Parasitic and Fungal Otitis Externa in the Cat: Managing Otodectes, Demodex, Malassezia and Associated Otic Diseases

Tammy P. Sadek, DVM
DABVP (Feline)
Kentwood Cat Clinic
3215 Breton Rd Se
Kentwood, MI 49512

Otitis externa is a frequent finding in cats, with a prevalence rate of 2-10 % of all feline patients seen in general practice.(1) Otodectes cyanotis, the ear mite, is responsible for approximately 50% of all feline otitis externa cases. However, other infectious and inflammatory conditions frequently either cause or predispose the cat to ear disease. In 20-30 % of chronic feline ear cases the initiating cause remains unknown.

In the cat as well as the dog, the causes of otitis externa can be divided into primary causes, predisposing causes, and perpetuating causes of ear disease.

Primary causes include parasitic diseases such as Otodectes cyanotis, Demodex cati and D. gatoi, Notoedres cati, Sarcoptes scabei, and Eutrombicula alfreddugesi (the chigger). (2), (3). Additional infectious causes of otitis externa in the cat include yeast infections, such as Malassezia pachydermatitis, and dermatophytes such as Microsporum canis and Trichophyton mentagrophytes, and the feline papilloma virus. Hypersensitivity reactions such as atopy, flea allergy, food allergies, contact allergies, and drug eruptions can all cause primary ear inflammation. Autoimmune diseases such as pemphigus may have lesions that extend into the ear canal. Foreign objects such as grass awns, cerumen concretions, and inspissated otic preparations can all cause primary ear disease. Inflammatory ear polyps, intraluminal tumors such as ceruminous gland tumors, mast cell tumors and squamous cell carcinomas all mechanically obstruct the ear and can initiate local inflammation if ulceration or necrosis occurs. Disorders of keratinization such as primary idiopathic seborrhea of Persians can induce hyperkeratitis and primary ceruminous otitis.(4)

The most important predisposing causes of feline ear disease are the retroviruses. Cats immunosuppressed by FeLV or FIV show a 2-5% prevalence of severe otitis externa with bacteria, Malassezia, and Demodex infections.(1) Other systemic illnesses such as hyperadrenocorticism, hypoadrenocorticism, and diabetes mellitus can change the local environment present in the ear canal and allow secondary pathogens to flourish. Increased environmental heat and humidity may also change the ear canal microenvironment, as can iatrogenic factors such as excessive cleaning of the ear. Anatomical problems such as ear stenosis and fibrosis from trauma may also encourage local inflammation.

Perpetuating factors of feline otitis externa include opportunistic bacteria that can overwhelm compromised ear canal defenses. Secondary bacterial infections of Staphylococcus spp, coliforms, Pseudomonas, and Proteus mirabilis have all been cultured from chronic feline otitis externa canals. Malassezia pachydermatitis is probably the most common complicating infection in feline allergic otitis, and thrives in the moist environment of the inflamed ear canal. Otitis media can serve as a source of continuing reinfection of the adjacent ear canal. Chronic ear canal hypertrophy, fibrosis and cartilage calcification can prevent the return of normal architecture to the ear.

Diagnosis of the cause of feline otitis externa is based on history, signalment, clinical findings on physical exam of the entire cat, bilateral otoscopic exam, microscopic exam and cytology of the ear canal contents, and occasionally ear culture and sensitivity, skin scrapings, biopsy of affected areas, blood analyses, fecal analysis, skull radiographs and /or allergy testing. Some cats require general anesthesia for complete otoscopic exam, especially if painful or fractious. Observation of the tympanum of each canal is recommended to ascertain the integrity of the membrane. Many otic cleaning preparations are contraindicated in the presence of a ruptured tympanum. Horner’s syndrome, vestibular disease and deafness are potential sequellae of over-aggressive or inappropriate cleaning treatments.

The retrovirus status of every cat should be known as it affects the long term prognosis for each case. Consequently, FeLV/FIV testing should be performed on all otitis externa patients whose retrovirus status is not known. Ear debris should be examined in both a mineral oil preparation to evaluate for ear mites as well as new methylene blue and Diff- Quik type stains for evaluation of Malassezia, bacterial and inflammatory cells present. A negative ear mite slide evaluation for ear mites does not rule out ear mite disease completely. Some cats develop an allergic hypersensitivity to the ear mite saliva and can develop marked otic pruritus in the presence of a very few mites. In resistant cases of otitis externa, scrapings of the affected ear canal may discover Demodex or Sarcops or Notodedres mites. Allergy testing, food allergy feeding trials, and routine hematological and biochemical testing including thyroid testing can be valuable in certain patients, especially when age or concurrent skin or systemic disease warrants. Patients with concurrent skin disease may also benefit from fungal cultures, skin biopsies, and skin scrapings. Patients with Notedres may have pinnal and pedal crusts and excoriations. Cats with Demodex may show evidence of overgrooming without inflammation, or may exhibit comedones in affected areas. Since pruritic cats often groom incessantly, and may swallow the inciting mites, routine fecal analysis may detect mites or their ova in the stool. Long haired or cattery born cats may benefit from a toothbrush type fungal culture of their coats, as dermatophytes may cause inflammation of the ear canal with mild or inapparent clinical signs elsewhere. Cats with feline papilloma virus may show friable debris in the ear canal. A therapeutic trial of an antiparasiticide such as selamectin (Revolution) or ivermectin can be useful in some cases. CT scanning is available in a referral environment for those cases where neoplasia or otitis media is suspected.

