Agents of superficial mycoses are fungi that colonize the keratinized outer layers of the skin, hair, and nails. Infections caused by these organisms elicit little or no host immune response and are nondestructive and thus asymptomatic. They are usually of cosmetic concern only and are easy to diagnose and treat.
PITYRIASIS (TINEA) VERSICOLOR
Pityriasis versicolor is a common superficial fungal infection that is seen worldwide. In tropical environments, it may affect 30% to 35% of the population. It is caused by the lipophilic yeast species of the Malassezia furfur complex: M. furfur, M. sympodialis, M. globosa, M. restricta, M. slooffiae, M. obtusa, M. dermatis, M. japonica, and M. yamatoensis. In routine clinical reporting referring to these organisms as members of the M. furfur complex is usually sufficient.
Morphology
When viewed in skin scrapings, members of the M. furfur complex appear as clusters of spherical or oval, thick walled yeastlike cells, 3 to 8 μm in diameter (Fig.1). The yeast cells may be mixed with short, infrequently branched hyphae that tend to orient end to end. The yeastlike cells represent phialoconidia and show polar bud formation with a “lip” or collarette around the point of bud initiation on the parent cell (Fig.2). In culture on standard media containing or overlaid with olive oil, species of the M. furfur complex grow as cream-colored to tan yeastlike colonies composed of budding yeastlike cells; hyphae are infrequently produced.
Fig1. Pityriasis versicolor. Periodic acid–Schiff–stained skin scraping showing yeastlike cells and short, infrequently branched hyphae that are often oriented end to end (×100). (From Connor, D.H., Chandler, F.W., Schwartz, D.A., et al., 1997. Pathology of Infectious Diseases. Appleton & Lange, Stamford, CT.)
Fig2. Scanning electron micrograph of Malassezia furfur demonstrating the liplike collarette around the point of bud initiation on the parent cell. (Courtesy S.A. Messer.)
Epidemiology
Pityriasis versicolor is a disease of healthy persons that occurs worldwide, but it is most prevalent in tropical and subtropical regions. Young adults are most commonly affected. M. furfur and other members of the species com plex are not found as saprophytes in nature, and pityriasis versicolor has not been documented in animals. Human infection is thought to result from the direct or indirect transfer of infected keratinous material from one person to another.
Clinical Syndromes
The lesions of pityriasis versicolor are small hypopigmented or hyperpigmented macules. The upper trunk, arms, chest, shoulders, face, and neck are most often involved, but any part of the body may be affected (Fig. 3). The lesions are irregular, well-demarcated patches of discoloration that may be raised and covered by a fine scale. Because species of the M. furfur complex tend to interfere with melanin pro duction, lesions are hypopigmented in dark-skinned individuals. In light-skinned individuals, the lesions are pink to pale brown and become more obvious when they fail to tan after exposure to sunlight. Little or no host reaction occurs, and the lesions are asymptomatic, with the exception of mild pruritus in severe cases. The M. furfur complex has also been associated with folliculitis, obstructive dacryocystitis, systemic infections in patients receiving intravenous lipid infusions, and seborrheic dermatitis, especially in patients with the acquired immunodeficiency syndrome (AIDS).
Fig3. Pityriasis versicolor. Multiple, pale brown, hyperpigmented patches on chest and shoulders. (From Chandler, F.W., Watts, J.C., 1987. Pathologic Diagnosis of Fungal Infections. American Society for Clinical Pathology Press, Chicago, IL.)
Laboratory Diagnosis
The laboratory diagnosis of pityriasis versicolor is made by the direct visualization of the fungal elements on microscopic examination of epidermal scales in 10% potassium hydroxide (KOH) with or without calcofluor white. The organisms are usually numerous and may be visualized with hematoxylin and eosin (H&E) or periodic acid–Schiff (PAS) stains (see Fig. 1). The lesions will also fluoresce with a yellowish color on exposure to a Wood lamp.
Although not usually necessary for establishing the diagnosis, culture may be performed using synthetic mycologic media supplemented with olive oil as a source of lipid. Growth of yeastlike colonies appear after incubation at 30°C for 5 to 7 days. Microscopically, the colonies are comprised of budding yeastlike cells with occasional hyphae.
Treatment
Although spontaneous cure has been reported, the disease is generally chronic and persistent. Treatment consists of the use of topical azoles or selenium sulfide shampoo. For more widespread infection, oral ketoconazole or itraconazole may be used.
TINEA NIGRA
Tinea nigra is a superficial phaeohyphomycosis caused by the black fungus Hortaea werneckii (formerly Exophiala werneckii).
Morphology
Microscopically, H. werneckii appears as dematiaceous (melanin [brown to black] pigmented), frequently branched, septate hyphae, 1.5 to 3.0 μm wide. Arthroconidia and elongate budding cells are also present (Fig. 4). H. werneckii also grows in culture on standard mycologic media at 25°C, on which it is a black mold-producing annelloconidia (conidia possessing annelids or rings), which often slide down the sides of the conidiophore.
