Moon Blindness (Equine Recurrent Uveitis) 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.

Over time, ERU results in chronic symptoms.

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

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 causes white blood cells to accumulate in the eye, resulting in inflammation.

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

Symptoms of vision loss include

• Shying away from visual stimuli
• Increased ‘startle’ responses
• Hesitant movement, especially in an unfamiliar environment
• Bumping into obstacles

Diagnostics include:

• Physical examination
• Ophthalmological exam
• Blood work
• Diagnostic imaging, including ultrasound of the eye

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.

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

• Medication
• Surgery