Ascarids (Parascaris equorum) are large worms (adults are 4"-15" long, pale worms that look like bean sprouts), which are the most important parasite of young horses. The adult stage live in the small intestine of growing horses. Small numbers of the parasites do not cause much problem in young horses, but large worm burdens cause classic signs of intestinal parasitism- poor growth, pot belly, poor hair coat, etc. This parasite is uncommon in adult horses.

"Ascarid impaction" usually occurs after worming a young horse that has a large parasite load. Huge numbers of these large parasites die en masse, drift downstream and obstruct the intestine, causing signs of colic. The dead parasites also cause an intense inflammatory response in the small intestine,damaging the lining. Severely obstructed horses can even require colic surgery to remove the obstruction.

There is increasing evidence of ascarids being resistant to ivermectin, and some farms are seeing increasing problems.

It is important not to start worming young horses before 60-70 days of age. Benzimidazole wormers are more effective against ascarids than ivermectin is, and kills the parasites more slowly, thereby making impaction of dead worms less likely.