Malassezia infections

 Pityriasis versicolor is a highly prevalent, chronic superficial infection of the stratum corneum caused by species of the lipophilic yeast, Malassezia. These yeasts can be isolated from normal skin and scalp and are considered part of the cutaneous mycobiota. Thus, infections are likely caused by endogenous strains. There are 14 currently recognized species of Malassezia, but the vast majority of cases of pityriasis versicolor are caused by Malassezia globosa, Malassezia furfur, or Malassezia sympodialis. The infection is characterized by discrete, serpentine, hyper-, or hypopigmented maculae that develop on the skin, usually on the chest, upper back, arms, or abdomen. These patches of discolored skin may enlarge and coalesce, but scaling, inflammation, and irritation are minimal. Indeed, this common affliction is largely a cosmetic problem. Pityriasis versicolor affects all ages, and the annual incidence is reportedly 5–8%. Predisposing conditions include the immune status of the patient, genetic fac tors, and elevated temperature and humidity.

Most species of Malassezia require lipid in the medium for growth. The diagnosis is confirmed by direct KOH microscopic examination of scrapings of infected skin. Short unbranched, nonpigmented hyphae and spherical cells are observed. The lesions also fluoresce under Wood’s lamp. Pityriasis versicolor is treated with daily applications of selenium sulfide. Topical or oral azoles are also effective. The goal of treatment is not to eradicate Malassezia from the skin but to reduce the cutaneous population to com mensal levels.

The species of Malassezia noted above, as well as Malas seziarestricta, have been implicated as causes or contributors to seborrheic dermatitis (ie, dandruff). This etiology is supported by the observation that many cases are alleviated by treatment with ketoconazole. Rarely, Malassezia may cause an opportunistic fungemia in patients—usually infants— receiving total parenteral nutrition (TPN), as a result of contamination of the lipid emulsion. In most cases, the fungemia is transient and eliminated by replacing the TPN and intravenous catheter. In addition, another less common manifestation of Malassezia is folliculitis.

Tinea Nigra

Tinea nigra (or tinea nigra palmaris) is a superficial chronic and asymptomatic infection of the stratum corneum caused by the dematiaceous fungus Hortaea (Exophiala) werneckii. This condition is more prevalent in warm coastal regions and among young women. The lesions appear as a dark (brown to black) discoloration, often on the palm. Microscopic examination of skin scrapings from the periphery of the lesion will reveal branched, septate hyphae and budding yeast cells with melanized cell walls. Tinea nigra will respond to treatment with keratolytic solutions, salicylic acid, or azole antifungal drugs.

Piedra

Black piedra is a nodular infection of the hair shaft caused by Piedraia hortae (Figure1B). White piedra, due to infection with Trichosporon species, presents as larger, softer, yellowish nodules on the hairs (see Figure 1A). Hair of the axilla, genitalia, beard, and scalp hair may be infected. Treatment for both types consists of removal of the infected hair and application of a topical antifungal agent. Piedra is endemic in tropical countries.

Fig1. Piedra. A: White piedra hair with nodule due to growth of Trichosporon. 200×. B: Black piedra hair with a hard, black nodule, caused by growth of the dematiaceous mold, P. hortae. 200×.

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مواضيع ذات صلة

Histoplasmosis

Coccidioidomycosis

Endemic Mycoses

Phaeohyphomycosis

Chromoblastomycosis

Mycetoma

Sporotrichosis

Cutaneous mycoses

Laboratory Diagnosis of Mycoses

General Properties, Virulence, and Classification of Pathogenic Fungi

Conventional Yeast identification methods

Commercially available yeast identification systems