GI parasites of horses are more completely covered in Gastrointestinal Parasites of Horses Gastrointestinal Parasites of Horses
The parasites most likely to cause chronic diarrhea in horses and foals are large and small strongyles and Cryptosporidium spp. The condition associated with small strongyles in horses is termed cyathostomiasis and has been reported to result in recurrent colic, diarrhea, and weight loss.
Cryptosporidia Cryptosporidiosis have been identified in the feces of both healthy and diarrheic foals. There is evidence that Cryptosporidium spp can cause diarrhea and even death in immunocompetent foals; cryptosporidia have been described as a cause of outbreaks of foal diarrhea on some farms. Giardiasis (Giardia)