Corneal Ulcers in Horses

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.

The cornea is composed of multiple layers of cells that have the important quality of being transparent. The outermost layer of cells is called the epithelium, the inner layers are called the stroma, and the innermost layers are called Descemet's membrane and the endothelium.

Although the cornea has no blood vessels, it has many nerves – as anyone who has had a speck of dust in his eye can attest. This very extensive innervation is important – it reminds us to blink our eyes to prevent injury and to keep the tear layer over the cornea – but it causes intense pain with even the most minor of eye injuries.

The endothelium has no ability to heal, so it is very important to prevent ulcers from penetrating through that layer. Most ulcers are the result of trauma – most frequently due to a foreign body, such as a piece of hay or other small debris. However, many ulcers can quickly become seeded with bacteria or fungi, which makes treatment more difficult.

Bacteria that produce destructive enzymes, such as Pseudomonas (a gram negative bacterium) and Streptococcal species are most likely to cause melting ulcers. The cornea will actually start to look as though it is melting – it will have the appearance of wax dripping down the side of a candle. The destructive enzymes are usually collagenases, or enzymes that destroy collagen connective tissue.

Corneal ulcers are most difficult to treat when they are down to Descemet's membrane, have a fungal component (mycotic ulceration), are indolent, or are in an immune-compromised horse or a horse that has been treated with corticosteroids.

A descemetocele refers to a corneal ulcer that is so deep that it extends to Descemet's membrane – the single layer of cells that forms the very innermost layer of the cornea. This single layer is much thinner than the thinnest hair – and is all that lies between the anterior chamber of the eye and the outside world. Once that inner membrane – Descemet's membrane – is ruptured, the contents of the inner eye will begin to leak out, and the inside of the eye will rapidly become infected. At this point, it is very difficult to repair the damaged, delicate cornea, and a surgical repair will be necessary. Whereas simple, uncomplicated corneal ulcers usually heal well with a minimum of treatment, some ulcers become deep, chronic, or simply non-responsive.

Infected Ulcers

Corneal ulcers most frequently become infected with bacteria in warm, wet conditions or in eyes that have been treated with corticosteroids, which decreases the eye's ability to fight off infection, or with long-term or multiple antibiotics. Antibiotic treatment can lead to fungal infection because it may kill off all the bacteria that naturally compete with the fungi.

Fungal infections are usually seen several days to weeks after initial ulceration, and may appear as a sudden worsening of an ulcer that had apparently been getting better. They often appear to be very deep, and often have a whitish to yellowish plaque of accompanying material – this is actually cellular debris and the fungi themselves. The fungi have a preference for Descemet's membrane, which contributes to the ulcers becoming very deep and extensive.

Fungal ulcerations must be treated with anti-fungal agent – most of which are not approved for eyes, but are useful nonetheless. Anti-fungal treatment can be very expensive. When ulcers have a fungal infection they can take months of treatment in order to save the eye.

Surgical treatment is often necessary with mycotic ulcers.

Indolent Ulcer

An indolent ulcer is one that does not heal properly despite appropriate treatment. Indolent ulcers are more common in older horses, horses with equine Cushing's disease (ECD), and horses that have been on corticosteroids treatment.

Equine Cushing's disease makes indolent ulceration more likely because it increases the body's own production of corticosteroids, thus causing immunosuppression and poor wound healing. Indolent ulcers often do not have the acute, painful appearance of an infected ulcer. In fact, the vessels that are so critical for ulcer healing often fail to appear.

With indolent ulcers, the cornea produces the cells that are necessary to heal the ulcer, but these ulcers lack the ability to stick to the underlying membrane, and merely slide off. It is often necessary to perform a grid keratectomy or a conjunctival flap in order to encourage healing.

What to Watch For

• Tearing is the eye's way of ridding the eye of any foreign body – in a way, it flushes out the eye. It is due to stimulation and inflammation of the tear ducts.
• The typically cloudy eye is actually corneal edema. This refers to fluid being retained between the layers of cells in the cornea, and tells us that the cornea is not functioning properly.
• The swollen eyelid is often the result of the horse rubbing his eye in an attempt to relieve himself of the pain. Unfortunately, the result is often that the horse does even further damage to his eye.
• The reddened eye reflects both inflammation and formation of new blood vessels. With deep, extensive, infected ulcers, the eye will not be able to heal until blood vessels grow into the cornea and can help to heal the cornea as well as helping to deliver antibiotics and anti-inflammatory drugs. These vessels only grow one millimeter each day in the best of situations – consequently the healing process is limited by the growth of the blood vessels. The blood vessels are a double-edged sword, however – although they help to heal the eye, they also interfere with vision. In treating corneal ulcers, we are often torn between welcoming the appearance of blood vessels and wanting to limit the excessive growth of blood vessels so that minimal disruption to sight occurs.

