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Figure 1 Otoscopic examination of left and right ears.

Sponsored by an unrestricted educational grant from Bayer

Click here for product information regarding Baytril OticClick here for product information regarding Claro

A 2-year-old, 30 kg, male Labrador retriever was presented with chronic, bilateral otitis for the past year, with near-constant head shaking. Previous treatments included several commercial otic medications and glucocorticoid injections; however, the owner had not seen any reduction in signs. Otoscopic examination showed severe, hyperplastic changes and both waxy and purulent exudate in both ear canals (Figure 1). The integrity of the tympanic membranes could not be assessed. 

Key Points

  • Cytology is a key initial and continuing therapeutic indicator for otitis.
  • Chronic otitis presents a greater possibility for resistant infections, as many medications may have been used during its course.
  • Infectious components may vary from veterinary visit to visit.
  • There is almost always an underlying cause of otitis—commonly for dogs, atopic dermatitis.

What diagnostic tests are indicated? 

The key diagnostic procedure for this patient now is cytology. Likely, a culture and susceptibility test is warranted given the history of chronicity and treatment with multiple antimicrobial agents. However, cytology must be performed first to determine if micro-organisms are present and to direct initial treatment.

Samples of exudate from each ear were collected from the horizontal canals using cotton-tipped applicators. The exudate from each ear was then firmly rolled onto 2 glass microscope slides: one was stained with Romanowsky stain (eg, Diff-Quik®) and the other with Gram stain. Gram-positive and -negative rods and Gram-positive cocci were identified. A sample was then collected from the horizontal ear canal to submit for bacterial culture and susceptibility testing (C&S). Results were expected in 5 to 7 days. 

Immediate therapeutic plan

Several factors must be considered when selecting treatment for chronic otitis while awaiting C&S results. These include: 1) cytologic findings, 2) previous therapy, 3) integrity of the tympanic membrane, and 4) specific goals. The bacterial infection is mixed and we will likely find significant resistance to some antimicrobial agents, given the past treatment history. Consider also that the hyperplastic changes will interfere with delivery of topical medications deep into the canal. Thus, treatment recommendations were 1) topical Baytril® Otic (enrofloxacin/silver sulfadiazine) Emulsion and 2) oral prednisone (30 mg, orally, once daily for 5 days, then every other day for 5 doses).

Baytril® Otic was recommended for the following reasons:

  1. It contains both enrofloxacin and silver sulfadiazine, a very broad-spectrum antiseptic. The concentration of enrofloxacin (0.5% or 5000 mcg/mL) is high enough to address pathogens with high MIC that still have susceptibility to fluoroquinolones. 
  2. It offers the flexibility to separately prescribe systemic anti-inflammatory therapy on a case by case need.
  3. It has an aqueous base with low viscosity, which makes it more likely to distribute deep into the ear canal.  

Prednisone was prescribed for the following reasons:

  1. A glucocorticoid is necessary to reduce swelling and inflammation to: a) facilitate movement of medications down the canals, b) allow for more thorough visual inspection, and c) facilitate cleaning. 
  2. The dog has a history of pain, as reflected by head shaking and rubbing its ears. 
  3. Systemic medication is more likely to reduce inflammation. Topical glucocorticoids may be inadequate to reach the deepest aspects of the ear canals. 

Culture results showed presence of Escherichia coli, Proteus mirabilis, Corynebacterium auriscanis, and Streptococcus canis. All were sensitive to enrofloxacin, except the Proteus spp, which was reported as intermediate in sensitivity. 

At day 10, otoscopy showed significantly reduced overall edema and hyperplastic changes, but some areas of the left ear still showed severe, raised, irregular hyperplasia. Repeat cytology showed cocci, rod-shaped bacteria, and yeast organisms; however, they were present in lower numbers than previously. The dog was anesthetized for computerized tomography and deep ear cleaning using a squalene-based otic cleanser and sterile saline. The middle ears and bullae appeared normal on the CT images. Biopsies were taken from the remaining unusual hyperplastic areas in the left ear and submitted for histopathologic evaluation. The changes were interpreted as severe hyperplasia with lymphocytic and eosinophilic dermatitis. 

The Baytril® Otic (enrofloxacin/silver sulfadiazine) Emulsion treatment course was completed, and the prednisone was slowly tapered. At a subsequent re-check examination on day 49 (after initial presentation), the outward appearance of both ears was markedly improved. However, cytology revealed low numbers of cocci in the left ear, and low numbers of yeast in the right ear. Based upon cytology results, both ears were treated with Claro® (florfenicol, terbinafine, mometasone furoate) Otic Solution, selected because its florfenicol and terbinafine are very effective in staphylococcal and yeast infections, respectively. In addition, the mometasone is ideal to help further reduce the inflammation. 

The owner reported continued improvement. At a subsequent presentation, there was a more marked reduction in hyperplasia. Very low numbers of yeast remained in the left ear, and an additional treatment with Claro® was recommended, as was regular long-term use (eg, 1-2 times weekly) of T8 Keto® Flush for maintenance cleaning until the underlying factors (ie, causes) of the otitis could be identified and controlled. 

Discussion

Several concepts are important in managing otitis externa. First, cytology is a key diagnostic tool in making a rational choice for initial therapy. Cytology should be repeated at EVERY recheck examination to allow evaluation of the therapy and to guide adjustments. Infective agents may vary from episode to episode or visit to visit. 

Second, rational selection of treatment is based on several factors:

  • infectious agent(s) 
  • integrity of the tympanic membrane
  • level or severity of hyperplastic changes 
  • goals of therapy (killing organisms, reducing pain or swelling, etc)
  • overall health of the patient
  • ability of the owner to properly instill medications 

Third, the veterinarian must adapt the treatment plan to ongoing changes. As we cleared the initial infections, a different infection developed. Such changes are not unusual and are generally readily managed, if detected early. 

Fourth, there is almost always an underlying cause of canine otitis. Often it is atopic dermatitis, which is very common in the Labrador retriever. The patient did have a history of licking its feet, and some of the head/ear rubbing may have been a reflection of allergic pruritus. If the underlying cause is not identified and controlled, it is very likely that otitis or some other secondary problem, such as pyoderma or Malassezia spp dermatitis, will develop. 

CAUTION: Federal (U.S.A.) law restricts Baytril® Otic to use by or on the order of a licensed veterinarian. Federal law prohibits the extra label use of this drug in food-producing animals. CONTRAINDICATIONS: Baytril® Otic is contraindicated in dogs with suspected or known hypersensitivity to quinolones and/or sulfonamides.

CAUTION: Federal (U.S.A.) law restricts Claro® to use by or on the order of a licensed veterinarian. CONTRAINDICATIONS: Claro® should not be used in dogs known or suspected to be allergic to Claro® or any of its ingredients.

T8 Keto® Flush is for topical use on dogs, cats and horses. Avoid contact with eyes. If eye contact occurs or skin irritation develops, rinse thoroughly with water, discontinue use and contact your veterinarian.

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