Rida in sheep. Pathological and clinical aspects

Summary

In Rida there is a symmetrical affection of a certain number of brain stem nuclei many of which are related to the cerebellar structures. This consists of neuronal vacuolation with varying degrees of fibrillary gliosis and very slight cell loss.

Considering the slight degree of neuronal cell loss, the severity of the gliosis exceeds what would be expected to be a normal response. The gliosis always covers a larger area than that occupied by vacuolated cells. The gliosis also affects a certain number of myelinated structures such as the inferior and middle cerebellar peduncles and the transverse fibres of the medulla and the pons. It is but of proportion to the slight neuronal loss and is only rarely accompanied by a microglial reaction. The way in which the gliosis affects the brain stem nuclei and the myelinated fibres connecting or surrounding them is at certain levels, suggestive of a system disorder.

Two nuclei belonging to the cerebellar system are more affected in Rida than in Scrapie: the dentate nucleus and the red nucleus.

A slighter degree of neuronal vacuolation may be found in other motor nuclei of the brain stem but it never reaches the same severity as in the nuclei mentioned above.

Considering the interpretation of these lesions, which have been so well discussed by Beck Daniel and Parry in their work on Scrapie, we have nothing to add. Although we found the same general alterations as they describe, Alzheimer type II cells were not demonstrated in our cases.

Apart from a few details mentioned above, the neuropathological changes in Rida and Scrapie are identical. Perhaps they are not entirely the same condition, but they certainly belong to the same group, at least from the neuropathological point of view.