Large Roundworm / Ascarids

These are the largest parasites reaching a length of up to 30cm long and about the thickness of a pencil. Their eggs, which contain the infective larvae, are well protected by a thick sticky outer coat that enables them to survive for years on pasture and in stables. Horses and foals become infected by swallowing the eggs picked up from the pasture or from cracks and crevices in contaminated stabling.

The larvae migrate through the bloodstream to the liver and lungs before returning to the small Intestine to develop into adults again. These can be present in the foal's small intestine from 12 weeks of age. Adult worms can stop weight gain and lead to the emaciation of the foal. Heavy burdens can block the gut causing digestive problems and in rare cases a fatal blockage or rupture of the gut.

Migrating larval stages can cause 'summer colds' with fever, coughing and loss of appetite as they migrate through the lungs. Foals develop their own immunity to these worms at about 18 months of age.