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Corneal Ulcers in Horses

By: Dr. Melissa Mazan

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The cornea is the visible, glistening, clear, external portion of the eye. In humans, a portion of the eye called the sclera, or the white of the eye, is also clearly visible. We rarely see much of the sclera in the horse, although in Apaloosas, paints and some other horses the sclera is visible, giving the appearance of a somewhat wild eye.

The cornea is extremely thin, less than 2 millimeters, or approximately the thickness of a hair. Horses' eyes have a very prominent place on their faces, which makes them very attractive but prone to injury. Since the cornea has no blood vessels, light travel through it easily. However, that same lack of vessels means that the cornea heals very slowly when it is injured.

Corneal ulceration occurs when one or more layers of the cornea are damaged. Unfortunately, the horse's cornea heals more slowly than any other species. This is complicated by the fact that horses can be very resistant to treatment.

Corneal ulcers are usually caused by trauma. For instance, a piece of hay or dust may be caught in the horse's eye, or your horse may bump his eye on something protruding in his stall, or be involved in a paddock fight.

Most corneal ulcers start out by being sterile – that is, they are not infected. However, most will become secondarily infected. Once a corneal ulcer is infected, the bacteria and their products can cause the cornea to break down at an accelerated rate. This is known as a melting ulcer and can result in rapid destruction of the cornea. The most common bacteria are gram positive, but a wide variety of bacteria may be present. Uncomplicated, non-infected corneal ulcers usually heal in 2 to 6 days. However, complicated ulcers may take many weeks or months to heal.

What to Watch For

  • Swelling of the eyelids and mucous membranes
  • Tearing
  • Cloudy eye (corneal edema)
  • Sensitivity to bright lights
  • Reddened eye
  • Constricted pupils
  • Spasm of the eyelids
  • Pus-like ocular discharge
  • Head-shyness

    Diagnosis

    Your veterinarian will first take an in-depth medical history and will concentrate on trying to find out how your horse developed the ulcer in the first place. He will need to know if your horse has been treated with any type of corticosteroids in the recent past, as use of these drugs can seriously inhibit healing of corneal ulcers. Your veterinarian may also recommend the following:

  • Examination for any underlying medical problems, such as equine Cushing's disease, that might result in increased susceptibility to corneal ulceration or poor healing.

  • Careful examination of the eye with a penlight. He will also examine the insides of the lids and inside the nictitating membrane (third eyelid) to look for a possible foreign body that may be trapped against the eye.

  • Staining of the affected eye with fluroscein. This will help to determine the extent and depth of the ulcer. The fluroscein has a green color which attaches to the inner layers of the cornea (the stroma), but does not stain the outermost or the innermost layers of the cornea. Thus, a lack of stain uptake usually indicates that no ulcer is present. However, if the ulcer is very deep and penetrates to the innermost layer (or Descemet's membrane) of the cornea, it also will have no stain uptake. This situation, termed a descemetocele, is very serious indeed.

  • A culture of the corneal ulcer to determine if, and with what bacteria, the ulcer is infected

  • A corneal scraping for microscopic examination of the cells in order to try to tell what type of bacteria or fungi may be present

    Treatment

    Your veterinarian will usually prescribe several drugs, among them, atropine, anti-inflammatory drugs and antibiotics.

  • Atropine is important in dilating the eye and decreasing some of the pain caused by corneal ulceration.

  • Anti-inflammatory drugs can be applied both topically (meaning to the eye itself) and systemically. The most commonly used systemic drug is Banamine. Anti-inflammatory drugs help to decrease pain as well as decreasing inflammation.

  • Antibiotics should be applied topically, and may need to be applied as infrequently as every 6 hours in simple ulceration, or as frequently as every hour or two in complicated ulceration. Typical choices include triple antibiotic and gentomycin.

    Your veterinarian will often advise that your horse wear an eye mask or at least a fly mask. This will serve to protect the eye, and will shade him from bright sunlight. Remember that atropine dilates the pupil, and bright sunlight is painful to a dilated eye. Eye masks are preferable to fly masks because they have a hard, protective covering over the eye, and horses with painful eyes often make matters worse by rubbing their eyes in an effort to get rid of the pain.

    Home Care

    It is extremely important to follow your veterinarian's instructions exactly. If you are instructed to treat your horse every four hours, then you must treat every four hours throughout the day and night, and not just when it is convenient.

    It is important not to administer any eye ointments that contain corticosteroids; this will inhibit healing and may result in a much larger and more severe ulcer.

    If your horse is being treated with atropine, then you should monitor his appetite and manure production every day. Atropine can be absorbed systemically from the eye and can cause the gastrointestinal system to shut down.

    It is important to keep your horse out of hot, dusty areas. In fact, avoid any situation that would make his eye itchy and would make your horse want to rub his eye.

    Monitor your horse carefully for signs of recovery. He should look more comfortable every day. Any signs of increased pain, swelling, ocular discharge, or eyelid spasm may mean that the ulcer is not healing as well as it should, and you should call your veterinarian immediately.

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