Why Does My Dog Keep Getting Ear Infections?

Your dog shakes its head, the ear is red and smells wrong, and the vet prescribes drops. Within a week or two the ear looks normal again, and a few weeks or months later you are back in the same consulting room. Most owners in that position conclude that the infection has returned, and that stronger drops or a longer course are what is needed.

That is a reasonable reading, and often an incomplete one. Veterinary dermatologists have long treated the organisms in a sore ear as one layer of the disease and not its origin. Three recent studies, each with real limitations, add detail to that view and point towards a different kind of long-term management.

An ear infection is usually the second thing to go wrong

Otitis externa means inflammation of the external ear canal, and the Merck Veterinary Manual separates what drives it into four groups. Primary causes create disease in a normal ear: allergy (both atopic dermatitis and adverse food reactions), parasites, foreign bodies and hormonal disease among them. Secondary causes create disease only in an ear that is already abnormal, and this is where bacteria and Malassezia yeast sit. Merck describes these as often chronic or recurrent problems that develop when the primary cause is not addressed.

The other two groups explain why some ears are harder than others. Predisposing factors are present before any disease, such as pendulous ear flaps, hairy or narrow canals and moisture from swimming. Perpetuating factors are the damage that inflammation leaves behind: swelling and narrowing of the canal, enlarged glands, calcified tissue and middle ear disease. A spaniel that swims has two predisposing factors, but neither explains what first inflamed the ear. Small numbers of cocci and yeast live in most healthy canine ears, and they become a problem when the conditions around them change.

What one antibacterial study found

At the Royal Veterinary College, Juhola and colleagues followed 29 dogs with bacterial ear infections, most of them referred with chronic disease after licensed products had failed. Ears were swabbed before and after topical treatment, which lasted between 8 and 36 days, with one of two antibacterials. Both reduced bacterial counts. In the piperacillin–tazobactam group (11 dogs, 14 courses of treatment, 19 ears), yeast was cultured before treatment on one occasion in 14 and afterwards on all 14.

The other group matters as much. Dogs given a fluoroquinolone solution, which was combined with the steroid dexamethasone, went from yeast on three occasions in 15 to six in 15, a change that was not statistically significant. The dogs were not randomly assigned, so the study cannot say why the two groups differed. What it shows is that a shift towards yeast can follow one potent antibacterial used on its own, and the authors' advice was to monitor such cases for yeast overgrowth. It does not show that antibacterial ear treatment in general causes yeast infections.

A year of combination drops that did not hold

The second study, by Ramos and colleagues in Brazil, is a different kind of evidence and should not be read as a repeat of the first. It followed 45 dogs (90 ears), all with a history of bilateral recurrent otitis and at least three relapses a year, after excluding 13 dogs with signs of atopic dermatitis. For the first year the dogs received a conventional combination ear drop containing an antibiotic (ciprofloxacin), an antifungal (clotrimazole) and a steroid (betamethasone), given twice daily for 21 days and repeated every three months.

Bacterial scores fell over that year. Yeast scores rose, despite the antifungal in the product. The most severe otoscopy scores became rarer, but moderate scores became more common, and the ears did not return to normal. The authors called this a failure of the stabilisation phase under their conditions, and suggested several possible contributors, including a shift in the ear's microbial population and reduced susceptibility of the yeast to clotrimazole. Those are proposed explanations, and the study did not test them.

The phase that worked

The same 45 dogs then moved to a year of what the authors called proactive management: a steroid-only solution, hydrocortisone aceponate, applied to each ear twice a week, with no antibiotic or antifungal. Ears were scored on a 12-point otoscopy scale on which 0 or 1 is considered healthy. From the 90-day check onwards, 96.7% of scores were 0 or 1, compared with 2.7% during the year of combination drops, and most ears showed no detectable yeast or bacteria under the microscope.

That is a striking difference, and it needs careful handling. There was no placebo or control group, every dog received the phases in the same order, and all were treated at a single clinic. No side effects were observed, though hormone testing was not carried out. The result is promising evidence that controlling inflammation can achieve what repeatedly targeting organisms did not. It is not proof.

A second study pointing the same way

Independent support comes from Lorente-Méndez and Alonso-Miguel, who reviewed the records of 63 dogs (115 ears) seen at two dermatology referral centres. These ears had chronic or recurrent otitis but no active infection, most having been enrolled after an infection had been cleared. Each was treated with hydrocortisone aceponate until inflammation settled, then kept on a maintenance schedule matched to its severity.

Over an average follow-up of 202.6 days, 91 of the 115 ears (79.1%) had no recurrence of infection. Of the 24 ears that relapsed, seven did so after treatment had been stopped, whereas only one of the 91 recurrence-free ears had stopped treatment. Relapse was statistically associated with stopping therapy and with recent bacterial infection. This was a retrospective study without a control group, and its authors called for randomised trials. Two imperfect studies agreeing is useful, but it is not the same as a controlled trial.

Why "infected or not" is too simple

Sequencing studies help explain why clearing one organism does not restore a healthy ear. In a study of 40 dogs at a US referral hospital, every type of otitis examined, whether dominated by cocci, rods or yeast, was associated with lower fungal diversity than in healthy ears. The researchers also found that cytology and culture did not predict the full range of organisms present in Pseudomonas or Malassezia otitis. An earlier study of 26 dogs found lower bacterial diversity in affected ears. A diseased ear is a simplified community, and removing one member of it does not rebuild the rest.

What this means at the vet's

None of this is an argument against treating infection. Painful, infected ears need treatment chosen by a vet, and Merck advises repeat examination and cytology every week or two until no infection remains, noting that many owners stop when the ear looks better but before it has resolved. The eardrum should be checked before drops are used, because some ingredients can damage hearing if it is ruptured.

For a dog with repeated episodes, the conversation should widen. Merck is direct that primary causes must be managed to reduce recurrence, and that when allergy is the primary problem there are likely to be other signs beyond the ear. Whether a maintenance anti-inflammatory plan suits your dog is a decision for your vet, based on what is driving the disease.

Try this today

Look at what your dog's ears do between episodes. Check whether they stay quiet, or whether there is still occasional head shaking, scratching or extra wax when no infection has been diagnosed. Then look beyond the ears for paw licking, redness between the toes, face rubbing or itching elsewhere. Write down the date of each episode and which ear was affected, and take that record to your next appointment. Avoid reaching for leftover drops, vinegar solutions or frequent cleaning at home, because Merck lists overcleaning as a cause of ear disease in its own right and warns that home remedies can irritate an inflamed canal.

The Bottom Line

A recurring ear infection is usually a recurring inflammation with an infection on top of it. Treating the organisms is necessary and often works for that episode, but recent research suggests it may not be what breaks the cycle. In the two studies that tried it, keeping inflammation under control between episodes was associated with far fewer relapses, though neither was a controlled trial. If your dog is on its third or fourth sore ear of the year, the most useful question for your vet is why the ear keeps becoming inflamed.

This article is for general information and does not replace veterinary advice. A painful ear, marked swelling or discharge, a head tilt, loss of balance, reduced hearing or recurrent ear disease should be assessed by a vet.