Diagnosis

• Slit lamp. A slit lamp a very bright, focused source of light that allows the ophthalmologist to assess the depth and extent of the ulcer, and allows a better look at the inner structures of the eye.
• Ultrasound. If the eye has severe corneal edema, to the extent that the inner portion of the eye cannot be seen, and the ophthalmologist suspects that there is further damage to the inner portion of the eye, she may choose to look at the eye using ultrasound. This is performed with the horse's eyelid closed, and can help to determine if complications such as retinal detachment or cataracts are present.
• Bacterial studies. In a complicated infected corneal ulcer, it is important to try to determine what is causing the infection. This way, treatment can be specific. When the cornea appears to be melting, we usually suspect Streptococcal species (gram positive bacteria) or Pseudomonas (a gram negative bacterium).
• Corneal scrapings. It is also very important to determine what type of fungus might be present, but the search for the fungus is often less rewarding. This is partially because the fungi tend to be so deeply imbedded in the cornea that a corneal scraping only yields inflammatory cells and dead corneal cells, but no infectious agents.
• Virus check. We sometimes suspect that a virus may be the cause of a superficial persistent ulcer – especially if there are multiple tiny ulcers. The most likely virus is equine herpes virus.

Medical Treatment

• Antibiotics. For initial treatment, we try to find an antibiotic that has broad spectrum properties (meaning that it targets both gram negative and gram positive bacteria) and has good penetration into the eye. Most antibiotics do not penetrate the eye well, although when the eye is inflamed it is more receptive to antibiotics. A good initial choice is triple antibiotic.
• Anti-fungal medication. The only anti-fungal medication that is actually approved for use in eyes is natamycin. However, many others, including miconazole vaginal cream and the intravenous form of fluconazole, can be used successfully in the eye.
• Collagenase inhibitors. When a corneal ulcer starts to melt, or when we suspect that this will happen, it is important to use therapy that will prevent collagenases that are produced by bacteria and fungi from functioning. One of the most useful anti-collagenases is the horse's own serum.

We harvest the serum by drawing approximately 2 to 3 milliliters of blood. The blood is spun down, and the serum drawn off and stored in a sterile container. This is usually only practical in a hospital setting, as it must be drawn fresh and prepared every day. Serum is a wonderful growth medium for bacteria, so it cannot be allowed to sit, even in the refrigerator, for more than one day.

The other commonly used anti-collagenase is acetylcysteine. This can be stored for longer periods of time, and is more commonly prescribed for use at home.

• Debridement. If there is cellular debris at the area of the ulcer, it will be difficult to impossible for new, healthy corneal cells to thrive. Debridement refers to gently removing these dead cells so that the new cells can move in. It can be done with a sterile swab or, if a more aggressive approach is necessary, with a specialized scalpel blade.
• Subpalpebral lavage. Many horses become resistant to treatment – their eyes hurt, and the medication is often stinging. A subpalpebral lavage system acts as a catheter into the eye, and allows us to instill the medication from a port that is located at the level of the withers. It can be placed in the standing horse under heavy sedation.

Surgical Treatment

• Conjunctival flap. If the blood vessels are slow to arrive, there is a descemetocele, or a severe and extensive ulcer, the ophthalmologist may choose to use a portion of the horse's conjunctiva (the pinkish fleshy inner portion of the eyelid) as a 'band-aid' for the cornea. This will make healing more rapid, and may help to save the eye, but it is important to know that it may result in more scarring.
• Grid keratectomy. In indolent ulcers, the new cells have trouble attaching to the underlying corneal stroma. Your veterinarian may be able to aid the cells in attaching by making tiny channels in the stroma with a sterile needle. This can be done in the field under heavy sedation.
• Corneal transplant. In some cases, the cornea has endured such massive damage that a corneal transplant presents the best option for saving the eye. This must be done in a referral center by an ophthalmologist.

Corneal ulcers should always be considered emergencies. If your horse has a swollen, weepy eye, if his eye is cloudy or red, or he keeps his eye clamped shut, you should call your veterinarian immediately.