NEUROAXONAL DEGENERATION IN SHEEP GRAZING SORGHUM PASTURES

During the fall of 1992, 250 (10%) of 2,500 Rambouilet cross feeder la mbs grazing Sorghum bicolor developed neurologic signs including weakn ess, ataxia, head shaking, knuckling of the fetlocks, inability to ris e, and opisthotonos. One hundred fifteen (46%) of the affected lambs d ied. Twenty of the surviving lambs exhibited residual neurologic signs of ataxia when stressed. At the same time, 275 (25%) of 1,100 ewes gr azing a nearby sudex pasture (S. sudanese x S. bicolor) gave birth to lambs that were weak and unable to rise. Newborn lambs exhibited exten sor rigidity and opisthotonos when assisted to a standing position. Th e dystocias that occurred were due to lambs with contracted limbs (art hrogryposis). All affected lambs died or were euthanized. Histologic e xamination of the brains of 3 feeder lambs and 9 newborn lambs reveale d similar microscopic lesions. The predominant change was the presence of focal axonal enlargements (spheroids) in the proximal segments of axons, which were restricted to the nuclei of the medulla, cerebellum, and midbrain. In addition, the spinal cord contained spheroids in the ventral horn gray matter of the 6 newborns examined. Ultrastructurall y, the spheroids were composed of aggregates of neurofilaments, mitoch ondria, vesicular bodies, and dense bodies bounded by a thin myelin sh eath. There was mild gliosis in the more severely affected animals of both groups. There was minimal Wallerian degeneration in the white mat ter adjacent to affected nuclei in the brain and the ventromedial and dorsolateral funiculi of the spinal cord. This is the first detailed r eport of Sorghum toxicity in sheep.