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Veterinary Forum February 2008 (Vol 25, No 2)

WWVC Update: Managing canine otitis externa

by Paul Basilio

    RENO, Nev. — Dermatologists are asked a lot of questions about ear disease, and some of the cases can be particularly frustrating to deal with, according to Jon D. Plant, DVM, DACVD, assistant professor at the Oregon State University College of Veterinary Medicine, at a session held here at the Wild West Veterinary Conference.

    Thorough assessment of an otitis case may require anesthesia and deep ear flushing, Plant said. "Flushing is partly a diagnostic test. I explain that to owners and tell them that I don't know what I'm going to find once I flush the ear. There may be otitis media, or I may find a tumor that I need to biopsy, but I won't know until I get all that exudate out."

    Plant explained that radiographs are not particularly helpful in diagnosing otitis. "Sometimes [a radio­graph] can confirm how severe the calcification is on a cocker spaniel's ear, and you can decide from that if you're dealing with end-stage otitis that needs surgery and ablation," adding that a veterinarian could probably find that during a thorough palpation.

    A cytology sample using a cotton swab should be taken from as deep as is safely possible in the dog's horizontal canal, then evaluated for yeast, bacteria and inflammatory cells, Plant said. "Sometimes you'll find a cesspool appearance that has all sorts of bacteria, both rods and cocci," he added. "Everything you could hope for."

    Plant said that there are no strict guidelines for how much yeast is too much and that occasional yeast on a normal-looking ear is probably okay. If the cytology shows 10 or more yeast organisms per high-powered field, Plant recommends treatment.

    "I have sometimes failed to treat those cases with occasional yeast, and the owner comes back 3 days later and says, 'I told you so. My dog's ear was infected and you didn't treat it, and now it's getting worse.' You need to take it seriously when inflammation is there and it's associated with some organisms."

    In cases in which an animal has Pseudomonas infection and systemic therapy is being considered, culture and susceptibility testing may be necessary. Plant warned, however, that several recent studies have shown that it is difficult to get accurate results from these types of tests. He noted that in one of the studies, three laboratories were sent cultures from the same infected ears of a population of dogs. Even though all of the cultures had been taken at the same time and should have been identical, the laboratories only agreed on the presence of Pseudo­monas 83% of the time. Plant said that the two laboratories that conducted minimum inhibitory concentration (MIC) testing found susceptibility agreement that ranged from 56% to 81%, meaning they had identical patterns only 56% of the time.

    "There's a little problem with overinterpreting test results," Plant said, "because you can't even get the same results taken from two swabs from the same ears at the same time."

    To help treat ear disease, Plant said it is important to define and control the predisposing, primary, secondary and perpetuating factors, as well as treat any underlying infections (see box).

    Plant prefers to use a video otoscope to treat otitis. "Once I got a video otoscope, I was surprised at how badly I was doing before I had it," he joked. "I would get to the point at which I thought I had everything flushed out, but in retrospect, I'm sure that I wasn't doing as good of a job because I never flushed out as much junk as I do now."

    Plant recommends using copious amounts of warm saline to flush the ear. In some severe cases, there is a risk of rupturing the tympanic membrane, but Plant said that in those cases, the membrane is often compromised or ruptured before flushing.

    Some veterinarians will add a cerumenolytic to the saline to help break up waxy exudates, but Plant recommends that regardless of the solution used during lavage, straight saline should be the last thing employed to remove any chemicals that are instilled into the middle ear.

    Plant said that most ears require some sort of topical treatment and that it is rare when he relies on systemic therapy in dogs. "You need to get the ears cleaned out," he said. "Many medications are going to be less effective if we put them on top of a bunch of pus or exudate. You need to get them cleaned out so we have good contact and so that we don't have inactivation of antibiotics."

    Plant noted that frequent reevaluation is important in otitis treatment, "especially for a Pseudomonas [infection in an] ear. You're going to want to get the dog back in a week or less. They are so ulcerated and painful and there is so much exudate that they may need to be flushed out sooner rather than later."

    Since Pseudomonas organisms are tenacious and can survive virtually anywhere, Plant recommends thoroughly disinfecting the otoscope cone. He noted that one study showed how simply wiping off a cone with alcohol or placing it in an ear speculum cleaner did not rid the cone of Pseudomonas organisms. "You really should put it in a cold sterile [solution] or sterilize it to get rid of Pseudomonas," he said.

    Plant also suggested that a good way to become familiar with ear disease is to understand the healthy ear.

    "One of the most important things for my students and for young veterinarians is to get familiar with what a normal ear canal should look like," Plant said. "I encourage them to look at every patient's ears during every exam. I think it's simple enough to do an ear exam, and it makes sense to check it out. Sometimes you'll find otitis externa even when the owner doesn't appreciate anything going wrong with the dog."

    For more information:

    Plant JD. Managing otitis externa. Presented at: The Wild West Veterinary Conference. Reno, Nev.; Oct. 10-14, 2007.

    Click below to see exclusive videos from Dr. Plant's otoscope.

    Otoscope video 1
    Otoscope video 2
    Otoscope video 3
    Otoscope video 4
    Otoscope video 5

    Dr. Plant reported no potential conflict of interest relevant to this article.

    References »

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