Most adult horses contain a parasite called strongyle in their gastrointestinal tract. The small strongyles actually cause damage by migrating inside the wall of the intestinal tract of horses. Horses acquire this type of parasite by eating pasture grass that has been contaminated by parasite larvae, which develop from eggs shed in horse manure. These eggs can survive for long periods of time on the pasture, and the contamination can be very severe.
For more information, please read the article Equine Gastrointestinal Parasites.