Corneal Ulcers in Horses
Dr. Melissa Mazan
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.
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.
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.
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.
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.