Histological findings in corneal stromal abscesses of 11 horses: correlation with cultures and cytology.

Histopathology was compared to culture results and cytology from horses with corneal stromal abscess at the Auburn University and the Ohio State University Veterinary Teaching Hospitals. Significant bacteria were not isolated in culture or seen on histopathology in any of the horses. Although most bacteria infecting equine corneas can be isolated with blood and MacConkey's agars, failure to detect bacterial growth may not rule out infection because anaerobic or intracellular bacteria would not be isolated. The inability to visualise bacterial organisms on histological sections did not rule out their presence in the tissue, because there is often destruction of bacteria by neutrophils, macrophages and antibiotic therapy greatly reducing their numbers. Fungal keratitis was diagnosed by histopathology in 4 of 11 eyes (36%) and keratitis with no aetiological agent in 7 of 11 eyes (64%). Nine of 11 horses (82%) had a prominent neutrophilic stromal infiltrate and 2 (18%) had a predominantly pyogranulomatous reaction. Two of the 4 lesions that showed histological evidence of fungal infection were positive for identifiable fungi on culture and cytology. Fungal cultures of the other 2 cases with histological evidence of mycotic keratitis were negative or grew unidentifiable fungi which were considered pathogenic because, on histopathological sections, fungal hyphae were found deep in the corneal stroma surrounded by an inflammatory reaction. In 3 of 6 cases where fungi were recovered on culture, they were considered contaminants based on lack of evidence of organisms in histopathological sections. Histopathology and the use of special stains were important in the interpretation of culture and cytology results.