There are more than 40 different species of small strongyles, or cyathostomes, and most of these parasites are smaller than the large strongyle worms. The smaller parasites, however, are no less harmful. Small strongyles can significantly damage a horse's cecum and colon by feeding on the superficial lining of the intestines called the mucosa. Ulcer formation and inflammation of the cecum and colon can occur when the small strongyle larvae move into the wall of the intestines. The tissues of the intestinal wall will encyst, or surround the larvae, protecting them so that they can live inside the mucosa for a prolonged period of time. It is the encysted larvae that cause continued irritation and damage to the intestines.
During late winter and spring, the small strongyle larvae living in the wall of an infected horse's cecum and colon come out of dormancy as mature adults. Large numbers of adult worms can be present within the intestines at once. The presence of numerous adult parasites within the intestines can cause clinical signs such as diarrhea and colic. In severely affected horses and ponies, death can result.
