Disease Overview

Horse ascarids (Parascaris equorum), or roundworms, are stout yellow or white worms that can reach up to 15 inches long and primarily affect foals. What’s more, they are the most significant parasite threat to young horses. Foals ingest infective larvae. If enough larvae are present in the body, they can cause respiratory signs, or “summer colds,” as they migrate. Adult roundworms can cause physical damage from intestinal blockage or intestinal rupture.

Merck Animal Health Solutions

Parasite Life Cycle

Infection affects the central nervous system.

  • Adult ascarids living in the horse’s small intestine lay eggs.
  • One-celled eggs exit the horse’s body in feces.
  • Eggs become infective and can remain viable for years in contaminated soil in pastures, paddocks or stalls.
  • The horse ingests infective larvae that migrate to the liver and lungs.
  • Larvae are coughed up and swallowed, entering the small intestine again.
  • Larvae become egg-laying adults.

Clinical Signs

  • Lack of thriftiness and poor growth
  • Lack of energy
  • Coughing
  • Nasal discharge
  • Colic signs associated with small intestinal impaction or rupture

Risk Factors

  • Age 2 years old or less
  • Breeding operation environment
  • Contamination of stalls from previous year’s foals
  • Compromised immune system


“AAEP Internal Parasite Control Guidelines,” American Association of Equine Practitioners, 2019,

“Ascarids in Horses,” Extension Foundation, Jan. 22, 2020,

Thomas R. Klei, “Gastrointestinal Parasites of Horses” Merck Veterinary Manual, June 2019,

Important Safety Information

Do not use in horses intended for human consumption.