Treatment of otitis externa in the cat is directed to the precipitating cause(s) when known. In the 50% of cases where ear mites are the primary cause of the otitis, treatment with selamectin (Revolution) or ivermectin (Acarexx) is typically safe and effective and relatively easily performed by most clients. Retreatment 2-4 weeks later is usually needed. Some cats with ear mites develop sufficient secondary inflammation and/or infection with opportunistic organisms that topical steroid/ antibiotic otic preparations can be helpful.

Demodex induced otitis externa may be treated with systemic ivermectin and/ or lime sulfur dips weekly. Treatment of any systemic disease such as diabetes mellitus is imperative in the control of Demodex. and often times dermatophyte infections.

Flea control with selamectin (Revolution), imidacloprid (Advantage) or fipronil (Frontline) will address the flea allergic otitis externa cases.

Pain management in severely affected patients, especially those with concurrent otitis media, is an often overlooked adjunctive treatment that can dramatically improve the patient’s quality of life. The author prefers injectable or sublingual buprenorphine as needed for several days.

Restoring the ear canal’s microenvironment may by itself reduce the numbers of opportunistic organisms such as Malassezia and some bacteria. Mildly acidifying cleansers such as Alocetic and Oticalm, or a 1:1 vinegar: water dilution can be effective in certain cases. Antifungals such as topical miconazole can be very effective in some cases of Malassezia. Systemic antifungals such as itraconazole are useful in more severe cases, as well as those cases with generalized dermal involvement. Dermatophyte induced otitis externa may also be treated with topical miconazole in mild cases, systemic itraconazole, or oral lufenuron (Program).

Systemic antibiotics are indicated in cases of non parasitic moderate to severe otitis externa, especially when otitis media is suspected. Antibiotics used should be based on culture and sensitivity results, with the duration determined by the severity and chronicity of the infection.

Topical antibiotics are indicated in most cases, with solutions or suspensions preferred as the vehicle of choice especially if stenosis of the ear canal is present. Many topicals are inactivated by debris in the canal, so cleaning is an important step in the therapeutic process. Topicals may be used for10-14 days in mild infections up to 6 weeks in more chronic conditions.

Topical or systemic steroids are indicated for most patients where atopic or hypersensitivity reactions are involved. Steroids must be used with caution even when topically applied in diabetic cats. Budenoside is the systemic steroid of choice in the diabetic cat if required. Oral antihistamines such as chlorpheniramine and Omega 3 fatty acid supplementation are useful adjunctive therapies in many of these pruritic cats.

The removal of foreign objects, inflammatory ear polyps, and ear tumors dramatically decreases the local reaction present in the ear canal. Some causes of otitis externa may be self limiting, such as chiggers and the feline papilloma virus. Some predisposing causes of otitis externa will persist despite the best efforts of client and clinician. These conditions, such as retroviral infection, atopy, and primary idiopathic seborrhea of Persians will require life long management.

In general, the diagnosis and treatment of many feline patients with otitis externa will be straightforward and relatively routine. Cats with chronic otitis, immunosuppressed cats, allergic cats and those cats for whom an initiating cause cannot be found present a greater challenge diagnostically and therapeutically for the practitioner.

 References:

Logas, D. Causes of otitis externa. 2003 Proceedings of the AAHA 70th Annual Meeting Scientific Program: 67-72.

Kontos, V; Sotiraki, S: Himonas, C. Two Rare Disorders in the Cat: Demodectic Otitis Externa and Sarcoptic Mange. Feline Pract 26[6]: 18-20 Nov/Dec ’98.

Van Poucke, S. Ceruminous otitis externa due to Demodex cati in a cat. Vet Rec 149[21[:651-2 2001 Nov 24.

Werner, Alexander H. Otitis Externa and Media. In: Rhodes, Karen H. ed. The 5-Minute Veterinary Consult Clinical Companion Small Animal Dermatology. Baltimore: Lipincott, 2002: 144-151.