Case 1 - more than meets the eye

Dr. McCleery was called to give this Rocky Mountain mare a health check upon arrival to her new home. Her previous owners had mentioned that the mare could be a flighty at times, but was otherwise a good horse with no known health problems.
The mare (seen at left) was bay silver in color and her general condition was fine, but upon closer examination Dr. McCleery noticed several abnormal findings on the ophthalmic portion of the exam.

Do you notice anything abnormal about this mare's eye?

The mare's eyes did not appear to be in any pain, as she was holding them open normally and had no tearing or discharge. After examining the mare's eyes, Dr. McCleery had the mare navigate over a pole placed under the shade of a tree. The mare could see the pole, but was reluctant to walk over it - after some coaxing she carefully walked around the edge of it. When moved into full sun, the pole did not pose a problem. So the mare was visual, but her sight appeared to be impaired, especially under suboptimal lighting conditions.

Based on her findings, Dr. McCleery made a presumptive diagnosis of equine Multiple Congenital Ocular Anomalies (MCOA) syndrome. Congenital means that a disease or physical abnormality is present from birth. MCOA syndrome consists of a variety of problems that can affect the cornea, iris, ciliary body, lens, and retina of the eye.

The abnormalities are genetically linked to coat color, and horses with chocolate (also termed silver) coat color and a white mane and tail are more severely affected. The syndrome has been described in the Rocky Mountain Horse, Kentucky Saddle Horse, American Miniature Horse, Icelandic Horse, and Mountain Pleasure Horse breeds.

The two pictures below compare our mare with the eye of a normal horse. The mare had similar abnormalities in both of her eyes.

Normal equine eye.

Eye of the mare in our case.

What do you notice?

1. Cataract - Clouding of the lens (you are seeing the lens when you look through the pupil - white arrow in the picture below). This mare's entire lens is cloudy.
   
2. The granula iridica (the little flap of tissue seen over the pupil in the normal eye above) is missing in this mare (yellow lines in picture below).
3. Iris hypoplasia - The iris (brown colored portion of the eye) appears thin and forms an irregularly shaped pupil. The pupil does not function - in fact it appears stuck at this size and does not open and close with changing light conditions like it would in a normal animal.

Horses affected by MCOA can also have uveal cysts within their eyes, bulbous enlargement of the eye, and, rarely, detachment of the retina. The severity of the problem will vary by horse, with some horses being only mildly affected. On cursory exam these horses will appear normal and have normal vision. Detection of the syndrome requires close examination of the internal structures or an ultrasound of the eye.

Unfortunately, there is no treatment for MCOA. The good news is that the disease is not painful or progressive and although vision can be compromised to varying degrees, many horses will lead full, productive lives. It is important not to breed horses affected with MCOA as there is a high probability that ocular issues will be passed onto offspring.

Genetic research has shown that the mutation that codes for MCOA can be closely linked with the silver dapple trait that gives us the beautiful bay silvers like the mare in our case or the stunning chocolate coat color seen at right. Careful screening of breeding prospects can help prevent passing on ocular anomalies along with the beautiful coat colors.

Want to learn more about MCOA? Check out this article Multiple congenital ocular anomalies in Icelandic horses.