Moon Blindness (Equine Recurrent Uveitis) in Horses

Key Takeaways

Equine Recurrent Uveitis (ERU), or moon blindness, is an uncommon condition affecting horses, characterized by inflammation within the eye that occurs repeatedly. 

• ERU cycles through periods where it is active, aggressive, and painful, as well as mild phases which often show no symptoms

• An affected eye may present with cloudiness, blood within the eye, increased tearing, squinting, and vision loss

• Diagnostics include an eye examination, blood work, and diagnostic imaging

• Treatment is not curative, and instead looks to manage symptoms

• Treatments include medication such as NSAIDs, antiinflammatories, or antibiotics

• In severe cases, or cases where costs are a concern, surgical removal of the eye is indicated to improve quality of life

• Prognosis for ERU is guarded, and many horses go blind even with appropriate treatment

A Closer Look: What is ERU in Horses?

Uveitis describes inflammation within the eye, particularly affecting the uvea, which consists of the iris (colored ring around the pupil), ciliary body (muscles inside the front of the eye), and choroid (lining towards the back of the eye). Except for the iris, this tract is not externally visible. When uveitis is persistent and recurring in horses, this is referred to as Equine Recurrent Uveitis or moon blindness.

Active periods of ERU present with overt symptoms which last for weeks. In between these active phases, the inflammation is more mild, and horses may show no symptoms.

Active ERU symptoms include

Cloudiness to the eye

• Light sensitivity

• Excessive squinting

• Excessive tearing

• Pus leaking from the eye

Over time, ERU results in chronic symptoms. These include:

• Discoloration of the skin around the eyes (hyperpigmentation)

• Scarring of the surface of the eye 

• Irregularly shaped pupil 

• Excessive pupil constriction (miosis)

• Discoloration of the eye

• Cloudiness to the eye

Risk Factors

ERU is uncommon, but it is the most common cause of blindness in horses. ERU is often a highly painful condition, and any horse presenting with symptoms benefits from immediate veterinary intervention.

Some breeds of horse are more likely to develop ERU, which suggests a genetic predisposition. Appaloosas, Warmbloods, and draft horses are predisposed. ERU is more common in temperate or tropical climates.

Other contributing causes may include:

• Bacterial infections, including infections elsewhere in the body

• Parasitic infection

• Viral infections

• Eye injuries

Possible Causes

Currently, ERU is poorly understood, as are its underlying causes. ERU is associated with Leptospira bacteria, which suggests an autoimmune reaction to the bacterial invaders. This immune response results in white blood cells accumulating in the eye, resulting in inflammation.

Main Symptoms

Symptoms of ERU affect one or both eyes, and include:

• Frequent squinting (blepharospasm)

• Excessive tearing

Cloudy eye

• Blood accumulating within the eye

• Redness or irritation in the eye

• Light sensitivity (photophobia)

• Discoloration or misshaping of iris

Vision loss can be subtle, and manifest in different ways:

• Shying away from visual stimuli

• Increased ‘startle’ responses

• Hesitant movement, especially in an unfamiliar environment

• Bumping into obstacles

Testing and Diagnosis

Diagnostics include:

• A physical examination

• Ophthalmological exam

• Bloodwork

• Diagnostic imaging, including ultrasound of the eye

Steps to Recovery

Treatment for ERU is not curative, and instead looks to control symptoms by reducing inflammation and controlling pain. This may include:

• Anti-inflammatories

• Antibiotics

• Painkillers (NSAIDs)

• Corticosteroids

Weekly follow-up is required to monitor treatment progression, and make adjustments to protocols as needed. 

In cases where medical intervention is unsuccessful, or owner costs become a concern, removal of the affected eye is the best option for improving quality of life. 

Most cases of ERU require lifelong management. Early identification and treatment provides the best chances for recovery and protection of vision, however even with appropriate treatment, many horses permanently lose their vision.


ERU is difficult to prevent, although including eye exams in regular health checkups may enable early detection.

Is ERU Common in Horses?

While an uncommon condition, moon blindness is the most common cause of blindness in horses.

Typical Treatment

• Medication

• Surgery

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