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
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:
Your veterinarian will usually prescribe several drugs, among them, atropine, anti-inflammatory drugs and antibiotics.
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.
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.
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.
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
Gentocin is often a first choice, although it is not effective against gram positive bacteria. In the case of complicated bacterial infections, your veterinarian may choose to use ciprofloxacin or chloramphenicol. These are both good broad spectrum antibiotics, although ciprofloxacin does not target Streptococcal species well. Chloramphenicol has the virtue of penetrating the eye well, and it is very broad spectrum. However, it is more toxic than the other antibiotics, so it is usually reserved for difficult or unresponsive cases.
If the eye is very inflamed, and there is good neovascularization, your veterinarian may choose to place your horse on systemic antibiotics. In most cases, it is not only unnecessary but useless, as systemic antibiotics will not be able to reach most corneal ulcers.
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.
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.