The lens of the eye is a clear body that lies behind the opening in the colored part of the eye, the pupil. Its function is to focus an image on the retina in the back of the eyeball.

Cataract is defined as opacity or loss of clarity of the lens. It appears as a white or grayish area within the lens, visible through the pupil.

Cataract can be hereditary, congenital, or acquired and range in size from a tiny spot to covering the entire lens.

The only truly hereditary cataract has been proven in the Morgan Horse. Congenital cataract is more common in that breed and the Arabian than others, but can occur in any breed.

Acquired cataract can occur through trauma to the eye and is commonly seen as part of Equine Recurrent Uveitis (ERU). When vets see a cataract, we always ask ourselves whether it is changing or static. Most congenital cataracts do not worsen.

Large cataracts that cover a majority of the lens will interfere with vision, whereas small peripheral cataract often does not cause appreciable loss of vision. Complete cataract causes blindness.

Older horses develop a graying of the cornea, called nuclear sclerosis. This usually does not interfere with vision significantly.

Diagnosis of cataract and the cause of cataract requires careful history and ophthalmic examination with an ophthalmoscope.

Treatment now includes surgical removal of cataract using a variety of means. Specialty procedures like this are usually performed by board certified veterinary ophthalmologists.