Antibacterial susceptibility patterns for microbial isolates associated with infectious keratitis in horses: 63 cases (1986-1994).

Seventy-three aerobic bacterial isolates were cultured from 64 eyes of 63 horses with infectious keratitis. Forty-two (58%) of the organisms isolated initially were gram-positive (g+, 10 genera) and 31 (42%) were gram-negative (g-, 5 genera). After local antimicrobial treatment, repeat cultures from samples obtained from 15 eyes of hospitalized horses yielded 21 secondary bacterial isolates. Staphylococci spp and Streptococci spp were the most common g(+) isolates and accounted for 79% of g(+) organisms isolated initially. Antibiograms revealed ticarcillin to be the most efficacious antibiotic tested on g(+) organisms, with 28 of 30 (93%) being susceptible. Of commercially available topical ophthalmic antibiotics tested on g(+) organisms, erythromycin was the most efficacious, with 32 of 35 (91%) isolates being susceptible. Pseudomonas spp, Escherichia coli, and Acinetobacter spp accounted for 68% of g(-) organisms isolated initially. Gentamicin, tobramycin, polymyxin B, and neomycin were highly effective in vitro against initial g(-) isolates. Chloramphenicol was ineffective against g(+) and g(-) organisms isolated initially. A significantly (P < 0.05) higher frequency of g(-) organisms was noticed on repeat cultures after intensive topical antimicrobial treatments as compared to organisms isolated at initial examination. Pseudomonas organisms isolated from second cultures were resistant to gentamicin, but susceptible to ciprofloxacin. Overall, secondary g(-) isolates were more susceptible to ciprofloxacin, neomycin, tobramycin, or amikacin than to gentamicin. Fungi were isolated in 24 of 63 (38%) horses in the study. Twenty-five filamentous fungi and 2 yeasts were identified from 24 eyes.(ABSTRACT TRUNCATED AT 250 WORDS)