Roundworms in Horses

Apr 01, 2011
Preventative/Preparedness parasites | roundworms

Roundworms (aka Ascarids) are a common parasite of even well-managed young horses. These large, white worms in their adult form are sometimes passed in horse’s manure (often just after deworming) and are noticed because of their size and stark color. Adult Roundworms are the largest parasite that infects horses. Weanlings, yearlings and 2 year olds are commonly affected, although Roundworms are sometimes present in horses older than 2 years of age and geriatric horses are susceptible as immunity wanes with age.

Roundworm infestation can cause young horses to look unthrifty with poor hair coats and/or slow growth and gain or show upper respiratory symptoms such as nasal discharge and coughing as larvae migrate throughout the lungs and trachea. Sometimes, there are no outward signs at all! This is why it is important to have fecal examinations performed prior to deworming, especially on young horses housed together or with older horses. Because Roundworms have demonstrated resistance to Ivermectin, Fecal Egg Reduction Count Tests (FERCTs) are often performed 2 weeks after deworming to determine dewormer efficacy. Roundworm infection can have deadly consequences and therefore, making sure your deworming protocol is effective is of utmost importance!

Roundworm Lifecycle

  • Roundworms live in the small intestines of young horses (rare in horses > 2 years of age). Horses usually develop immunity against Roundworms by 2 years of age.
  • Infective larvated eggs in horses’ manure and/or in environment are accidentally ingested by other horses, larvae hatch and undergo tracheal, liver and/or lung migration (coughing may be observed) and then mature into adult worms in small intestine – this entire process takes approximately 10 – 12 weeks.
  • Adult worms produce 200,000 eggs/day that are passed in their host horse’s manure.
  • Roundworm eggs may survive for years in the environment but take approximately 2 weeks – months to develop into infective larvated eggs.

Signs of Roundworm Infestation

  • Many foals and young horses can show symptoms of Roundworm infection as early as 10 – 13 weeks of age.
  • Large numbers of adult Roundworms can cause intestinal obstruction, intussusception (when one piece of bowel gets bunched up inside itself) and possibly rupture of the small intestine resulting in peritonitis and abscess formation.
  • Sometimes young horses will spontaneously pass adult Roundworms at 28 weeks or so.
  • As Roundworm larvae migrate through lungs fever, nasal discharge, coughing and even pneumonia can occur.
  • Roundworm infection of the small intestine can cause young horses a chronic, unthrifty appearance, dull hair coat, pot belly, slow growth (small size for their age and breed), dry mucus-covered stools and colic symptoms.

Parasite Control

  • Control measures include assuring clean pastures and evaluating pasture contamination by running fecal exams on horses housed on group pasture.
  • Roundworm eggs (which are microscopic) can stick to a mare’s udder and should be washed with gentle soap and water before foals nurse for the first time and on a regular basis thereafter to prevent transmission to foals.
  • Deworm young horses every 2 months starting at 6 – 8 weeks of age with Strongid (Pyrantel pamoate) or Panacur (Fenbendazole) until 1 year of age (can start using Ivermectin at 6 months of age) to prevent overwhelming Roundworm infections. Yearlings can start Adult horse deworming protocol.
  • In young horses with known overwhelming Roundworm infection , the best treatment may be Fenbendazole or Piperazine at low doses for a few days so as to avoid Ascarid impaction.
  • Please make your veterinarian aware if you are concerned about the possibility of Roundworm infection so they can be available to evaluate, guide and assist in order to avoid potential deadly complications.
Jessica Marsh

Written by Jessica Marsh, DVM