Bilateral colobomas involving the optic discs in a quarterhorse.

This research article presents the case study of a young horse suffering from a rare ocular condition known as bilateral colobomas of the optic discs, which resulted in blindness from birth.

Case History

• The subject of the study is a four-month-old Quarterhorse colt who has been blind since birth.
• The horse’s blindness was first noticed when it displayed abnormal behavior – showing dependence on its mother and appearing disoriented in unfamiliar environments.
• The colt was referred to the University of Missouri Equine Center for examination.

Clinical Observations

• Despite its young age, the horse is physically larger than its peers.
• Aside from its eyes, the horse appears normal.
• The horse has blue eyes, a stark contrast to its deep chestnut coat.
• The eyes are the normal size, with well-pigmented eyelids, but the pupils are only partially dilated and minimally respond to direct light stimulus.
• The horse lacks a menace response, an involuntary blinking reflex triggered by rapid movements close to the face, further suggesting visual impairment.
• Examination of the back of the eye (fundus) reveals large colobomas, or gaps, involving the optic discs.
• The depth of the colobomas is obscured by pigmented tissue.
• Due to the severity of the blindness, the owner elected for euthanasia and the eyes were removed for further examination.

Pathological Evaluations

• The post-mortem examination didn’t reveal any abnormalities outside of the eyes.
• The eye dimensions were normal, ruling out microphthalmia (abnormally small eyes).
• However, the colobomas caused dilation in the region of the optic nerve head, extending about 6-7 mm from each eye’s back.
• The retina separated from the back of the right eye upon dissecting the globes.

In conclusion, this study presents a rare case of a horse born with bilateral colobomas, contributing to the limited body of knowledge about such occurrences in equine species. It highlights the need for further research on rare ocular conditions in animals.