KEY FACTS Malassezia Dermatitis

Malassezia yeast infections are recognized with increasing frequency in veterinary medicine. These opportunistic yeasts cause secondary ear and skin infections that can be extremely pruritic. Within the past 10 years, the Malassezia genus has been divided into seven species. This article describes the new species of Malassezia and their relevance to companion animals. Additionally, recognition, diagnosis, and treatment of Malassezia infections are discussed. Yeasts fitting the description of Malassezia have been recognized on human skin since 1846. Their importance as members of the normal cutaneous biota and their role in disease continue to be prominent topics in human and veterinary medical literature. It is now generally accepted that Malassezia yeasts are commensal organisms of mammals and may also be associated with various cutaneous diseases. The predisposing factors and infectious properties of Malassezia are still a focus of research and debate. The following is a review of Malassezia yeasts and their role in cutaneous disease. CLASSIFICATION Due to their lipophilic nature, Malassezia species other than Malassezia pachydermatis can be very challenging to culture. This difficulty may account for the initial confusion in classifying the organism (it was identified based on cytologic characteristics alone for over 50 years). These yeasts, previously known as Pityrosporum, are now classified in the genus Malassezia. The genus Malassezia is taxonomically located in the family Cryptococcaceae. Based on the structure of the yeast cell wall (laminar with distinctive spiraling ridges), positive urea hydrolysis, positive staining with diazinon blue B, and resistance of the cell wall to lysis by β-(1,3)-D-glucanase, Malassezia have been placed into the Basidiomycetes class. Malassezia are globose to ellipsoidal, lipophilic yeasts (Figure 1). Asexual reproduction occurs through monopolar budding from the same site (except for Malassezia sympodialis, which also exhibits sympodial budding). Each bud is separated by a septum. Every separation leaves a scar, or collarette, that can be visualized with light microscopy. Currently, seven species of Malassezia have been identified: M. pachydermatis, M. furfur, M. sympodialis, M. globosa, M. obtusa, M. restricta, and M. slooffiae. The species are defined by morphologic, physiologic, and molecular differences. M. pachydermatis is usually isolated from animals, particularly carnivores. It is rarely identified on human skin but has been associated with fungal septicemia CE 224 Vol. 24, No. 3 March 2002 V n Of the seven species of Malassezia yeast, Malassezia pachydermatis is the most common species found on