Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Periodic ophthalmia is a disease of the eye affecting horses, mules and asses only. Etiology of this disease is not known, although existing evidence points to a virus as the possible cause. It is characterized by a recurring inflammation which may effect one eye alone, or both eyes either simultaneously or alternately, or one eye until blindness occurs and then the other eye.
"A Summary of BLindness in Domestic Animals Part II,"
Iowa State University Veterinarian: Vol. 18
, Article 2.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol18/iss2/2