(described by McGinnis and Schell in 1985)

Kingdom: Fungi
Phylum: Ascomycota
Class: Euascomycetes
Order: Dothideales
Family: Dothioraceae
Genus: Hortaea

Description and Natural Habitats

Hortaea werneckii is a dematiaceous yeast that inhabits the soil, particularly in tropical and subtropical climates. It is halophilic and has also been isolated from saltwater fish. Hortaea werneckii is an occasional cause of human infections [2202].

Species

Hortaea werneckii is the only species classified in the genus Hortaea. This particular fungus has previously been classified in various other genera and the genus Hortaea appears as its most currently accepted classification. See the list of obsolete names and synonyms for older names of Hortaea werneckii.

Pathogenicity and Clinical Significance

Hortaea werneckii is the causative agent of Tinea nigra. Tinea nigra is a superficial infection of stratum corneum. The infection is mostly acquired via direct inoculation of the fungus onto the skin due to contact with soil, wood, and decaying vegetation. The lesions of tinea nigra are usually located on palms but may occasionally involve other parts of the body, such as soles of the feet. These lesions are typically brown to black, flat, not scaly, and with irregular contours. They resemble and must be differentiated from those of malignant melanoma and junctional nevus [150, 970, 2261, 2464].While most of the patients do not have any symptoms related to these lesions, some may have itching.

Macroscopic Features

The colonies of Hortaea werneckii grow slowly and mature within 21 days. From the front, they are initially pale in color, moist, shiny, and yeast-like. In time, these colonies become velvety, olive black, and are covered with a thin layer of mycelium. From the reverse, the color is black. Hortaea werneckii is halophilic. It can tolerate and grow in existence of 10{64e6c1a1710838655cc965f0e1ea13052e867597ac43370498029d1bc5831201} NaCl. Significantly, it does not grow at 37°C [531, 1295, 2144, 2202].

Microscopic Features

Septate hyphae, (bicellular) yeast-like conidia, and chlamydospores are observed. The yeast-like conidia (2-5 x 5-10 µm) are the initial structures observed in the early phase of the colony development. These cells have a round end and a tapered and elongated annelidic neck part. They are hyaline initially and become pale olivaceous in time. They function as annellides and produce new annelloconidia. Some of the yeast-like conidia have a central septum and are bicellular. They may gradually be converted to chlamydospore-like cells. Septate, thick-walled, and brown hyphae (up to 6 µm wide) are formed as the colony ages. The annelloconidia are formed at intercalary and lateral annellidic points along the hyphae [531, 1295, 2144, 2202].

Histopathologic Features

Hyperkeratosis (abnormal thickening of the cornified epidermis) is observed. The layers of cornified epidermis are separated by branched hyphae.

Compare to

Aureobasidium
Exophiala
Wangiella

Some biochemical and growth characteristics, such as decomposition of casein and tyrosine, growth in 15{64e6c1a1710838655cc965f0e1ea13052e867597ac43370498029d1bc5831201} NaCl, KNO3 assimilation, and maximum growth temperature aid in differentiation of Hortaea werneckii from Exophiala jeanselmei and Wangiella dermatitidis [1295]. Also, the annelated zones of Hortaea werneckii are much broader than those of Exophiala [531].

Laboratory Precautions

No special precautions other than general laboratory precautions are required.

Susceptibility

Very few data are available. MICs of terbinafine, amphotericin B, itraconazole, and voriconazole tend to be low for isolates of Hortaea werneckii [1490, 1492].

Scraping of the visible affected skin constitutes the primary step in treatment of tinea nigra. It should be performed prior to the application of topical antifungal agents. Topical agents , such as ketoconazole [344] and miconazole [1080, 1437], cicloprox olamine [2038], or terbinafine [2085] are used to treat tinea nigra. Interestingly, topical application of thiabendazole suspension has also been shown to clear the lesions of tinea nigra [380, 2172]. Systemic therapy is not recommended and usually not required [2038]. Oral itraconazole has been succesfully used in some cases [953].

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