Parascaris equorum (Ascarids or Roundworms)

What you need to know

Eggs are passed in the dung and become infective larvae in the grass. While grazing, the horse swallows the larvae, which hatch and burrow into the walls of the intestine. From there, they are carried by the bloodstream into the liver and lungs. The horse coughs up the larvae and swallows them again. Larvae mature into egg-laying adults in the intestine. Ascarids are especially dangerous to foals 6 months or younger. Severe infection in young horses can build up quickly and lead to liver and lung damage, poor growth and even death. Larvae in the bloodstream can cause coughing, fever, pneumonia, bleeding lungs and respiratory infections. In the adult stage, ascarids live in the small intestine where they can cause colic, blockage, ruptured gut and death.

Effective wormer chemicals: Ivermectin, Moxidectin, Oxfendazole, Oxibendazole, Pyrantel pamoate, Fenbendazole (may require more than 1 syringe for a heavy horse).

More information

Adult Parascaris equorum are stout, whitish worms, up to 30 cm long, with three prominent lips. The life cycle is similar to that of Ascaris suum (the roundworm of pigs, Ascaris sp), with a prepatent period of 10-12 wk. Large numbers of infective eggs can remain viable for years in contaminated soil. Adult animals usually harbour very few worms. The principal sources of infection for young foals are pastures, paddocks, or stalls contaminated with eggs from foals of the previous year.

In heavy infections, the migrating larvae may produce respiratory signs ("summer colds"). In heavy intestinal infections, foals show unthriftiness, loss of energy, and occasionally colic. Intestinal obstruction and perforation have been reported. Diagnosis is based on demonstration of eggs in the faeces. If disease due to prepatent infection is suspected, diagnosis may be confirmed by administration of an anthelmintic, after which large numbers of immature worms may be seen in the faeces.

On farms where the infection is common, most foals become infected soon after birth. As a result, most of the worms are maturing when the foals are ~4-5 mo old. Treatment should be started when foals are ~8 wk old and repeated at 6- to 8-wk intervals until they are yearlings. All of the broad-spectrum equine anthelmintics are effective against the adult and immature worms in the small intestine and, therefore, ascarids are readily controlled by an appropriate worm control program. In cases in which verminous pneumonia due to Parascaris migration has occurred, therapeutic benefit may be achieved by treatment with ivermectin or fenbendazole (the latter at 10 mg/kg/day for 5 consecutive days) concurrent with appropriate antimicrobial therapy. Parascaris infection can be effectively prevented by daily administration of pyrantel tartrate once foals consume grain on a regular basis.