Equine Corneal Ulcer Cytology: Why and How

Corneal ulcers—loss of or damage to tissue on the outer surface of the eye—occur commonly in horses. While some corneal ulcers might not be particularly challenging for veterinarians to treat, many others are complicated by infection with either bacteria or fungi, foreign bodies, inflammatory reactions that cause tissue to swell and “melt,” and more, and require more intensive diagnostics and treatment.

Ann Dwyer, DVM, a private equine practitioner at Genesee Valley Equine Clinic, in Scottsville, New York, and a former American Association of Equine Practitioners (AAEP) president, has a special interest in ophthalmology and often presents on the topics at veterinary meetings. The 2017 AAEP Convention, held Nov. 17-21 in San Antonio, Texas, was no exception. There she shared her wisdom gained from treating many equine eye problems.

Dwyer said one of the first items a veterinarian reaches for when faced with a painful eye is fluorescein stain—a green dye that does not adhere to normal corneal epithelial tissue but binds with deeper layers of the stroma of the eye to show the location and extent of a corneal defect.

Once veterinarian diagnoses an ulcer, then what? Starting treatment without first addressing the underlying cause, can set the horse and owner up for a prolonged treatment period and put the horse’s vision at risk.

“Successful resolution of corneal ulcers requires targeted therapy, and this can only be achieved by performing corneal cytology (sampling cells to view under a microscope),” said Dwyer. “(This) is a simple stall-side procedure that provides essential information needed to prescribe rational therapy.”

She added that equipment needed to perform corneal cytology is affordable and readily available: a cytology sampling tool (a kimura spatula, the base of a scalpel blade, or a cytology brush) and a head lamp (one from a camping store will do just fine, she said).

To collect a sample, the veterinarian must:

  • Appropriately sedate the horse, provide head support, and use local eyelid blocks and topical anesthetic for the cornea (preferably using tetracaine or proparacaine, but lidocaine will do in a pinch, she said).
  • Be prepared! Have everything on hand to maximize “bang for buck” during sedation (and not have to resedate the horse).
  • Use the sampling device properly. Pretend to frost a cake, she advised. Hold the tool at a 45° angle, and use firm, even strokes. The cornea is tougher than it appears, so be certain to apply sufficient pressure. Veterinarians often do not collect enough cells.

After obtaining the cytology sample, use several sterile dry cotton swabs to debride the lesion and remove loose epithelium. Dwyer warned, “Large sections of epithelium are sometimes removed during debridement … if this happens, don’t panic! The tissue was diseased and needed to go.” She also noted that while swabs are good for debridement, they are not suitable cytology sampling devices.

Once slides are prepared, veterinarians can stain them with a simple set of Romanowsky stains (Dif-Quik) using either a rapid-dip or soaking method. The veterinarian can then Gram-stain samples showing infectious agents. Dwyer said a fingernail drier can dry slides rapidly for immediate evaluation. She reminded attendees to examine the entire slide so as not to miss foreign bodies or infectious agents and to use low and high microscope power, as well as oil immersion (a technique to increase a microscope’s resolving power of a microscope which involves immersing both the cells and the objective lens in a transparent oil with a high refractive index,).

“If the cornea was healthy, all you should see is corneal epithelial cells that look like sheets of fried eggs; there should be nothing else,” said Dwyer. Even a single fungal hyphae (a long, branching filamentous structure of a fungus) can be significant and require treatment.

When examining the slides, the main questions the veterinarian considers include:

  • Are there non-native or inflammatory cells (neutrophils or eosinophils)?
  • Are there any infectious agents, such as fungal hyphae or bacteria? If there are infectious agents, are they inside or outside the cells?
  • Is any foreign material (such as plant material, mineral crystals, and external parasites) present?

“Slides can easily be photographed (through the microscope lens) with any cell phone, making it easy to share information with the client and consult with other veterinarians,” Dwyer said.