The roundworm parasite of horses is Parascaris equorum. These eggs are very resistant and ubiquitous in horse environments. Virtually all young foals are exposed to roundworm parasitism. By approximately 10 months of age, the young horse has developed immunity and essentially eliminates the adult worm in the intestinal system. Roundworms are rarely seen in the intestinal tract of adult horses. Clinical problems associated with roundworm parasitism include failure to thrive, diarrhea, poor growth, impaired performance, anemia, weight loss and increased risk for colic. One should be careful not to give a young horse that hasn't been wormed regularly a full dose of dewormer, that could kill all the worms simultaneously and block the intestines.
For more information, please read the article Equine Gastrointestinal Parasites.