Image Quiz: A chronically ear-scratching, head-shaking golden retriever (Sponsored by Dechra)

A topical fluoroquinolone/steroid-containing medication is correct!

When treating otitis externa, the first-line antimicrobials include polymyxin, neomycin, and miconazole. (In addition to its antimycotic activity, the spectrum for miconazole includes gram-positive cocci.) Fluoroquinolones should be considered a second-line antibiotic. Other second-line topical antibiotics include amikacin, tobramycin, ticarcillin, and chloramphenicol. With regard to fluoroquinolones, there is evidence that resistant mutants can be selected with relative ease, leading to relapse and treatment failure. Fluoroquinolones are associated with an increased incidence of Methicillin-resistant Staphylococcus aureus (MRSA), and they predispose a patient to infection with, or carriage of, MRSA.1-6

In this case, the ear cytology revealed rod-shaped bacteria. The most common rod-shaped bacterium in otitis externa cases is Pseudomonas aeruginosa. Either polymyxin or gentamicin would be appropriate choices in this case, with EDTA used as adjunctive therapy. When otitis externa due to gram-negative rods fails to respond to appropriate first-line antibiotics, second-line antibiotics including fluoroquinolone therapy should be considered. However, the response to treatment in this case was undetermined since a follow-up examination during therapy was not performed.

An actual "failure to respond to treatment" means that the owner came back for a recheck while the dog was still being treated and that the infection was still present. Unfortunately, a more common scenario, such as probably was the case with Sammy, is that an owner treats the dog for 5 to 10 days, the clinical signs improve or resolve, and the owner stops treating the ears, even though the infection may still be present. The owner then brings the dog back weeks to months later for "another" ear infection. These dogs don't need second-line antibiotics but indeed need to be treated with a first-line antibiotic and then rechecked while still receiving the drug.

Tris-EDTA is an appropriate adjunctive therapy for cases of otitis externa complicated by a gram-negative organism. Tris-EDTA has direct bactericidal activity; it disrupts the cell membrane of gram-negative bacteria, leading to leakage of cell contents and cell death. This disruption also allows antibiotics to more effectively penetrate the bacterial cytoplasm. For maximum effectiveness, pretreat an infected ear with tris-EDTA and, after 5 to 10 minutes, instill the topical antibiotic.

In cases of unresolved/resistant gram-negative infections, topical silver sulfadiazine (mixed 1:1 with water) should also be used. When using both tris-EDTA and silver sulfadiazine, do not administer them within one hour of each other. When given together, the tris-EDTA may reduce the effectiveness of silver sulfadiazine by chelating the silver.

When treating otitis externa, it is very important to re-examine the animal during therapy to document resolution of the infection. Resolution of clinical signs is NOT adequate for assessing the effectiveness of a treatment. Another important component of successful treatment is to be sure that the owner can actually put drops in the dog’s ears. Many owners are afraid they will harm the dog. In addition, identifying and managing the underlying cause (most commonly allergic skin disease) are key to long-term treatment success.

REFERENCES

1. Weber SG, Gold HS, Hooper DC, et al. Fluoroquinolones and the risk for methicillin-resistant Staphylococcus aureus in hospitalized patients. Emerg Infect Dis 2006;12(9):1398-1405. 2. MacDougall C, Powell JP, Johnson CK. Hospital and community fluoroquinolone use and resistance in Staphylococcus aureus and Escherichia coli in 17 US hospitals. Clin Infect Dis 2005;41(4):435-440.

3. MacDougall C, Harpe SE, Powell JP, et al. Pseudomonas aeruginosa, Staphylococcus aureus, and fluoroquinolone use. Emerg Infect Dis 2005;11(8):1197-1204.

4. Graham KK, Hufcut RM, Copeland CM, et al. Fluoroquinolone exposure and the development of nosocomial MRSA bacteremia [abstr S-M3-06]. In: Program and Abstracts of the Fourth Decennial International Conference on Nosocomial and Healthcare-Associated Infections; Atlanta, Ga: Centers for Disease Control and Prevention, 2000;52.

5. Crowcroft NS, Ronveaux O, Monnet DL, et al. Methicillin-resistant Staphylococcus aureus and antimicrobial use in Belgian hospitals. Infect Control Hosp Epidemiol 1999;20:31-36.

6. Venezia RA, Domaracki BE, Evans AM, et al. Selection of high-level oxacillin resistance in heteroresistant Staphylococcus aureus by fluoroquinolone exposure. J Antimicrob Chemother 2001;48:375-381.