Otitis Externa en Gatos. Diferenciales y Manejo

Otitis externa (OE) is an acute or chronic inflammation of the ear canal and tympanic membrane. The prevalence of OE in the cat is lower than that described for the dog. Moreover, factors involved in the development of OE in cats are less numerous and different from those described in dogs.

Etiopathogenesis

The classic subdivision of the factors involved in the pathogenesis of OE in the dog, named primary or secondary causes and predisposing factors is also used in the description of the pathogenesis of OE in the cat. Perpetuating factors such as otitis media (OM) or proliferative changes in the ear canal (glandular hyperplasia, calcification of cartilage), are very rare in the cat.

Primary Causes

The etiological agents associated with the direct induction of an inflammatory condition of the ear canal in cats are relatively less numerous than in the dog. Mite infestations (Otodectes cynotis and Demodex spp.) and allergic dermatitis are included among the most common.

Less frequent primary causes are dermatophytosis and immune mediated diseases such as pemphigus foliaceus or proliferative and necrotizing OE.

Secondary Causes

In this category bacterial infection by cocci and/or bacilli, overgrowth by bacteria and/or Malassezia spp. and irritant or allergic contact reactions to locally applied drugs are included.

Cytological examination is undoubtedly the most accurate way to determine the meaning of the microorganisms present in the discharge. The presence of cocci or bacilli (intra- and/or extracellular) with inflammatory cells suggests an infection, whereas the absence of inflammatory cells with a large number of microorganisms suggests a microbiological overgrowth.

Predisposing Factors

The most common predisposing factors in cats are inflammatory polyps followed by epithelial tumours or ceruminous cystomatosis in the ear canal.

Perpetuating Factors

Otitis media (OM), contrary to what happens in dogs, is rarely secondary to infectious OE. In fact, in cats bacterial OM is commonly caused by upper respiratory disease or by inflammatory polyps.

Diagnostic Approach

It is important to perform a complete history, a detailed physical and otic examinations along with proper diagnostic tests.

Based on otic examination and clinical history, the clinical problem may be defined as a variable pruritic erythemato-ceruminous OE, usually bilateral, or mono or bilateral simil-purulent OE.

Variable Pruritic Erythemato-Ceruminous OE

Usually associated with ear mites or allergic otitis and less frequently to dermatophytosis, ceruminous cystomatosis or proliferative and necrotizing OE with or without bacterial or Malassezia overgrowth.

1.  Microscopic examination of the cerumen should be performed to look for ear mites.

2.  Cytological examination of cerumen should be perfumed in order to investigate secondary infections or overgrowths.

3.  Fungal culture (to be performed based on clinical suspicion).

4.  Acaricidal trial even with a negative microscopic examination of cerumen should be performed in order to definitively rule out ear mites.

5.  Ones ear mites and dermatophytes have been rule out and in the absence of proliferative changes, consider an eventual allergic otitis.

Simil-Purulent OE

Usually associated with bacterial infection secondary inflammatory polyps, neoplasia, extension of an OM or less frequent with allergic otitis or pemphigus foliaceus.

1.  Cytological examination of cerumen.

2.  Dermatopathological examination (to be performed based on clinical suspicion).

3.  Compete ear canal examination under general anaesthesia and after ear canal flushing with physiologic solution in order to remove any purulent exudate and rule out masses and/or OM.

4.  Imaging with computed tomography or magnetic resonance imaging.

5.  If any specific cause is not found during examination under general anesthesia, acaricidal trial and specific antibiotic therapy based on bacterial culture and sensitivity test should be performed.

References

1.  Miller WH, Griffin CE, Campbell LK. Muller and Kirk's: Small Animal Dermatology. 7th ed. Philadelphia, PA: WB Saunders Co; 2013.

2.  Klose TC, MacPhail CM, Schultheiss PC, Rosychuk RA, Hawley JR, Lappin MR. Prevalence of select infectious agents in inflammatory aural and nasopharyngeal polyps from client-owned cats. J Feline Med Surg. 2010;12(10):769–774.

3.  Borio S, Massari F, Abramo F, Colombo S. Proliferative and necrotising otitis externa in a cat without pinnal involvement: video-otoscopic features. J Feline Med Surg. 2013;15(4):353–536.