Corneal ulcers are the most common ophthalmic condition equine practitioners see in the field. Some are simple scratches that heal quickly with appropriate treatment, while others are more complex, involve infection, and can take longer to heal. Regardless of their severity, and they must be managed appropriately to prevent/treat infection, control pain, and speed healing.
“When an ulcer is slow to heal, one must carefully examine the eye to determine what kind of ulcer you are dealing with,” said Catherine Nunnery, DVM, Dipl. ACVO, during a presentation at the 2017 American Association of Equine Practitioners Convention, held Nov. 17-21 in San Antonio, Texas.
Classifying ulcers correctly will ensure the timely application of appropriate therapy.
Nunnery, currently an equine ophthalmologist practicing primarily in Maryland and Virginia, said corneal ulcers in horses can be divided into six types:
- Nonhealing (indolent) ulcers, which occur due to abnormal adhesion of corneal epithelial cells to the anterior stroma (the layer of tissue beneath the outer surface of the cornea);
- Corneal mineralization due to calcium deposits. Patients with this type of ulcer have either recurrent anterior uveitis or chronic/recurrent keratitis (inflammation of the cornea). The calcium deposits are recognized as multiple white opacities in horizontal patterns across the central or ventral (lower) cornea;
- Eosinophilic keratitis, which researchers believe results from exposure to either an allergen or parasites. In these cases, one or both eyes can be involved. Fluorescein staining (a test that uses dye to detect corneal damage) results can be faint and potentially missed without the use of a cobalt blue light. Cytology (viewing cells under a microscope) performed on corneal scapings reveals eosinophils—a type of white blood cell that responds to allergic and/or parasitic stimuli—often without evidence of infection in a relatively nonpainful eye;
- Corneal edema, characterized by fluid in the stromal layer located in the surface of the eye. These are caused because either the corneal ulcer allows fluid to reach the stroma or due to dysfunction of the deepest layer of the cornea (e.g., glaucoma) causes fluid buildup in the stroma and corneal ulcers to form;
- Corneal infection, most commonly cause by bacteria (Staphylococcus, Streptococcus, Psudomonas) and fungi (Aspergillus, Fusarium), and;
- Keratomalacia and stromal loss, the dreaded melting ulcer.
The first four types of corneal ulcers in horses are generally treated similarly; therapy generally involves administering systemic non-steroidal anti-inflammatory drugs (e.g., phenylbutazone or flunixin meglumine equivalent); the cycloplegic atropine to relieve ciliary spasm, which is essentially a “charlie horse” in the muscle that controls pupil dilation; antibiotics effective against both Gram-positive and -negative agents; antifungal agents; and antiproteases such as serum or ethylenediaminetetraacetic acid (more simply called EDTA). As your veterinarian will warn you, any ocular medication containing a corticosteroid is contraindicated for the treatment of ulcerated corneas.
In some cases, a veterinarian might need to remove the damaged tissue using a process called debridement.
“The underlying cause of ulcers can be very difficult to ascertain,” said Nunnery, adding that corneal cytology plays a key role in diagnosing and treating the underlying disease.