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).

 
I

II

III

IV

V

VI
positive for bacteria

positive for yeast

positive for bacteria and yeast

positive for bacteria, not for yeast

positive for yeast, note for bacteria

completely negative
79,0%

61,6%

49,8%

29,2%

11,8%

9,2%

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.

  
Bacteria speciespotency in %Most potent drug
Staphylococcus
intermedius
 
 
96,2
69,6
61,9
1,7
Gentamicin
Chloramphenicol
Neomycin
Polymyxin B
ß-hemolytic
Streptococci

94,0
42,6
8,9
4,2
Chloramphenicol
Gentamicin
Polymyxin B
Neomycin
Pseudomonas
aeruginosa 

75,9
53,5
43,0
3,8
Gentamicin
Neomycin
Polymyxin B
Chloramphenicol
Proteus mirabilis


93,2
73,6
63,8
0,6
Gentamicin
Neomycin
Chloramphenicol
Polymyxin B
Escherichia coli


 
94,1
77,6
76,6
62,3
Gentamicin
Chloramphenicol
Neomycin
Polymyxin B

Table 1: most commonly isolated species and potency of drugs used topically in otitis externa

In the ear in general it is difficult to reach a substancial concentration of drugs given systemically. In severe cases this mode of application is the method of choice though. For all bacterial species frequently isolated from the ear Chinolones prooved to be the first choice drug.

 
  
Bacteria speciespotency in %Most potent drug
Staphylococcus
intermedius
94,9
91,5
78,1
Chinolones
Amoxicillin/Clavulanic acid
Cefalosporins(Cefaclor/Cefalexin)
ß-hemolytic
Streptococci 
97,2
93,4
51,4
Chinolones
Amoxicillin/Clavulanic acid
Cefalosporine (Cefaclor/Cefalexin)
Pseudomonas
aeruginosa
74,9
0,6
0,3
Chinolones
Amoxicillin/Clavulanic acid
Cefalosporine (Cefaclor/Cefalexin)
Proteus mirabilis

89,3
82,2
37,8
Chinolones
Amoxicillin/Clavulanic acid
Cefalosporine (Cefaclor/Cefalexin)
Escherichia coli

91,9
13,4
6,9
Chinolones
Amoxicillin/Clavulanic acid
Cefalosporine (Cefaclor/Cefalexin)

Table 2: most commonly isolated species and potency of drugs used systemically in otitis externa

The commonly used drugs Econazol, Miconazol, Ketokonazol and Amphotericin B prooved to be effective in all cases of Malassezia pachydermatis examined. Nystatin and Clotrimazol were of less efficacy. Flucytosin and Natamycin should be avoided in the treatment of Malassezia otitis (see table 3).

 
Antimycotic drugSensitive Resistent Intermediate
Econazol164 00
Ketokonazol16400
Miconazol16400
Amphotericin B16400
Nystatin15806
Clotrimazol15491
Flucytosin01640
Natamycin01640

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







 

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