The roundworm larvae are returned to the small intestine where they mature to egg producing adults, completing the life cycle. The life cycle takes about three months. Physical damage such as inflammation and scarring of liver and lung tissue occur in the horse during migration. Adult roundworms can cause physical damage, ranging from mild digestive upset and lower feed absorption, to severe colic, due to intestinal blockage or intestinal rupture. Clinical signs of roundworm infection may include unthriftiness, potbelly, rough haircoat, and slow growth.
Some young horses develop nasal discharge accompanied by a cough as a result of larvae migration.
Strongyloides westeri is an intestinal parasite that can infect foals as early as four days of age. A foal becomes infected by ingestion of larvae in the dam s milk or by penetration of the foal s skin by infected larvae in the bedding. However, the larvae are not present in colostrum. The larvae migrate through the lungs and the small intestine. The life cycle can be completed in less than two weeks.