Clotrimazole reduces fungal dysbiosis risk without compromising antibacterial therapy for canine otitis externa, study shows

30 September 2026

Adding the antifungal drug clotrimazole to piperacillin-tazobactam treatment for canine bacterial otitis externa markedly reduces the risk of fungal dysbiosis, while maintaining excellent antibacterial effectiveness, a new study in the Journal of Small Animal Practice has found.

While antibacterial therapy can be highly effective in treating canine bacterial otitis, fungal dysbiosis such as Malassezia is commonly seen after treatment when using potent broad-spectrum drugs, especially piperacillin-tazobactam.

Researchers at the Royal Veterinary College, funded by BSAVA PetSavers, investigated treatment outcomes in dogs with refractory bacterial otitis externa. They compared 21 dogs treated with topical piperacillin-tazobactam (PT) combined with intermittent administration of 1% clotrimazole (CTZ) against 14 dogs receiving PT alone.

The addition of CTZ to the protocol and associated reduction of the frequency of PT application did not compromise the antibacterial effectiveness of treatment and substantially reduced the risk of fungal dysbiosis by 81%, compared with PT therapy alone.

Bacterial counts fell markedly with both PT and PT-CTZ treatments, with no meaningful differences in antibacterial response between the two protocols. Whilst yeast counts increased in every dog treated with PT alone, an increase was observed in only one-third of dogs receiving the PT-CTZ combination.

The findings demonstrate that a combination of clotrimazole with piperacillin-tazobactam is highly effective at treating refractory bacterial otitis, while substantially reducing the risk of treatment-associated fungal dysbiosis.

Although the addition of clotrimazole did not completely eliminate fungal dysbiosis risk, researchers recommend that clinicians should consider the concurrent use of clotrimazole when using piperacillin-tazobactam to minimise the risk of secondary yeast overgrowth.

The study formed the research component of Dr Georgina Gallow’s dermatology residency programme under the supervision of Professor Ross Bond. Dr Gallow, lead author, commented: “My predecessor, Dr Jonna Juhola and team, showed that fungal dysbiosis was a common issue in dogs with chronic otitis externa treated with potent antibacterial monotherapy, mostly for refractory pseudomonad infections. It was most satisfying to extend the work and identify a solution.”

Professor Bond added: “We are most grateful to BSAVA PetSavers for kindly agreeing to extend their original funding to enable the assessment of clotrimazole in this clinical scenario. Whilst dysbiosis tends not to occur in routine cases managed with conventional polypharmacy ear drop products, the results have enabled us to adopt preventative therapy to improve the outcome of the most challenging cases of otitis.”

Read the paper in the Journal of Small Animal Practice.