Equine recurrent uveitis (ERU), also known as moon blindness, is an autoimmune disease and the leading cause of blindness in horses. [1] The disease results from the immune system repeatedly attacking the structures of the eye causing tissue damage, inflammation, and eventually blindness.

The underlying cause of equine recurrent uveitis is usually unknown, but Appaloosa horses, Warmblood horses, and Icelandic horses appear to have a genetic predisposition. Horses that develop leptospirosis or other infectious diseases also have an increased risk of ERU.

Symptoms of ERU include cloudy eye (corneal edema), abnormal pupil responsiveness, and loss of vision. Some horses experience “flare-ups” of uveitis with more severe symptoms such as squinting and yellow-green fluid within the eye.

ERU is a lifelong disease often requiring ongoing management by owners. Treatment focuses on reducing inflammation and preventing flare-ups in affected horses. Treatment options include medical management, intraocular injections, and surgical intervention. Despite appropriate treatment, the prognosis for vision in an affected eye is poor.

Causes of Moon Blindness

Equine recurrent uveitis (moon blindness, periodic ophthalmia) is an autoimmune disease, meaning the horse’s own immune system attacks the tissues of the eye and causes damage. [2]

Under normal circumstances, the eye contains no white blood cells or other immune cells. [3] The blood-ocular barrier, a thin layer of cells lining the blood vessels of the eye, prevents white blood cells from entering the organ. [3]

Equine recurrent uveitis occurs when there is a disruption to the blood-ocular barrier, allowing white blood cells to enter the eye. [3] Since white blood cells are not normally present within the eye, the immune system identifies the eye tissues as foreign and targets them for destruction. [3] This results in widespread damage to the eye’s internal structures. [3]

The historical term “moon blindness” likely originated from the belief that horses developed the condition during the full moon, though there’s no scientific evidence supporting this notion. It’s more likely that the increased awareness of the condition during the night, when horses would display symptoms such as increased sensitivity to light, led to this association.


Autoimmune Disease Development

Like many other autoimmune diseases, the precise cause of ERU is usually unknown. [3] Studies of autoimmune disorders suggest there is an initial trigger that results in the immune system targeting antigens (recognizable proteins) on normal body tissues. [3] The immune system then identifies these tissues as abnormal and attempts to destroy them. [3]

Although it is not clearly understood, proposed causes for autoimmune disease development include: [3]

  • Molecular mimicry: a pathogen produces similar antigens to a normal body tissue
  • Bystander activation: the immune system activates against normal body tissues accidentally while trying to clear an infection
  • Epitope spreading: the immune system’s response to an infection starts targeting other proteins, including those produced by normal body tissues

Connection to Infectious Diseases

Studies show that leptospirosis, an infectious disease affecting the whole body, can trigger development of ERU. [2] Horses become infected by ingesting contaminated water or food containing urine from a Leptospira-infected animal. [3]

The development of ERU after Leptospira infection is likely due to molecular mimicry, where the Leptospira bacteria produce a protein that is similar to normal eye tissue proteins. [2] As the immune system attempts to clear the Leptospira infection, it also targets the eyes, resulting in ERU. [2]

For this reason, some researchers recommend vaccinating horses with a high risk for ERU against Leptospira. [3][4] Vaccination of horses currently affected with ERU is not recommended, as studies show it does not slow the progression of ERU. [3][5]

Other infectious diseases that could potentially trigger ERU include: [6]