Fig4. Tinea nigra. Dematiaceous hyphae of Hortaea werneckii (hematoxylin and eosin, ×100). (From Connor, D.H., et al., 1997. Pathology of Infectious Diseases. Appleton & Lange, Stamford, CT.)
Epidemiology
Tinea nigra is a tropical or subtropical condition. It is likely contracted by traumatic inoculation of the fungus into the superficial layers of the epidermis. It is most prevalent in Africa, Asia, and Central and South America. Children and young adults are most often affected, with a higher incidence in females.
Clinical Syndromes
Tinea nigra appears as a solitary, irregular, pigmented (brown to black) macule, usually on the palms or soles (Fig. 5). There is no scaling or invasion of hair follicles, and the infection is not contagious. Because of its superficial location, there is little or no discomfort or host reaction. Because the lesion grossly may resemble a malignant melanoma, biopsy or local excision may be considered. Such invasive procedures may be avoided by a simple microscopic examination of skin scrapings of the affected area.
Fig5. Tinea nigra. Darkly pigmented macules with irregular edges present on the palm. (From Chandler, F.W., Watts, J.C., 1987. Pathologic Diagnosis of Fungal Infections. American Society for Clinical Pathology Press, Chicago, IL.)
Laboratory Diagnosis
Tinea nigra is easily diagnosed by microscopic examination of skin scrapings placed in 10% to 20% KOH. The pigmented hyphae and yeast forms are confined to the outer layers of the stratum corneum and are easily detected on H&E stained sections (see Fig. 4). Once fungal elements are detected, skin scrapings should be placed on mycologic media with antibiotics. A dematiaceous yeast like colony should appear within 3 weeks, becoming velvety with age. Microscopic examination reveals two-celled, cylindric, yeastlike cells and, depending on the age of the colony, toruloid hyphae.
Treatment
The infection responds well to topical therapy, including Whitfield ointment, azole creams, and terbinafine.
WHITE PIEDRA
White piedra is a superficial infection of hair caused by yeastlike fungi of the genus Trichosporon: T. ovoides (causes pubic hair white piedra), T. inkin (causes most cases of scalp hair white piedra), and T. asahii.
Morphology
Microscopic examination reveals hyphal elements, arthro conidia (rectangular cells resulting from the fragmentation of hyphal cells), and blastoconidia (budding yeast cells).
Epidemiology
This condition occurs in tropical and subtropical regions and is related to poor hygiene.
Clinical Syndromes
White piedra affects the hairs of the groin and axillae. The fungus surrounds the hair shaft and forms a white to brown swelling along the hair strand. The swellings are soft and pasty and may be easily removed by running a section of the hair between the thumb and forefinger. The infection does not damage the hair shaft.
Laboratory Diagnosis
When microscopic examination reveals hyphal elements, arthroconidia, and/or budding yeast cells, infected hair should be placed on mycologic media without cycloheximide (cycloheximide will inhibit Trichosporon spp.). Trichosporon spp. will form cream-colored, dry, wrinkled colonies within 48 to 72 hours on incubation at room temperature. The various species of Trichosporon can be identified in the same manner as other yeast isolates. Sugar assimilations, potassium nitrate (KNO3 ) assimilation (negative), urease production (positive), and morphology on cornmeal agar (both arthroconidia and blastoconidia are present) should be determined.
Treatment
Treatment may be accomplished by the use of topical azoles; however, improved hygiene and shaving of the infected hair are also effective and usually negate the necessity of medical treatment.
BLACK PIEDRA
Another condition affecting the hair, primarily the scalp, is black piedra. The causative agent of black piedra is Piedraia hortae.
Morphology
The organism grows as a pigmented (brown to reddish black) mold. As the culture ages, spindle-shaped ascospores are formed within specialized structures (asci). These structures (asci and ascospores) also are produced within the rock-hard hyphal mass that surrounds the hair shaft.
Epidemiology
Black piedra is uncommon and has been reported from tropical areas in Africa, Asia, and Central and South America. It is thought to be a condition of poor hygiene.
Clinical Syndromes
Black piedra presents as small, dark nodules that surround the hair shafts. It is asymptomatic and generally involves the scalp. The hyphal mass is held together by a cement-like substance and contains asci and ascospores, which is the sexual phase of the fungus.
Laboratory Diagnosis
Examination of the nodule reveals branched, pigmented, hyphae held together by a cement-like substance. P. hortae can be cultured on routine mycologic media. Very slow growth may be observed at 25°C and may begin as a yeast like colony, later becoming velvety as hyphae develop. Asci may be observed microscopically, usually ranging from 4 to 30 μm and containing up to eight ascospores.
Treatment
Treatment of black piedra is easily accomplished by a hair cut and proper, regular washings.
