Clinical, cytologic and histopathologic manifestations of protothecosis in two dogs.

Two cases of sudden onset of blindness associated with ocular protothecosis in dogs are reported. Both were adult, spayed female, mixed-breed dogs that lacked the usual clinical signs of systemic infection with Prototheca species. Physical abnormalities at the time of presentation were limited to the affected eyes which had serous discharge, hyperemic conjunctiva, and aqueous flare. The pupillary light reflexes were slow, and the menace reflexes were absent. Both dogs had glaucoma. Results of complete blood counts and serologic titres for antibodies to Blastomyces dermatitidis and Histoplasma capsulatum were within reference intervals. Protothecosis was diagnosed by cytologic analysis of vitreous humor and was confirmed at necropsy. These two cases were unusual because of their presenting signs and prolonged course of disease progression.