Peripheral nerve lesions in a case of equine motor neuron disease.

A male 14-year-old Arab horse was pathologically diagnosed as equine motor neuron disease (EMND), which was kept as a breeding horse on a farm in Tokachi district of Hokkaido in Japan. On examination of the peripheral nerves, the most characteristic feature was Wallerian-type degeneration revealed by myelinoclasis associated with myelin ovoids which were sometimes infiltrated by macrophages. The other abnormalities were axonal swellings which were surrounded by thin myelin sheaths. Ultrastructurally, the axonal swelling was due to an accumulation of neurofilaments, and was accompanied by a thin and degenerating myelin sheaths. In teased nerve fiber preparations, the most conspicuous change was myelinoclasis represented by segmentation into myelin ovoids or balls. Occasionally, segmental demyelination and axonal degeneration characterized by multifocal axonal swelling were observed.