Uveitis in horses is a condition in which the uvea layer of the eye becomes inflamed. It is the most common cause of blindness in horses.

It is estimated that as many as 25% of horses worldwide are affected by some form of uveitis. Some types of uveitis are due to an immune-related disorder and can be difficult to treat.

Uveitis can be recurring – referred to as Equine Recurrent Uveitis (ERU) – or it can be acute with a single episode of limited duration.

Recurrent uveitis often affects both eyes and eventually leads to blindness if left untreated. This form of uveitis is considered an autoimmune disease, where the body’s immune system attacks normal tissues. [1]

ERU is also commonly called Moon Blindness, a term coined in the 1600s when it was thought that recurring episodes of eye problems were related to the phases of the moon.

Causes of Uveitis

Uveitis involves inflammation of the uveal tract of the eye, which is the blood vessel-filled tissue beneath the sclera (white of the eye).

Uveitis can be acute, meaning it happens suddenly and lasts only a short period of time. More often, it is a recurring condition that gradually worsens over time.

Acute Uveitis

There are several possible causes of acute (sudden onset) uveitis in horses, including: [2]

  • Traumatic injuries to the eye
  • Bacterial infections
  • Septicemia, a bacterial infection in the bloodstream
  • Tooth root abscesses
  • Tumors in or around the eye
  • Corneal ulcers
  • Abnormal structure of the eye present at birth

These causes all result in irritation in or around the eye, which triggers an inflammatory response by the body’s immune system. [3] The activated immune cells infiltrate the eye and damage the ocular structures as they try to resolve the source of irritation. [2]

Equine Recurrent Uveitis

Three types of Equine Recurrent Uveitis have been described: classic ERU, insidious ERU and posterior uveitis.

Classic ERU

Classic recurring uveitis is the most common form of uveitis and is characterized by periods of inflammation within the eye, followed by periods of dormancy.

With each inflammatory phase, the disease progressively worsens and vision may eventually be lost. [4] Warmbloods and Icelandic horses appear to be at higher risk of developing Classic ERU.

Insidious ERU

Horses with insidious ERU develop a low level of inflammation within their eye that never completely resolves. This form of uveitis leads to the gradual destruction of ocular tissues and degeneration of structures in the eye. [5] Insidious ERU is commonly seen in Appaloosas as well as some draft breed horses.

Horses with insidious ERU do not typically demonstrate eye discomfort. Owners may not recognize that their horse is experiencing uveitis until a cataract develops or the horse’s vision becomes impaired. [6]

Posterior Uveitis

Posterior uveitis is most commonly seen in Warmbloods and horses in Europe. It is characterized by changes and degeneration in the vitreous fluid and retina of the eye. [4]

The inflammation is mainly in the back or posterior part of the eye, hence the name.

Cause of ERU</h3 > Many different microorganisms have been found to trigger uveitis, but the most common is the Leptospira bacteria. In fact, this bacteria can be found in about 60% of horses with ERU. [7]

Though researchers are still working to understand how Leptospira triggers the disease, one theory is that it penetrates the eye where it is deposited in the ocular tissue, causing infection. [8]

The main carriers of Leptospira are small rodents, which excrete the bacteria in their urine. Swampy pastures present a higher risk for infections.

Oral mucous membranes, conjunctiva, nasal mucous membranes, and skin lesions may also be possible entry sites for pathogens. [1]

Other pathogens, viruses and illnesses that have been found to trigger uveitis include:

Parasitic infection from Onchocerca spp., Strongylus spp., and Toxoplasma spp. have also been associated with uveitis. [9]

These infections activate T cells, one of the main white blood cell types within the body. When the T cells are activated, they overcome the blood retinal barrier to invade the eye. [10]

The T cells then cause severe inflammation within the eye, especially in the retina and iris. With each inflammatory episode, the retina further degrades until eyesight is eventually lost. [11]

This may also help to explain why some horses have episodes of inflammation following routine vaccinations or anthelmintic treatments when the immune system is stimulated. [6]

Genetic Predispositions
A genetic component has been linked to ERU in German Warmbloods. A mutation on chromosome 20 may affect important components of the immune system to predispose these horses to ERU. In Appaloosas, another genetic mutation has been found affecting the major histocom