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Equine ileocolonic aganglionosis, which is also called lethal white foal syndrome (LWFS), is a severe congenital condition characterized by the unsuccessful colonization of neural crest progenitors in the caudal part of the small intestine and the entire large intestine. LWFS, which is attributable to a mutation in the endothelin receptor B gene, is the horse equivalent of Hirschsprung's disease in humans. Affected foals suffer from aganglionosis or hypoganglionosis of the enteric ganglia resulting in intestinal akinesia and colic. In other species with aganglionosis, fibers of extrinsic origin show an abnormal distribution pattern within the gut wall, but we have no information to date regarding this occurrence in horses. Our present aim is to investigate the distribution of extrinsic sympathetic and sensory neural fibers in LWFS, focusing on ileum and the pelvic flexure of the colon of two LWFS foals compared with a control subject. The sympathetic fibers were immunohistochemically identified with the markers tyrosine hydroxylase and dopamine beta-hydroxylase. The extrinsic sensory fibers were identified with the markers Substance P (SP) and calcitonin gene-related peptide (CGRP). Since SP and CGRP are also synthesized by subclasses of horse intramural neurons, LWFS represents a good model for the selective study of extrinsic fiber distribution. Affected foals showed large bundles of extrinsic fibers, compared with the control, as observed in Hirschsprung's disease. Furthermore, altered adrenergic pathways were observed, prominently in the pelvic flexure. The numbers of SP- and CGRP-immunoreactive fibers in the muscle, a target of enteric neurons, were dramatically reduced, whereas fibers deduced to be extrinsic sensory axons persisted around submucosal blood vessels. Fiber numbers in the mucosa were reduced. Thus, extrinsic innervation, contributing to modulate enteric functions, might also be affected during LWFS.