Otitis Externa

As in human beings, the skin disorders are widespread in dogs that are especially susceptible to inflammatory skin disease (dermatoses). Though not fatal, these may cause a lot of discomfort both to the dog and the owner. The external auditory canal is the portion of the external ear located between the pinna (ear flap) and the tympanic membrane.

The external ear is considered to be a specialized region and its infection (otitis) is thus considered as one of the skin diseases. Otitis externa has been traditionally described using terms such as reactive, inflammatory, allergic, or infectious.The external ear canal of the dog is long and narrow and halfway along its length it turns from a vertical to a horizontal direction. It is lined by squamous epithelium which contains sebaceous and modified apocrine glands (the ceruminous glands). The ceruminous glands secrete a white, odorless fluid. Along with desquamated epithelial cell, these secretions combine to form cerumen. Bacteria and yeast are found in small numbers in normal ears of dogs and cats. Bacteria commonly found in normal canine ears include Staphylococcus epidermidis, Staphylococcus intermedius, Micrococcus species and occasionally coliform bacteria. Malassezia pachydermatis yeast commonly inhabit the oral cavity, lip commissures, external ear canals, interdigital webs, perineum and anal sacs. These yeasts belong to the normal cutaneous microflora but may behave as opportunistic pathogen whenever alterations in the microclimate of skin occur.

Mane factors have been implicated in the development of otitis. Primary factors are those that are capable of initiating inflammation in otherwise normal ears which include:

  • Parasites (Otodectes cyanotes, Demodex canis, Sarcoptes scabiei, Notoedres cati, ticks (e.g. Otobius megnini, the spinous ear tick of dogs), flies (e.g. Stomoxys calcitrans) and fleas (e.g. Echidnophaga gallinacea)).
  • Foreign bodies
  • Hypersensitivity disorders
  • Keratinization disorders
  • Auto-immune skin disease
  • Glandular disorders
  • Viral diseases
  • Miscellaneous conditions

Perpetuating factors are those that are responsible for continuing the inflammatory response even though the original primary factors may no longer be present or active. These include:

  • Bacteria
  • Yeasts
  • Otitis media
  • Progressive pathologic changes in the ear canal

Predisposing factors of otitis include:

  • Conformation of the Ear Canal. The ear type is an important factor in determining an animal's risk of developing otitis externa. Faulty ear conformation has been considered to be the most important causative factor. The pendulous pinnae of many breeds of dog restrict free circulation of air within the ear canal and may inhibit the radiation and convection of heat from the external ear canal.
  • Stenosis or swelling of the opening of the external ear canal is a real problem in some breeds, such as the Pug, Chow Chow, English bulldog and Chinese Shar-Pei .
  • Dense hairs found within the external auditory meatus of some breeds, such as the Poodles and Maltese terriers, impair ventilation and clearance of normal secretions.
  • Environmental factors. Frequently swimming or getting wet by bathing stimulates activity of the ceruminous glands thus obstructing the ear canal with secretions and eventually favoring microbial proliferation.
  • Climate. The micro- environment of the ear canal becomes more suitable for bacterial and yeast growth with any alteration in the environmental heat and humidity.
  • Treatment Factors. Traumatic use of cotton-tipped applicators for removing exudates from the ear canal and vigorous plucking of hairs from the meatus, necessary or unnecessary, can lead to swelling and erosions of the otic epithelium, predisposing it to opportunistic infections. The overuse of certain otic cleansing solutions and strong antiseptics causes maceration of the ear canal (Bass, 2004). Improper use of topical antimicrobial preparations causes colonization of the ear canal with opportunistic gram-negative bacteria, such as Pseudomonas aeruginosa or Proteus mirabilis.
  • Hypersensitivity disorders such as atopy, food allergy and contact dermatitis.

Clinical signs of otitis externa include inflammation, increased discharge (with or without odor), and pawing at the ear. The therapy aims at elimination of the underlying cause, cleaning the ear canal and middle ear, and administering systemic medications. A number of antibacterials have been suggested for use and have been found to be effective in the treatment of otitis externa, which include gentamicin, sulphadiazine in combination with trimethoprim, ampicillin, ampicillin in combination with cloxacillin, enrofloxacin, amoxycillin, cephalexin and cefadroxil. Treatment regimen for fungal ear infections includes ketoconazole, and miconazole. However, despite the advancement in the therapeutic approaches, otitis externa can be resistant to antifungals. Emergence of drug resistance is an important contributing factor. In view of these facts, herbal ear preparations may be of therapeutic efficacy.

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