Merck Manual

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Pet Owner Version

Disorders of the Lens 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 lens is a soft, transparent tissue that sits behind the iris. It helps refract incoming light onto the retina. Common disorders of the lens include those that affect its transparency (such as cataracts), and those that affect the placement of the lens.


Cataracts are a condition in which the lens becomes cloudy, which affects sight and eventually causes total blindness.

In foals, cataracts are the most common congenital defect of the eye. They are inherited in Belgian and Morgan horses. Cataracts usually occur in both eyes. When these cataracts interfere with vision in healthy foals, surgery followed by topical therapy is successful in a majority of cases.

In adult horses, most cataracts occur as a result of the inflammation of the anterior uvea associated with 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 . Horses older than 20 years of age may develop so-called senile cataracts that interfere with vision. Surgical removal of the lens is the only definitive treatment available.

Lens Displacement

Lens displacement (also called lens luxation) can occur in horses. The displacement may be due to trauma, longterm inflammation of the uvea (as occurs in equine recurrent uveitis), or glaucoma. The only effective treatment is surgical removal of the lens.

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