Microbiology in canine otitis externa 

Introduction

Otitis is a frequent problem in dogs - in many cases we deal with a recurrent problem.
Looking at the etiology we have to take into account bacterial infections, yeasts (Malassezia spp.), parasites (Otodectes cynotis as well as Demodex species) and foreign bodies. Less often we deal with neoplasia, allergies (especially food intolerance), autoimmune diseases or endocrinopathies that induce otitis, most commonly complicated by bacteria or yeasts. In contrast, cats rarely suffer from otitis externa. Most common cause in cats are parasites or allergic diseases including atopy and food intolerance.
Bacteriological and mycological examination is needed instead of routine application of antibiotics since a large percentage is complicated by yeast. An underlying cause always has to be considered in order to obtain a successful treatment.

Normal findings in the ear

Knowing about the normal bacteriological environment in the ear is crucial for the interpretation of bacteriological results. Staphylococci (especially Staphylococcus intermedius) and a-haemolytic Streptococci are frequently found in ears of dogs without clinical symptoms. Pseudomonas spp., Proteus spp., b-haemolytic Streptococci are rare in the healthy ear and their detection has to be discussed as prone to cause disease.

Malassezia spp. yeasts are found in healthy ears and on the skin, they can cause disease when the circumstances change towards warmer and more moist environment or primary lesions are present.

Microbiological findings in cases of otitis

Bacteriological examination
We looked at 903 samples for bacteria as well as yeasts. 93,0 % of the samples came from dogs, 7,0 % from cats. 79,0 % were bacteriologically positive. Most common was Staphylococcus (S.) intermedius (38,8 %), followed by ß-haemolytic Streptococci and the coagulase negative Staphylococci (S. haemolyticus, S. felis, S. epidermidis). Less frequently isolated were Pseudomonas aeruginosa, Proteus mirabilis, a-haemolytic Streptococci, Enterococcus spp., Escherichia coli, Enterobacter spp., Pasteurella spp. as well as Pseudomonas spp. In 38,6 % of the positive samples we isolated just one species (commonly S. intermedius, coagulase negative Staphylococci and Pseudomonas aeruginosa). Mixed cultures contained most often consisted b-haemolytic Streptococci together with S. intermedius, Proteus mirabilis or Pseudomonas aeruginosa.



Figure 1: Percentage of bacteriological and mycological isolates in swabs from ears (n = 903).

Therapy concepts for Otitis externa

For a successful management an intensive cleaning is essential especially in the first phase of therapy. A short term application of steroids initially helps reduce inflammation and pain. The application of other drugs thus is made easier and the ventilation of the ears is improved further due to reduced production of cerumen.

Resistograms of isolated bacteria species differ substancially for the most often used topical antimicrobial agents (see table 1 and 2). Gentamicin was the drug with best results for all of the most frequently isolated species; with ß-hemolytic Streptococci less than 50% efficacy was seen though. Especially for infections with Pseudomonas spp. an individually prepared resistogram is highly recommended.

Table 3: Antimycotic drugs used in Otitis externa and their efficacy against Malassezia pachydermatis (n = 164).