Rose bengal positive epithelial microerosions as a manifestation of equine keratomycosis.

The study investigates the appearance of small corneal erosions as a result of keratomycosis in horses. The findings indicate that fungi might not need to completely remove or penetrate the corneal barrier to cause this disease, and that the changes in the mucin layer of tears caused by these fungi may result in these microerosions.

Methods and Results

• The authors conducted a clinical study of six horses that initially were presumed to have viral keratitis, a type of corneal inflammation caused by a virus. They noted punctate (spot-like), multifocal (appearing in several places) superficial corneal opacities, or areas of cloudiness in the outer layer of the cornea. These opacities were found to retain rose bengal, a dye used for diagnosing eye diseases.
• In three cases, faint fluorescein staining was also present. Fluorescein is another dye used in eye examinations. Five of the horses were successfully cultured for fungi, with the type varying from Aspergillus to Curvularia, and one unknown fungus.
• Three cases were found to have mixed bacterial infections. None of the cases showed successful equine herpesvirus tissue culture inoculations, meaning there were no signs of viral keratitis.
• Two horses showed progression to deep melting corneal ulcers which required surgery. However, in the other four horses, the microerosions remained at the surface level.
• All six horses were treated topically with natamycin, an antifungal medication. Electron microscopy from one case revealed an intact corneal epithelial cell layer, disruption of the mucin layer, and joining of the fungus to deteriorating epithelial cells.
• Despite the considerably lengthy healing period, the visual outcome was positive for all six horses. The non-surgery cases took an average of 48.5 days to heal, whereas the surgical ones took an average of 62 days.

Conclusions

• The researchers conclude that complete removal or full-thickness penetration of the corneal epithelial cell barrier might not be necessary for fungi to adhere and initiate keratomycosis. Rather, fungi may induce changes in the mucin layer of the tear film, resulting in rose bengal positive microerosions on the superficial corneal epithelium.
• Moreover, the researchers advise that horses presenting with painful eyes and superficial, multifocal corneal opacities should have their corneas stained with both fluorescein and rose bengal. This is because fungal microerosions may stain weakly, or not at all, with fluorescein, and could be mistakenly diagnosed as ‘viral keratitis’.