Corneal Ulcers In Horses

Overview
The cornea is the clear part of the front of the eye that light passes through. A corneal ulcer is an erosion of the outer layer of the cornea that creates a cloudiness blocking the passing of light into the eye.
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The clear part of the eye is made of three parts – the one cell layer on the outside called the epithelium, the middle part called the stroma, and the inside part called the endothelium. This together is called the cornea of the eye.

Interestingly, there is no fluid in the cornea because the epithelium and the endothelium maintain a waterproof seal. As long as there is no water in the stroma, the corneas remains clear.

With an erosion of the epithelium, water enters the stroma and the familiar cloudiness occurs.

Bodies of humans and horses are filled with fluids because blood is carried to every part of it. However, with the cornea there is no blood flow and hence it stays dry and clear. In order for the eye to work though, it requires water on both sides of the cornea (tears on the outside). But for the stroma of the cornea to get rid of water that has accumulated within it (edema), it must first build a blood supply into the stroma. This can be seen in some of the photos as a fine, red lacy web that starts at the junction of the cornea and the white part of the eye (sclera) and moves across the cornea to the erosion (ulcer) that needs healing.

First the epithelium is healed and then the water is drawn out. With this the edema is resolved, the cloudiness becomes clear, and vision is restored.

Signs of a corneal ulcer include

  • Tearing from one eye.
  • Squinting or eyelid slightly to fully closed.
  • Pain or fully closing the eyelids as you approach it with your fingers.
  • Pain or fully closing the eyelids in response to sunlight or a light source.
  • A constricted pupil when compared to the other eye, especially in the dark.

There are two ways things go bad: 1) treatment is ineffective (or lacking) or 2) the wrong treatment is done.

Ineffective treatment includes not doing anything at all and keeping the eye exposed to the environment with all of its dirt. It also includes not removing the cause such as an ingrown eyelash or a chemically charged insect repellent strip hung from a brow band and remains caught on the eye). There are other things but the result often is scar development on the cornea (see in the pictures). This leads to a small to large solid white spot with permanent blood vessels coursing across the front of the eye.

The wrong treatment includes adding a medicine that allows bacterial infection of the ulcer. The common mistake is to add a steroid eye ointment to the ulcer which inhibits healing of the eye and allows for bacterial growth. The result can be catastrophic for the eye.

There are cases of corneal ulcers where action was taken within a few hours with an appropriate broad spectrum antibiotic, yet the eye does not respond to treatment. I had one case where within 24 hours of corneal ulcer discovery and appropriate treatment, the eyeball became severely infected, the globe doubled in size (very hard to imagine), and the horse was in agony. The only solution was enucleation (removal of the eye).

Treatment: Ideally, have the eye looked at as soon as possible. Your veterinarian will apply a stain to the eye that in a normal eye will run off. In an ulcerated eye, the stain will stick to the erosion and light up under ultraviolet light in a dark room. Stain uptake is proof of a corneal ulcer.

There are two parts to treatment. The first is choosing an appropriate antibiotic. Every case is different and every vet has a preference. Without a culture, it is unknown what antibiotic should be used. Most vets use a broad spectrum antibiotic because they need to treat the eye before getting a lab result back. The sooner treatment is started, the more effective it could be.

The second part of treatment is atropine which paralyses the muscles within the eye that causes the pupil to constrict. In other words it relaxes the iris causing it to dilate. This gives pain relief to the horse. But horses are very sensitive to atropine and in many horses, the pupil can remain dilated for days with just one treatment. This becomes an issue with horses that remain outside in the bright sunlight. My advice is to treat with atropine once and then with every subsequent, look at the pupil size. If it is dilated wide open (compare it to the other eye) and the horse is comfortable, skip the treatment and wait for the next treatment time and reassess the pupil size then.

The stain test should be repeated until there is no stain uptake, the time of which is determined by the size of the ulcer and the horse’s ability to heal.

A subpalpebral medication system can be used in long standing cases. Click this link for more information.

Summary: At the first moment of corneal damage, the sooner it is seen by your veterinarian the faster and better it will heal. Observe a damaged eye often during the treatment. While you can just put in some eye ointment and hope for the best, if at all possible, have it seen by a vet who has seen many of these cases and can determine the best course of treatment.


The Cornea Of The Eye, Part 1

The Cornea Of The Eye, Part 2

Images Of Corneal Ulcers


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