True Equestrian: Moon Blindness – What it is and How to handle it

By Lauren Mueller, Reporter

Equine Recurrent Uveitis, or “Moon Blindness,” is one of the most common inner eye disorders in horses. It is also the leading cause of blindness. Approximately 10 percent of horses suffer from this condition in either its chronic or acute form.

According to the Horse Owner’s Veterinary Handbook, third edition, by Thomas Gore, Paula Gore, and James Giffin, uveitis is defined as the inflammation of one or all of the inner pigmented structures of the eye, including the iris, ciliary body and choroid. There is also a secondary involvement of the cornea, lens and retina, and this is why the condition can cause permanent blindness in affected horses.

The most common causes of Moon Blindness are Leptospirosis and Onchocerciasis.
Leptospirosis is a bacterial disease that infects cattle, sheep, wild animals, rats, cats and humans. A horse would contract this infection through a break in the skin or through ingestion of water contaminated by urine containing the bacteria, usually because an infected animal urinated in the water source. When Moon Blindness is contracted this way, it can show symptoms either in tandem with the bacterial disease’s symptoms, or months or years later.
It is important to note that the Moon Blindness caused by Leptospirosis is not actually the result of an active bacterial attack. Instead, the authors of The Horse Owner’s Veterinary Handbook, third edition, suggest that the pursuant condition is the result of an immune hypersensitivity response, where the inner eye structures react to foreign proteins released to kill the bacteria.

The underlying cause of Onchocerciasis is a small, thread-like worm that lives in connective tissues in the horse’s neck. When these worms are adults, they release microfilariae that move beneath the skin and cause infections. During their movement, some of the worms end up in the horse’s eyelid and inner eye. This causes an inflammation in the eye in response to the adult worms dying out.

Ironically, according to Equine Ophthalmologist Dennis Brooks on The Chronicle of the Horse website, dewormers that kill worms like Onchocerciasis can actually aggravate the problem. Because the horse’s immune system reacts more violently to the dead microfilariae, when the dewormer kills off many of them at once the condition can be set off.
Other causes of Moon Blindness include, infrequently, strangles, viral arteritis, toxoplasmosis, other bacterial infections, fungal, viral and parasitic infections and trauma to the eye.

Moon Blindness usually appears in a way similar to an eye injury. The affected eye becomes red and may be puffy. The horse will “squint” this eye in much the same way as a human would squint if their eye was injured. There may also be tearing of the eye, and the eye may appear cloudy or “blind.” The horse’s pupil may also become small and contracted, and the eye will feel soft when slight pressure is applied.

As with all things equine, a trip to your veterinarian will be required to diagnosis Moon Blindness.

Once a horse has been diagnosed with Moon Blindness, there are several treatment options available.

According to The Horse Owner’s Veterinary Handbook, third edition, most treatments for Moon Blindness are directed at preserving vision and reducing inflammation and recurring flare-ups. Treatments include topical steroids, atropine and NSAIDS. Antibiotics may also be prescribed by a veterinarian if it is the first occurrence in the horse and any bacteria, such as Leptospirosis, is still present in the horse’s system.

The Veterinary Health Center at Kansas State University offers another, long-term option to treat Moon Blindness as well. This option is a cyclosporine implant. According to the Veterinary Health Center’s website: “A small disc-shaped implant filled with a medication called cyclosporine is surgically implanted within the white layer (sclera) of the eye. The implant then slowly releases the medication, which is an anti-inflammatory/immune-modulatory drug. A major goal of the cyclosporine implant is to reduce the requirement of topical and oral medications in horses treated with this procedure.”

While Moon Blindness is the most common cause of blindness in horses, it only occurs in 10 percent of horses worldwide. Treatment technology is rapidly evolving, and while the damage done by this condition is not yet preventable or reversible, the cyclosporine implant is a step in the right direction. The implant may last as long as five years, and can be replaced with a new one when depleted.

Veterinary medicine is working towards a way to prevent Moon Blindness from damaging a horse’s vision, and there is hope that this may happen in our lifetimes.
As always, if you as the reader have any suggestions or any topics you would like covered let me know. Happy Trails.