Malassezia dermatitis.

Malassezia pachydermatis is a saprophytic yeast commonly found on normal and abnormal skin of dogs. Malassezia pachydermatis and staphylococci play a significant role in seborrheic dermatitis. Malassezia dermatitis (MD) is common and should be considered in any case exhibiting erythematous, oily, and pruritic dermatitis (1). Specialty dermatology practices may see 2 or more cases of MD a week. Malassezia dermatitis is often the reason for therapeutic or diagnositc failures. When there is a lack of response to glucocorticoids, antibiotics, antiseborrheic shampoos, insecticides, or miticides, one should consider a diagnosis of MD (2). Some atopic patients that have failed to respond to desensitization have subsequently been diagnosed as having MD and, then, have responded well to desensitization after the MD was successfully treated. The diagnosis of food allergy relies upon the owner's observation of decreased pruritus while the patient is on an elimination food trial. If the patient has MD concurrently with the food allergy, the owner will not observe any decrease in pruitus until the MD has been successfully treated. Malassezia organisms can be cultured easily from normal dog skin, but cytological demonstration of the organism from normal skin is much more difficult. A decrease in the host's defenses or changes in the surface microclimate, such as, excessive sebum, accumulation of moisture, and disruption of the epidermal barrier, may lead to proliferation of Malassezia organisms. Allergic, hormonal, and bacterial skin diseases may be predisposing factors, as are longterm glucocorticoid administration and antibiotic administration (2). One study demonstrated that atopic dogs had higher numbers of yeast than did normal dogs, but not as many as dogs with MD. Factors associated with increased prevalence of higher counts of Malassezia pachydermatis were seborrheic dermatitis, recent antibiotic treatment, and certain breeds (3). Malassezia pachydermatis produces lipases that can liberate fatty acids and zymogen in the yeast cell wall, which activates complement, both actions may contribute to cutaneous inflammation (1). Two studies have suggested that the pathogenesis of MD may be related to the yeast antigens acting as an allergen and thus causing an immediate-type hypersensitivity reaction. The 1 st study showed that when seborrheic dogs were skin Figure 1. Malassezia dermatitis on the neck of a basset hound.