Veterinary advice should be sought before applying any treatment or vaccine.


Roundworms, Parascarosis

Ascaridiasis is the resulting disease caused by Ascarids (roundworms) which invade and live within the small intestine of the horse. Parascaris equorum is one of the largest types of roundworms that affects horses, which can grow up to 10.5 cm in length as mature adult worms. Ascarids are predominately found in younger horses, under 2 years of age. Ascaridiasis is rare in mature adult horses. Clinical signs differ on presentation depending on the number of worms within the foal's body. When there are large populations, it can often cause a problem with nutrient absorption, as the worms compete with foal for the amino acids and protein in food once ingested by the foal. Foals may develop hypoproteinemia and appear malnourished, with poor quality hair coats, pot-bellied appearance, and stunted growth. Without proper treatment, severe infestations can result in colic and eventually intestinal rupture followed by death.

Another problem affecting foals with ascaridiasis, is the risk of an impaction colic caused by the development and/or inflammatory reaction to the dead larvae in the small intestine, following anthelmintic treatment for the worms (which can occur in less than 24 hours to up to 6 days). The likelihood that the foal will suffer from impaction following treatment is not directly correlated with the number of worms in the intestines. It is thought to possibly be caused by a hypersensitivity reaction to the antigens in the roundworms.

Horses ingest the eggs while they are grazing on pasture grass. Once inside the horse, the eggs will hatch larvae that will immediately burrow into the horse's small intestines and migrate through other organs in order to get to the lungs. Once in the lungs, the larvae often get coughed up and swallowed again by the animal, where they are returned to the small intestines. As the larvae become adults, they begin producing eggs, which are passed by the foal in their feces. The total life cycle takes about 3 months. Many Ascarids have developed a resistance to many of the common chemicals in deworming medications.


Dull or rough haircoat
Pot bellied appearance
Nasal discharge
Stunted growth
Reduced gastrointestinal sounds


  • History
  • Fecal culture
  • Ultrasound of abdomen



Estimation of wormloadCan be assessed by your veterinarian using fecal testing results and an ultrasound examination of the abdomen
Anthelmintic therapyFoals may need to be initially administered a less efficient anthelmintic, to lessen the risk of an impaction caused by the large number of dead nematodes in the small intestines.
Medical therapySupportive therapy by administering mineral oil through nasogastric tubes and treatment of pain and shock
SurgeryMay be indicated if medical therapy fails to relieve the impaction.


  • Environmental management : Practice regular manure control in pastures and/or rotate
  • Group according to age: Group pastures according to age to reduce exposure to younger horses.
  • Feed daily deworming supplement: Products that contain pyrantel tartrate (2.64 mg/kg)

Scientific Research

General Overviews

Age Range

It predominately affects younger horses, under 2 years of age.

Risk Factors

  • Male foals are more at risk than females(67% and 33%, respectively)
  • History of recent treatment with anthelmintics for ascarids