Fungal dysbiosis following antibacterial monotherapy in canine otitis externa.

Objectives: To evaluate evidence of Malassezia overgrowth following successful topical antibacterial monotherapy of refractory canine bacterial otitis using semi-quantitative cultures.

Materials and methods: Twenty-nine dogs with bacterial otitis were treated topically with either fluoro-quinolone [0.5% enrofloxacin (18 dogs, 19 treatment events, 25 ears) or 0.1% marbofloxacin (1 ear), with 0.1% dexamethasone] ("FQ") SID, or 143 mg/mL piperacillin/18 mg/mL tazobactam ("PT") BID (11 dogs, 14 treatment events, 19 ears) for 8 to 36 days (mean 20 days). At visits 1 (V1) and 2 (V2), ear swab tips were washed in PBS + Triton X-100 and serial dilutions spread-plated onto blood, MacConkey (37°C, 48 hours) and modified Dixon's agar (32°C, 14 days) to generate semiquantitative counts. Microbes were identified by phenotype and MALDI-TOF.

Results: Prior to treatment, Pseudomonas aeruginosa was isolated alone or in combination with other bacteria in 14 FQ-treated ears and 18 PT-treated ears; the next most frequent bacteria were Streptococcus canis (8 FQ, 2 PT) and Staphylococcus pseudintermedius (8 FQ, 1 PT). The proportions of dogs' ears (excluding cross-over treatments and contra-lateral affected ears) from which bacteria were isolated were significantly reduced by treatment with both FQ (V1, 13/15; V2, 5/15) and PT (V1, 14/14; V2, 2/14). The proportions of dogs' ears from which yeasts (Malassezia pachydermatis, Candida spp.) were isolated were significantly increased by treatment in dogs treated with PT (V1, 1/14; V2, 14/14) but not FQ (V1, 3/15; V2, 6/15).

Clinical significance: Otitis cases that receive potent antibacterial monotherapy must be monitored for yeast overgrowth.