Merck Manual

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Pet Owner Version

Disorders of the Retina, Choroid, and Optic Disk (Ocular Fundus) in Horses


Kirk N. Gelatt

, VMD, DACVO, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida

Reviewed/Revised Apr 2019 | Modified Oct 2022

The ocular fundus is the back of the eye opposite the cornea and includes the retina, the membrane between the retina and the white of the eye (the choroid), and the optic disk (the spot where the optic nerve enters the eye). The choroid contains the tapetum (a reflective structure at the back of the eye that improves vision in dim light), the blood vessels that supply the retina, and colored (pigmented) cells. Diseases of the ocular fundus may occur on their own or as a part of generalized diseases.

Possible underlying causes for diseases of the retina in all species include:

  • inherited abnormalities

  • trauma

  • metabolic disturbances

  • generalized infections

  • tumors

  • blood disorders

  • high blood pressure

  • nutritional deficiencies

Inflammation of the Retina and Choroid (Chorioretinitis)

Inflammation of the retina and choroid is frequently a result of a generalized, body-wide infection. It is important as both a convenient diagnostic clue and a predictor of visual function. Unless the abnormalities are widespread or involve the optic nerve, they often are “silent” and produce no signs.

During eye examinations, your veterinarian will look for certain characteristic lesions in the eye. These include “bullet-hole” lesions (which suggest infection with equine herpesvirus), diffuse lesions (which may be caused by inflammation or severe head trauma), and “horizontal band” lesions (which may be caused by blockage of the blood vessels).

Inflammation of the retina and choroid may be present with bacterial (leptospirosis), algal, and fungal infections, and blood infections (septicemia) in foals, or caused by trauma or parasites (for example, onchocerciasis). Therapy is directed at the underlying generalized disease.

Retinal Detachments

When the retina becomes detached, it is separated from the back of the eye and from part of its blood supply, preventing it from functioning properly. In horses, detachment of the retina most frequently occurs with trauma, eye surgery, and equine recurrent uveitis Equine Recurrent Uveitis (Periodic Ophthalmia, Moon Blindness) The uvea (or the uveal tract) is the colored inside lining of the eye consisting of the iris, the ciliary body, and the choroid. The iris is the colored ring around the black pupil. The ciliary... read more .

Signs that the retina has become detached include excessive or prolonged dilation of the pupil, pupils of different sizes, vision impairment, and bleeding within the eye. Eye examinations need to be performed to confirm the diagnosis.

Detachments of the retina are treated medically with therapy directed at the primary disease or surgically to correct the detachment. Your veterinarian will select the treatment approach most appropriate for your horse’s condition.

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