Many horses are affected by uveitis, which is a painful, inflammatory eye disease that can lead to blindness. Our Leatherstocking Veterinary Services team commonly treats equine uveitis, and we provide information about this condition.

What is equine uveitis?

Uveitis is inflammation of the uvea, which is the middle eye layer between the sclera (i.e., the white part of the eye) and the retina (i.e., the light-sensitive area at the back of the eye). The uvea consists of three components:

• Iris — The iris is the colored part of the eye.
• Ciliary body — The ciliary body includes the ciliary muscle, which helps focus the lens, and the ciliary epithelium, which produces the transparent fluid (i.e., the aqueous humor) that maintains the eye’s ocular pressure and provides nutrition for some ocular tissues. 
• Choroid — The choroid is a vascular tissue layer between the retina and sclera that provides oxygen and nourishment to the retina.

What causes equine uveitis?

Any eye injury can result in uveitis, including blunt force trauma that doesn’t injure the cornea and corneal ulceration. Other causes include systemic infections, such as leptospirosis and Lyme disease, and Equine Recurrent Uveitis (ERU).

What is Equine Recurrent Uveitis?

ERU, which is commonly known as moon blindness, is a complex autoimmune disease that typically manifests as recurrent eye inflammation episodes. The condition is influenced by genetic and environmental factors, with Appaloosas eight times more likely to develop ERU than other breeds, and American Quarter Horse, Thoroughbred, Warmblood, Hanoverian, and American Paint Horse also at increased risk. ERU can lead to cataracts, glaucoma, and other ophthalmic complications and is the most common blindness cause in horses worldwide. ERU is classified into three syndromes:

• Classic — This is the most common form and is characterized by recurring painful episodes when the eye is actively inflamed. Between episodes, the eye is non-painful and quiet, or has low inflammatory levels. Repeated attacks can lead to vision loss.
• Insidious — This chronic form causes persistent low-grade eye inflammation that destroys ocular tissues, typically leading to blindness. Horses often do not exhibit outward pain.
• Posterior — Posterior ERU causes inflammation to the back of the eye and often leads to retinal degeneration.

What are equine uveitis signs?

Classic equine uveitis signs include squinting, tearing, sensitivity to light, bluish or cloudy corneal appearance, reddened sclera, and a constricted pupil. Horses with insidious or posterior ERU may not exhibit outward signs until the condition has significantly damaged the ocular structures, affecting the horse’s vision.

How is equine uveitis diagnosed?

Equine uveitis is primarily diagnosed by ophthalmic examination. Your veterinarian will use an ophthalmoscope to study every aspect of your horse’s eyes—examining both eyes is important anytime uveitis is suspected. Other needed diagnostics may include:

• Fluorescein stain — Applying a fluorescein stain strip to your horse’s eye allows your veterinarian to assess the cornea for abrasions and ulcerations.
• Blood work — In some cases, blood work (e.g., complete blood count, biochemistry profile) or leptospirosis or Lyme disease testing is necessary to rule out an infectious cause.
• Tonometer — Glaucoma is a potential sequela to equine uveitis, and your veterinarian may use a tonometer to check your horse’s ocular pressure.
• Ultrasound — If inflammation or a cataract prevents an assessment of the back part of your horse’s eye, your veterinarian may recommend an ocular ultrasound to assess the retina.

How is equine uveitis treated?

If equine uveitis is secondary to corneal trauma, your veterinarian will prescribe eye drops and ointments to ensure the defect does not get infected. Other potential uveitis treatments include:

• Pain management — Systemic non-steroidal anti-inflammatory drugs (NSAIDs) will alleviate uveitis pain and inflammation.
• Topical steroids — If the cornea is healthy, your veterinarian may recommend inflammation control with topical steroids, especially for a severe condition.
• Mydriatics — Topical medications are helpful to dilate the pupil, reduce pain, and prevent the pupil from scarring in a constricted position.
• Antibiotics — If your horse is diagnosed with leptospirosis or Lyme disease, they may need antibiotics to address the infection.
• Cyclosporine implants — A sustained-release cyclosporine implant inserted in the horse’s eye can help control inflammation and minimize ERU recurrences for up to three years. 
• Enucleation — If an eye is blind and the horse’s pain can’t be adequately controlled, your veterinarian may recommend eye removal.

How is equine uveitis prevented?

Researchers are performing studies to better understand ERU, but no definitive means to prevent ERU is currently available. Genetic tests are available to help determine a horse’s ERU risk, and horses who have uveitis or ERU risk according to the genetic test should not be bred. Early detection is the best way to protect your horse’s eyes, so a veterinarian should perform a thorough examination, including an ophthalmic assessment, at least once per year. UV exposure has also been linked to ERU, and fitting your horse with a UV-protective fly mask may reduce their risk.

If you are concerned about your horse’s eye health, or you would like to schedule a wellness exam, contact our Leatherstocking Veterinary Services team to schedule an appointment.