Yeast, a single-celled type of fungus, is a common opportunistic invader of the skin and mucous membranes of all animals. Malassezia pachydermatis is a species of yeast that is often cultured from skin and ear infections affecting dogs and cats. It is normally found in low numbers on healthy skin but quickly multiplies and invades inflamed, irritated, or damaged tissue. Malassezia infections are particularly itchy and malodorous, and cause thickening, pigmentation (darkening), and flaking of the skin. “Elephant skin” is an often-heard description of the affected tissue in cases of Malassezia dermatitis. These types of infections may recur spontaneously when conditions are ideal.
Malassezia yeast thrives in the presence of sebum, an oily discharge from the sebaceous glands that increases in production as a result of allergies and inflammation. The parts of the body that are abundant with sebaceous glands are the most susceptible to Malassezia. These areas include the ears, face, arm pits, flanks and inner thighs, groin and perineum, and down the middle of the back; however, any part of the body can become infected with yeast. Chronic otitis and seborrhea are common precursors to Malassezia infections.
Occasionally, an animal may have an immune deficiency that makes them more susceptible to yeast infections. Certain breeds are thought to have a genetic predisposition to Malassezia. They include: Bulldogs, West Highland Terriers, Basset Hounds, Cocker Spaniels, Australian Terriers, Scottish Terriers, Dachshunds, Shelties, and Silkies. Long-haired cats such as Himalayans and Persians may also be genetically susceptible.
Symptoms of Malassezia infections of the skin and ears are usually unmistakable. There is a pungent, musty odor typical with this yeast that is reminiscent of rancid bread dough or milk. The skin will be crusty, flaky, and very itchy (human dandruff is often caused by a species of Malassezia.) Chronic infections will cause darkening and thickening of the skin, ear flaps, facial folds, and skin.
Malassezia is easy to identify and confirm in the laboratory. Swabbing the skin or ear canals for a cytology analysis under the microscope is generally diagnostic. The organisms exfoliate (shed from the skin) easily, readily take up stain and have a typical “foot print” appearance, morphologically. Other tests that may be performed include tape impression smears, skin scrapings, and full-thickness skin biopsies. A biopsy may reveal an underlying skin disorder that has become secondarily infected with Malassezia.
Malassezia infections are treated topically as well as orally. Oral anti-fungal medications can be expensive and carry significant side-effects if used indiscriminately, so they are generally reserved for deep-seated or generalized infections. Topicals must be used frequently and persistently in order to be effective. Rapid recurrence of yeast infections are usually due to poor treatment compliance or stopping the anti-fungal before the infection is completely cleared. Malassezia infections are often secondary to other underlying skin diseases, so these must be ruled out in persistent yeast infections.
For the skin, shampoos containing ketoconazole, chlorhexadine, or miconazole are specifically formulated to kill yeast. They contain degreasing agents to remove the excess oils from the skin in which Malassezia thrives. They are used at least twice a week. There are also wipes, creams, and sprays that are used to spot-treat affected areas in-between baths.
Anti-fungal ointments are specifically made for ear infections caused by Malassezia. These medications often contain broad-spectrum anti-microbials to fight co-existing bacterial infection, as well as steroids to help with tissue inflammation. Proper technique and frequency of application into the ear canal is essential to successful treatment. The ear canals are often occluded by inflamed tissue and excess exudate, or discharge, which may need to be flushed out to facilitate treatment.
Before starting oral anti-fungal drugs, a blood test for liver function and complete blood cell count (CBC) is usually performed. Drugs like ketoconazole, griseofulvin, itraconazole, and other anti-fungals may cause liver problems and bone-marrow suppression. Ketoconazole can also increase the blood-levels of certain other drugs by reducing their rate of clearance from the body. These drugs are generally safe when used judiciously, and are often necessary to achieve a complete cure.