What are Roundworms?

Mature roundworms are white in color, females can grow up to 20 inches long and males grow up to 10 inches. Roundworms usually infect young horses, which have not developed an immunity to the parasite. Roundworms do not only cause damage to the liver and lungs, they also deprive the horse of valuable nutrients. The equine roundworm can only infect other horses and donkeys, it can’t infect other species of animals or humans.

When a horse ingests the roundworms (ascarids) larvae; the eggs hatch in the intestines.  The larvae travel through the bloodstream to the liver and heart, then migrate into the horse’s lungs.  Once in the lungs, the roundworms are coughed up, re-swallowed and enter the stomach where they begin to lay eggs; this becomes a recurrent cycle.

Symptoms of Roundworms in Horses

Symptoms may include:

  • Coughing
  • Pot belly
  • Rough hair coat
  • Slow growth in foals
  • Respiratory distress
  • Nasal discharge
  • Fever
  • Colic
  • Liver damage
  • Diarrhea
  • Pneumonia
  • Weight loss
  • Depression
  • Malnutrition
  • Stunted growth
  • A large number of roundworms can cause intestinal blockage or rupture


Causes of Roundworms in Horses

Roundworms in horses are caused by the ingestion of roundworm larvae.  The eggs are ingested by:

  • The horse ingests contaminated hay.
  • Drinking water that has roundworm eggs.
  • Feeding the horse on the stall floor, not using feeding trays
  • The horse forages in a pasture which has contaminated manure from another horse


Diagnosis of Roundworms in Horses

The veterinarian will want to discuss the deworming protocol typical for your horse. Clinical signs like coughing and rough haircoat may point to a roundworm infestation; in addition, a horse that looks malnourished will be suspected of having worms. Usually, the problems with roundworms are seen in young horses and because of this colic and impaction are a risk.

Diagnostic blood work may be recommended in order to rule out other conditions that present similarly to a roundworm infestation; a fecal sample may be taken although the larvae of the roundworm cannot be seen. The veterinarian may want to take x-rays of the lungs and an ultrasound of the abdomen. Roundworms will migrate to the lungs, causing breathing difficulties, and to the intestines where impaction can become a danger.



Treatment of Roundworms in Horses

If roundworms are diagnosed the veterinarian will recommend that the horse is dewormed. Horses and foals that share the stall with the patient should also be dewormed. Dewormers come in a paste or liquid suspension form.  Common equine deworming medications are ivermectin, panacur or pyrantel. Stalls, feeding trays and water buckets should be thoroughly cleaned. Additionally, the pasture where the horse grazes should be cleaned of any manure.

Horses that have a severe case of roundworm impaction will need to undergo surgery.  The roundworms must be removed before there is an intestinal rupture.  The veterinarian will refer you to an equine surgeon. Patients that have suffered malnutrition or that are anemic will need to be placed on dietary supplements and vitamins. The veterinarian may prescribe B-12 shots.



Recovery of Roundworms in Horses

Horses that are diagnosed with roundworm in the early stages have a better prognosis. Follow up visits will be necessary to monitor your horse’s progress.  The veterinarian may want to have bloodwork, fecal exam and x-rays retaken. If the roundworms are still present, additional dosages of a dewormer may be necessary.  

Horses that underwent surgery will need to stay hospitalized for 5-7 days. Medications and fluids are administered to the patient through an IV.  Once the patient is released the equine surgeon will provide you with post-operative instructions.  The horse will be on stall rest for 6-8 weeks. The veterinarian may prescribe corticosteroids to help reduce inflammation and antibiotics to help prevent a bacterial infection. The patient’s diet may be modified while he is recovering.  Sutures and or staples will need to be removed by the veterinarian.

It is important to avoid another infestation of roundworms.  The horse should be on a regular deworming preventative plan. Typically horses are dewormed every 6 months. If you have any questions about what dewormer to use or how often to administer it, discuss this with the veterinarian.  When administering a dewormer to a foal, it is important to make sure that the dewormer is approved for young horses. You may choose to have the veterinarian give the dewormer as some horses may resist the procedure.

Regular sanitation is another way to prevent parasites from harming your horse. Pastures and stalls should be cleaned daily of manure. Feeding troughs and water buckets should also be cleaned and scrubbed down on a regular basis.



Roundworms Questions and Advice from Veterinary Professionals



Quarter horse




Unknown severity


2 found helpful


Unknown severity

Has Symptoms

Worms In Poo

Just purchased.One worm in a pile of manure. Which worm please. Point end. Flat end. 5" round. I disected it.

Sept. 27, 2020



Dr. Michele K. DVM


2 Recommendations

Thank you for your question. . We typically look at the eggs of the worms under a microscope to see what worm it might be, as many of them look similar as adults. It would be best to have your pet seen by a veterinarian, as they can examine them, see what might be going on, and get any treatment that they might need.

Oct. 13, 2020

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