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In vitro efficacy of an ophthalmic drug combination against corneal pathogens of horses

Nicole C. ScottyUniversity of Florida Veterinary Medical Center, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32608.

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Dennis E. BrooksUniversity of Florida Veterinary Medical Center, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL 32608.

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Crystal D. Schuman RoseUniversity of Florida Veterinary Medical Center, Department of Small Animal Clinical Sciences, Clinical Microbiology, Serology, and Parasitology Laboratory Service, College of Veterinary Medicine, University of Florida, Gainesville, FL 32608.

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Abstract

Objective—To evaluate the in vitro efficacy of an ophthalmic drug combination against common corneal pathogens of horses.

Sample Population—Representative isolates of 3 bacterial and 2 fungal corneal pathogens of horses.

Procedures—Pathogens were subjected to minimum inhibitory concentration (MIC) testing of a drug combination that consisted of equal volumes of natamycin 3.33%, tobramycin 0.3%, cefazolin 5.5%, and equine serum. Proteinase inhibitory activity of the drug combination was assessed by use of a fluorescence microplate assay with gelatin and collagen I as substrates. The MICs of the drug combination were compared with those for each of the component medications and antiproteinase activity of the drug combination was compared with that of serum by use of paired t tests and a 2-way ANOVA, respectively.

Results—The drug combination was at least as effective as each medication separately for inhibiting microbial growth of all pathogens tested and was significantly more effective against B-hemolytic Streptococcus spp, Aspergillus spp, and Fusarium spp than the relevant medications separately. Serum and the drug combination both had significant antigelatinase activity, and serum had significant anticollagenase activity. Antiproteinase activity of serum was a concentration-dependent event, which enabled serum to achieve significantly greater activity than the drug combination after 3.5 and 4 hours of intubation for the gelatin and collagen I assays, respectively.

Conclusions and Clinical Relevance—Drug combinations have the attractive potential of minimizing the time, stress, and fatigue associated with topical treatment regimens consisting of multiple drugs used separately for horses with keratitis.

Abstract

Objective—To evaluate the in vitro efficacy of an ophthalmic drug combination against common corneal pathogens of horses.

Sample Population—Representative isolates of 3 bacterial and 2 fungal corneal pathogens of horses.

Procedures—Pathogens were subjected to minimum inhibitory concentration (MIC) testing of a drug combination that consisted of equal volumes of natamycin 3.33%, tobramycin 0.3%, cefazolin 5.5%, and equine serum. Proteinase inhibitory activity of the drug combination was assessed by use of a fluorescence microplate assay with gelatin and collagen I as substrates. The MICs of the drug combination were compared with those for each of the component medications and antiproteinase activity of the drug combination was compared with that of serum by use of paired t tests and a 2-way ANOVA, respectively.

Results—The drug combination was at least as effective as each medication separately for inhibiting microbial growth of all pathogens tested and was significantly more effective against B-hemolytic Streptococcus spp, Aspergillus spp, and Fusarium spp than the relevant medications separately. Serum and the drug combination both had significant antigelatinase activity, and serum had significant anticollagenase activity. Antiproteinase activity of serum was a concentration-dependent event, which enabled serum to achieve significantly greater activity than the drug combination after 3.5 and 4 hours of intubation for the gelatin and collagen I assays, respectively.

Conclusions and Clinical Relevance—Drug combinations have the attractive potential of minimizing the time, stress, and fatigue associated with topical treatment regimens consisting of multiple drugs used separately for horses with keratitis.

Contributor Notes

Dr. Scotty's present address is the Veterinary Medical Center, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, University of Minnesota, Saint Paul, MN 55108.

Presented in part at the 37th Annual Conference of the American College of Veterinary Ophthalmologists, San Antonio, Tex, November 2006.

The authors thank Scott Killian for assistance with antibacterial testing protocols and Annette Fothergill for assistance with antifungal testing protocols.

Address correspondence to Dr. Scotty.