Humpy back of sheep. Clinical and pathological observations

Humpy back, a disease of Merino sheep in western Queesland, occurs during mustering for shearing. It is usually seen in summer 6-10 weeks after substantial rainfall and is thought to be caused by the ingestion of a toxic plant. The disease is characterised clinically by a short-stepping, stilted gait of the hind limbs, followed by lowering of the head, arching of the back and inability to continue walking. Histopathological examination of 8 cases from 5 properties revealed a Wallerian degeneration of the white matter throughout the length of the spinal cord with the ventral and lateral columns most severely affected. A similar degenerative change was seen in the posterior cerebellar peduncles of 3 of the sheep. A more severe hind limb incoordination with more extensive degeneration of the white matter of the spinal cord, medulla and cerebellum was seen in a case of humpy back of two years duration. Similar, but much milder, spinal cord lesions were found in apparently unaffected sheep from the same group as the sheep affected with humpy back on 2 properties. Severe myodegeneration of hind limb muscle groups was seen in 3 affected sheep. It was thought to be associated with the long rail journey (1500 km) to the laboratory after the sheep were affected in the field.