Changes in bacterial and fungal ocular flora of clinically normal horses following experimental application of topical antimicrobial or antimicrobial-corticosteroid ophthalmic preparations.

The research sought to determine how topical antimicrobial and antimicrobial-corticosteroid preparations affect the ocular bacteria and fungi present in the eyes of healthy horses. It found that the use of these treatments resulted in a temporary reduction in the number and variety of these microorganisms, although repopulation occurred during and after treatment.

Research Methodology

• The experiment involved 40 horses. In each case, one eye received topical treatment three times daily for two weeks. There were four treatment groups: neomycin-bacitracin-polymyxin B ointment, 0.6% prednisolone-0.3% gentamicin ointment, neomycin-polymyxin B-0.05% dexamethasone ointment, or a control treatment of artificial tears.
• The eye not being treated in each horse served as a control.
• Bacterial and fungal samples were taken from the cornea and conjunctiva before treatment, after one and two weeks of treatment, and two weeks after the end of treatment. Culture growth quantity scores of bacterial and fungal species were calculated and compared across treatment groups.

Results and Findings

• Prior to the study, the most common bacterial and fungal species inhabiting the eyes of the horses were identified. These included various gram-positive and gram-negative bacteria, as well as certain types of fungi, like Aspergillus nidulans, Cladosporium spp, and Aspergillus fumigatus.
• The study found that in general, all treatments resulted in a decrease in the percentage of positive bacterial culture results, the quantity of growth of gram-positive bacteria, and the number of bacterial species isolated, after the first week. These numbers increased again in the second week.
• The growth quantity of gram-negative bacteria decreased throughout the treatment period, regardless of the treatment type.
• No significant differences were observed between the four treatment groups for all the measures examined.
• The study also found that fungal growth quantity decreased across all treatment groups during the treatment period.
• Repopulation of both bacterial and fungal species occurred during and after treatment across all groups.

Conclusions

• All the treatments successfully decreased the number of microorganisms including bacteria and fungi in the eyes of the horses, thus affirming their antimicrobial effects.
• Despite this, repopulation of normal flora occurred both during and after treatment, suggesting that the effects of such topical treatment are temporary, and continued application may be necessary to maintain their effects.