Threadworm (Foals) Average Cost

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What is Threadworm (Foals)?

Your foal can become infected with threadworms by the time they are only a few days old. Once the threadworm larvae are ingested by the foal, they migrate to the lungs where they move up the trachea to be swallowed. After they make it to the small intestine, they grow to maturity and lay their eggs, which get eliminated in the feces. Once they hatch, they start the process all over again. This process can all happen within two weeks. Infestation with threadworms can be a dangerous condition due to the damage they can do to the lungs and intestinal tract. Some of the complications include bleeding into the lungs and colic.

Threadworms in foals are caused by Strongyloides westeri (parasitic roundworms) transferred in the milk from mares while the foals are nursing. The worms are only about one centimeter long, but they can do serious damage to the foal when migrating through the liver and lungs, damaging the lining of the arteries. The intestinal lining is especially susceptible and the damage to it causes gastrointestinal symptoms such as diarrhea, appetite loss, and possible weight loss. Lung infiltration is also dangerous because of the damage these worms can cause. However, the most dangerous side effect of a large infestation is severe diarrhea which may produce dehydration. In a foal, any amount of dehydration can be fatal because of their size and frailty.

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Symptoms of Threadworm (Foals) in Horses

The symptoms of threadworm can happen fast and be mistaken for other illnesses such as respiratory or intestinal disorders. While drinking milk from the mare, your foal may be ingesting the larvae of these threadworms and they are transferred through the lungs and intestines. What starts out sounding like a respiratory infection but includes gastrointestinal symptoms in a foal will usually turn out to be threadworms.

  • Coughing
  • Wheezing
  • Trouble breathing
  • Loose, watery stool
  • Loss of appetite
  • Dermatitis (if worms enter through skin)
  • Dull hair coat
  • Depression
  • Lethargy
  • Weakness

Causes of Threadworm (Foals) in Horses

The causes of threadworm include:

  • Ingesting larvae from the threadworm in mother’s milk.
  • Threadworms that live in horse’s bedding can enter through the skin.

Diagnosis of Threadworm (Foals) in Horses

Diagnosing threadworms in your foal can be difficult because the symptoms are similar to many other conditions such as colic, toxic chemical ingestion, bacterial infection, salmonella infection, pneumonia, or infestations by other parasites. The only way to be able to get a definitive diagnosis is by finding eggs or parasites in your horse’s feces. However, if there are no eggs or worms in the stool, it does not mean that your foal does not have threadworms. Not all stool samples will have eggs or worms. It may take several samples before the test comes back positive for threadworms, but the veterinarian will most likely go ahead and start treatment.

In addition, routine blood tests will be ordered such as a blood count, chemistry panel, blood cultures, and a packed cell volume test to see if your foal is dehydrated. Other tests your veterinarian may perform to further rule out other illnesses are an ultrasound to check for intestinal damage or blockages, radiographs (x-rays), and possibly an endoscopic examination to be sure there are no other issues that need to be addressed.

Treatment of Threadworm (Foals) in Horses

The only way to successfully treat your foal for threadworms is by giving your horse medication such as macrocyclic lactones. However, other measures may be needed to treat side effects of threadworm infestation.


Some of the drugs that your veterinarian may use to treat threadworms are anthelmintics such as oxibendazole, benzimidazole, moxidectin, or ivermectin. Unfortunately, threadworms can be resistant to drugs and some are able to become dormant for an extended period by entering into the muscle tissues. Therefore, the veterinarian will likely have you treat your foal several times in order to eliminate them all.

Fluid Therapy

To prevent dehydration from diarrhea, the veterinarian may keep your horse in the hospital to administer fluids and electrolytes intravenously. The foal will remain in the hospital until they are no longer dehydrated, which usually only takes a few hours.

Recovery of Threadworm (Foals) in Horses

Chances of recovery are excellent, but since recurrent infections are likely, it is important to treat your horse again in six months and return to the veterinarian for another fecal examination and blood tests. To prevent this from happening again, it is recommended that you give your mare a deworming medication within 24 hours of her giving birth. This reduces the chance of transmitting the threadworm to her foal when nursing.