Congenital intestinal aganglionosis in white foals.

A congenital and probably hereditary neurological defect has been identified in the intestinal tract of six foals produced from the breeding of overo (a type of spotting pattern) horses. The foals had white hair and pink skin with the exception of occasional pigmented foci about the muzzle, ventral abdomen, and hindquarters. The foals appeared normal at birth, but within a few hours developed symptoms of colic. At necropsy, the only significant finding was a narrow, pale segment of large intestine. This abnormality either was confined to the small colon and rectum or involved the entire colon and rectum. Microscopically, myenteric and submucosal neuronal plexuses were absent throughout the large intestine and extensive portions of the small intestine. The only other significant finding was the lack of melanin in the skin.