles in Horses

When your equine veterinarian examines your horse, they will perform a complete physical evaluation of the animal, as well as a complete blood count and equine blood chemistry panel. If the patient has any internal bleeding occurring, the blood tests will likely show anemia. A definitive diagnosis is almost always made by using a fecal float test to identify to offending parasite. The eggs that are found in the feces can tell the examiner that a strongylus infestation is currently taking place, but the final determination of the variety of strongyles requires the direct evaluation of the larval worms that have been expelled from the digestive system. If the examining veterinarian believes there are unhealthy or dangerous levels of damage to the circulatory system, either an arteriography (an x-ray of the artery) or ultrasound may be recommended to detect ruptures or aneurysms that have been caused by the worms.

Treatment of Large Strongyles in Horses

If your horse appears to be in distress, supportive treatments are likely to be started right away. This may include intravenous fluids to prevent dehydration and to help balance the electrolytes and sugars in the blood. Dewormers such as moxidectin and ivermectin are generally successful at killing both the adult worms and the larval form of this parasite. Piperazine is not an effective defender against large strongyles. It is crucial to discuss the deworming schedule for your horse with a veterinarian and not attempt to self-treat this type of infestation. 

If the worms are present in significant numbers when the deworming medication is administered, then the worms may try to migrate all at once. This occurrence can cause the rupture of the intestines, which may cause death to the patient. Foals can be particularly sensitive to the effects of this parasite, which is transmitted to them in their mother’s milk. It is typically advised that broodmares who test positive for large strongyles receive treatment well before the foal is expected to be born to prevent the transfer of the parasites to the foal.

Recovery of Large Strongyles in Horses

One way to avoid having your horse pick up the parasites in the first place is to practice good pasture management. There are several ways that you can make your field a safer place to graze:

  • Alternating or co-grazing with other grazing animals - As the parasites that infect cattle are often very different from the parasites that infect horses, this can reduce the populations of both types of parasite in that pasture
  • Avoid overgrazing - The strongyles larva tend to live at the base of the plants, nearer to the ground; this makes it easier for your horse to pick up these parasites in pastures that are short due to overgrazing
  • Manure removal - Removing the manure from the pasture on a regular basis also reduces the parasite loads in the field and is particularly recommended for pastures frequented by high-risk equines such as foals and pregnant mares