Pathological evaluation of thigh muscle, sciatic nerve, and spinal cord in downer cow syndrome with emphasis on the prognostic significance

Abstract

To provide more reliable information for predicting prognosis of the downer cow syndrome and expand knowledge of the pathology of this disease, the thigh muscle, sciatic nerve, and spinal cord were investigated histopathologically. Eighteen Holstein-Friesian dairy cows varying in age from 2 to 13 years old (mean 6.3 years old), which presented with either anteparturient or postparturient onset of the downer cow condition, were culled because of poor prognosis. The duration of illness ranged from 1 to 19 days (mean 7.4 days). On histopathological examination, 4 (22%) of the 18 cows did not show any muscle lesions, but the others (78%) with disease duration of ≥ 3 days demonstrated bilateral or unilateral, widespread, obvious degenerative (necrotic) lesions in the thigh muscle. Sciatic nerve of 7 cows (39%) represented no significant lesions, whereas the nerve of the other 11 cows (61%) with disease duration of ≥ 3 days exhibited varying degrees of bilateral or unilateral degeneration of myelinated nerve fibers. This nerve fiber degeneration was identical to Wallerian-type changes of axons. Degenerative lesions in both the thigh muscle and sciatic nerve, which were thought to be extensive and severe to a sufficiently distinctive degree to develop clinical signs, could have occurred following disease duration of ≥ 3 days. Therefore, the possibility that time threshold for the development of the lesions in the thigh muscle and sciatic nerve was at least 3 days after the animals’ recumbency may be raised. Those 3 cows presenting with necrotic muscle lesions showed no lesions in the sciatic nerve, suggesting that, chronologically, the occurrence of sciatic nerve degeneration followed that of muscle lesion. Spinal cord had no significant lesions in 8 cows (44%), whereas in the other10 cows (56%), a minimal degree of axonal degeneration consistent with Wallerian-type degeneration was present in the spinal white matter. This nerve fiber degeneration was considered consistent with that previously documented in otherwise normal spinal cord in mature and aged domestic animals and was unlikely to have been implicated in the development of clinical signs in the present downer cows. With regard to the prevalence rate by lapse of time after the onset of clinical sign, there was no close relationship between the nerve fiber degeneration in the spinal cord and lesions of the thigh muscle and sciatic nerve.