In Vitro Evaluation of the Antibiotic Activity of Combinations of Ophthalmic Drugs Against Common Equine Ocular Pathogens

A subpalpebral lavage catheter is commonly used to administer medications in the treatment of infectious keratitis in horses. Drugs may mix in the subpalpebral lavage catheter, potentially affecting antimicrobial efficacy. The objective of this study was to determine the in vitro antimicrobial efficacy of combinations of commonly used ophthalmic preparations. An agar gel diffusion bioassay was used to determine the antibiotic activity of a number of antimicrobial, antifungal, mydriatic, and antiproteinase drug combinations. Drug mixtures were allowed to sit in a syringe at ambient temperatures and room lighting, for 1 (T1) and 6 hours (T2). The antibacterial efficacy was determined by measuring the zone of inhibition around wells containing the drug combinations after overnight incubation at 37°C and comparing them with the zone of inhibition around the antimicrobial alone. Combinations were tested against Pseudomonas aeruginosa and Streptococcus zooepidemicus. There was no significant decrease in the zone of inhibition, compared with controls, for the majority of combinations tested. Significantly smaller zones of inhibition were identified for the following combinations: (1) tobramycin, cefazolin, natamycin, and serum (T1) and (2) tobramycin, cefazolin, natamycin, serum, and atropine (T1) compared with tobramycin alone; (3) ciprofloxacin, cefazolin, natamycin (T1) compared with ciprofloxacin alone; (4) triple antibiotic ophthalmic solution (TAB) and natamycin (T6); (5) TAB, natamycin, and serum (T6); and (6) TAB, natamycin, serum, and atropine (T6) compared with TAB alone. Admixture does not affect in vitro antibacterial efficacy for the majority of ophthalmic preparations, although efficacy of TAB may be decreased compared with the drug alone.