Antibiotic treatment for otitis externa leads to yeast overgrowth in dogs’ ears, study shows

11 November 2024

Yeast populations may opportunistically increase in dogs’ ears following broad spectrum antibacterial treatment for chronic otitis externa, according to a new study.

Malassezia yeast overgrowth is often observed by cytological examination following successful topical therapy of canine bacterial otitis externa by antibacterial drugs, but this phenomenon is not well described.

A new study, conducted by researchers at the Royal Veterinary College and published in the Journal of Small Animal Practice, assessed 29 dogs with bacterial otitis externa. The dogs were treated topically with either fluoroquinolone or piperacillin-tazobactam, and ear swabs collected from affected ears before and after treatment for cytological evaluation and semi-quantitative culture.  These drugs were used in cases when conventional licensed polypharmacy products had failed or were contra-indicated due to rupture of the tympanic membrane.

The study found that while both antibacterial treatments significantly reduced bacterial counts, they also led to an increase in otic yeast populations, primarily with Malassezia pachydermatis, and less often Candida spp., particularly in cases treated with piperacillin-tazobactam. This highlights the potential for fungal overgrowth, particularly of Malassezia pachydermatis, following the use of potent antibacterial monotherapy in dogs.

These findings suggest that veterinary surgeons should carefully monitor for yeast overgrowth in dogs following antibacterial treatment for bacterial otitis. More research is needed to determine whether antifungal drugs or antiseptic ear rinses can be combined with antibacterial therapy to prevent fungal overgrowth.

Professor Ross Bond, corresponding author of the study, said: “We are very grateful to BSAVA PetSavers for kindly funding this study and to the owners of the dogs for agreeing to participate.  It was most gratifying to be able to confirm our previous observations by use of semi-quantitative cultures. Our next goal is to find treatment protocols that prevent the fungal dysbiosis without compromising the efficacy of the antibacterial therapy in these very difficult ear cases.”

You can access the full study here: http://doi.org/10.1111/jsap.13801.