Abstract
A 7-year-old male Thoroughbred horse exhibited recurrent falls followed by convulsive seizures. The horse was clinically diagnosed with epilepsy and phenobarbital treatment was initiated. However, as seizure control was unsuccessful, the animal was euthanized. At necropsy, yellow-brown linear lesions were found extensively at the U-fibres and cingulate gyrus within the cerebral white matter. Histopathologically, linear demyelination and occasional cavitation were observed. Glial cells with yellow-brown pigment granules, which were autofluorescent, positive to the periodic acid-Schiff reaction and stained with Masson-Fontana and Schmorl's stains, were frequently found within these demyelinating lesions. The pigment granules did not stain with Berlin blue, Luxol fast blue or Sudan III stains. Haematoxylin and eosin staining and immunohistochemistry of serial brain sections revealed that the pigmented glia were derived from glial fibrillary acidic protein-positive astrocytes. Based on these findings, the case was diagnosed as leucoencephalopathy with pigmented glia. This is the first report of the disease in any animal species.