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I had never noticed that my horse, Jack, had ocular cysts in both eyes until a veterinary intern saw them and pointed them out to me. That realization made a lot of sense and explained some unusual spooking behaviors. 

Corpora Nigra cysts are benign cysts that form in the irises of horses. It is believed to be a natural formation that acts as a shade to the sun, often occurring in horses who live outside and have limited protection from the sun. Mother Nature takes care of all her creatures in such incredible ways! They look like small, dark circles and often form in groups to resemble little clouds. 

For the most part, they are painless, benign, and of no consequence to the horse. However, they can create temporary blind spots so that what the horse sees may momentarily disappear and then reappear suddenly. This can cause a surprise that can trigger a spook. 

I wondered why Jack would be quiet and for no apparent reason side step, trot away in the field, then stop, puzzled, only to resume his quiet demeanor. This explains why he ended up in a rescue situation as he must have become unreliable as a working horse in rural communities. 

When I look into his lovely eyes in daylight or with a flashlight, I can see the corpora nigra cysts clearly. 

These cysts may be removed easily by laser, but as they are not harmful, nor bothersome to him, I will leave them alone. Mother Nature knows best. And, as with any condition, observation on a regular basis is key. Some cysts can be melanomas. Even when removed by laser, cysts can return. 

When I peer deep into his eyes, he must wonder what I am doing and what I might plan to ask of him. I cannot help but stand in awe of the science of evolution behind these tiny dark shades. So many wonders of nature are tiny or invisible to the naked eye, yet they play a vital role in the life of a horse or any other creature for that matter. There is so much we do not see, nor understand on which the natural world can shed a bright light.

Katharine MacCornack
Published on 07-10-2020
I started riding as a child and have always loved everything equine. I've been involved in training, breeding, and several disciplines over the years. I live on a small farm with my horses. I am a teacher and a writer.