Equine Recurrent Uveitis or ‘Moon Blindness’ in Horses

By:Dr. Lydia Gray

Updated on: 7/10/2019

Horses seem to be prone to eye problems, from injury and trauma to diseases and conditions. When certain internal structures of the eye known as the uvea become inflamed, the condition is called “uveitis.”

Like most medical problems, uveitis can be described as acute (a single, short-term bout) or chronic (persist long-term). A horse that suffers from repeated, intermittent bouts of uveitis over a long time –in one or both eyes -- may have developed Equine Recurrent Uveitis or “moonblindness,” an immune-mediated condition that is likely inherited.

Signs and Symptoms of Uveitis in Horses 

The clinical signs of uveitis in horses are not unique to this condition, but merely indicate that the horse is experiencing eye pain and include:

  • Squinting and a sensitivity to light
  • Tearing
  • The white of the eye (and surrounding membranes) becoming bloodshot, known as “red eye”
  • A cloudy appearance or bluish haze to the eye
  • Swelling of the eyelids

A horse displaying any of these signs of eye pain should be seen immediately by a veterinarian since early diagnosis and treatment may prevent the condition from getting worse or lead to blindness or loss of the eye in severe cases.

Equine Uveitis Diagnosis

History & Signalment

As with any physical examination, evaluating a horse’s eye begins with taking a history and signalment from the owner. In the medical field, “history” means the facts leading up to the patient’s current situation and includes:

  • When the problem started and how long it’s been going on
  • What signs and symptoms were observed
  • If any treatment was given
  • If this is the first time the horse has experienced this problem
  • What other problems the horse is currently having or recently had (e.g. illness or injury)
  • What medications and supplements the horse is receiving
  • The horse’s vaccination and deworming status

“Signalment” means the horse’s age, breed, and gender, and sometimes includes defining characteristics like color, markings, height, and weight. 

How might a vet use history and signalment as an aid in coming to a diagnosis of uveitis, and then to a more specific diagnosis of ERU? Knowing that a horse is an appaloosa -- a breed predisposed to ERU – and that the horse was recently infected with leptospirosis – a known trigger for ERU – puts the condition high up on the list of possibilities. This is especially true if this is not the first episode of eye pain and inflammation the horse has had.


The actual ophthalmological or eye exam in a horse involves looking at structures on the outside of the eye such as the eyelids and cornea, structures on the inside of the eye such as the pupil and lens, and structures in the back of the eye such as the retina. The vet will use a variety of tools and tests to evaluate the health and integrity of these different areas. These include: staining the cornea to check for scratches or ulcers, measuring the pressure inside the eye (similar to the glaucoma test human eye doctors perform), evaluating the pupil’s response to bright light, and more. The combination of history and exam hopefully produces a diagnosis for which a treatment plan can then be developed. Vet shining light into horse;s eye with a flashlight to see the pupil's response to light.Vet shining light into horse;s eye with a flashlight to see the pupil's response to light.

Treatment & Management


Horse wearing a flymask with UV protection to help protect his eyes.

The goal of ERU treatment is to:

  • Control the pain and inflammation
  • Slow progression and prevent vision loss
  • Treat underlying cause or trigger factor

In order to make the horse more comfortable and limit inflammation inside the eye, vets often prescribe non-steroidal anti-inflammatory drugs or NSAIDs like Banamine systemically (ie body-wide like an injection) along with steroids topically (ie applied directly to the eye itself).  Atropine is another medication that is often prescribed when dealing with the early stages of uveitis or an ERU flare-up. Because atropine dilates the eye, vets usually recommend that horses stay inside while this drug is on board or wear a fly mask outside to reduce sunlight.

It is still common practice to try and prevent future episodes of ERU by keeping horses affected 

with the condition on a low-dose, daily regimen of Banamine, bute, aspirin, or MSM. However, a new technique has been shown to both decrease the recurrence of uveitis as well as limit the severity and length of episodes. By surgically implanting a disk that provides a sustained release of the immunosuppressive drug cyclosporine, horses experience three to fours years of reduced signs which may slow progression of the disease. Surgical removal of the affected eye is another option for those horses who continue to suffer flare-ups and whose pain cannot be controlled.

Vets remind horse owners not to treat eyes without a diagnosis, even if uveitis is the likely culprit. That’s because the medications used to treat it will aggravate other conditions, like corneal ulcers. For that same reason, if a horse worsens during uveitis treatment, owners are urged to contact their vet right away for a recheck.


A black Kool Kurtain covering a horse's open top stall door to help reduce sun exposure.

Along with aggressive treatment during episodes of acute uveitis, experts recommend following these practices for horses diagnosed with ERU, some of which are just good general horse care advice and others are specific to this eye condition:

  • Provide proper health and wellness care including vaccines, parasite control, dental work along with a quality diet
  • Prevent eye trauma by selecting appropriate turnout buddies and ensuring stalls, paddocks, and pastures are safe
  • Reduce environmental triggers such as sun, wind, bugs, dust, etc. with fly masks, Kool Kurtains, and more appropriate bedding

When caring for a horse with ERU, extreme vigilance for subtle signs of an episode is necessary to get treatment started right away in an attempt to prevent irreversible eye damage.

SmartPak strongly encourages you to consult your veterinarian regarding specific questions about your horse's health. This information is not intended to diagnose or treat any disease, and is purely educational.

Article First Published Febuary 2012