Blindness in a wild American black bear cub (Ursus americanus)

An approximately six-month-old wild American black bear (Ursus americanus) was found wandering in Saskatchewan and was presented to the Veterinary Medical Centre of the Western College of Veterinary Medicine for apparent blindness. Clinical examination confirmed an inability to navigate a photopic maze, bilateral tapetal hyper-reflectivity, fundi devoid of retinal vessels, and small pale optic nerve papillae. Single-flash electroretinography revealed A and B-wave amplitudes of approximately 40 and 140 microvolts, respectively, in both eyes. Histologic abnormalities included bilateral optic papillary mineralization and bilateral segmental optic nerve degeneration, with occasional intralesional lymphocytes confirmed with immunohistochemistry for CD3+. There was also bilateral multifocal retinal dysplasia, gliosis, lymphocytic retinitis, a complete lack of retinal blood vessels, an intravitreal vascular membrane, and a mild lymphocytic-plasmacytic uveitis with small pre-iridal cellular membranes. The presence of a positive ERG in a blind bear with numerous retinal ganglion cells and degenerative changes in the optic nerve are most consistent with vision loss due to optic nerve injury, which given the young age of the bear likely occurred during ocular development. The presence of ocular inflammation suggests this injury resulted from an inflammatory/infectious process. The etiology could not be determined. Hepatic concentrations of vitamin A were within the normal reference range for domestic species. Pan-herpesvirus PCR and immunohistochemistry for canine distemper virus and Toxoplasma gondii were negative, although this does not rule out these or other infectious etiologies. This represents the first case report of neonatal or congenital ocular abnormalities in an ursid species.