Dematiaceous (Melanized) Molds

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Dematiaceous (Melanized) Molds

General Characteristics

The dematiaceous fungi indicate dark coloration as a result of their ability to produce melanin and are known agents of superficial and subcutaneous mycoses that involve the skin and subcutaneous tissues; less commonly, deeply invasive or disseminated disease may be caused by these fungi. These organisms are ubiquitous in nature and exist as saprophytes and plant pathogens. The etiologic agents are found in several unrelated fungal genera. Humans and animals serve as accidental hosts after traumatic inoculation of the organism into cutaneous and subcutaneous tissues.

In the mycology laboratory, these fungal species often are initially separated by growth rate into the slow-growing dematiaceous molds, which may require 7 to 10 days to grow, and the rapid-growing dematiaceous molds, which usually grow in less than 7 days. When nonsterile body sites are cultured, determining the significance of these organisms is difficult or impossible. If colonies of common saprophytic molds occur near the edge of the plate and are clearly away from the inoculum, they should be considered contaminants unless additional evidence of infection is present.

Epidemiology and Pathogenesis

Superficial Infections (Tinea Nigra and Black Piedra)

Tinea nigra is a superficial skin infection caused by Hortaea werneckii. It is manifested by blackish brown, macular patches on the palm of the hand or the sole of the foot. Lesions have been compared with silver nitrate staining of the skin. Black piedra is a fungal infection of the hair, scalp, and occasionally the axillary and pubic hair caused by the dematiaceous fungus Piedraia hortae. These diseases occur primarily in tropical areas of the world, with cases reported from Africa, Asia, and Latin America.

Mycetoma

A mycetoma is a chronic granulomatous infection that usually involves the lower extremities but may occur in any part of the body. The infection is characterized by swelling, purplish discoloration, tumorlike deformities of the subcutaneous tissue, and multiple sinus tracts that drain pus containing yellow, white, red, or black granules. The color of the granules is partly due to the type of infecting organism. The infection gradually progresses to involve the bone, muscle, or other contiguous tissue and ultimately requires amputation in most progressive cases. Dissemination of the organism may occur but is uncommon. Mycetomas usually are seen among people living in tropical and subtropical regions of the world whose outdoor occupations and failure to wear protective clothing predispose them to trauma.

Two types of mycetomas have been described. Actinomycotic (bacterial) mycetomas are caused by the aerobic actinomycetes, including Nocardia, Actinomadura, and Streptomyces spp. (The aerobic Actinomycetes are described in detail in Chapter 19.) Eumycotic (fungal) mycetomas are caused by a heterogeneous group of fungi that have septate hyphae. Eumycotic mycetomas are subcategorized as white grain mycetomas or black grain mycetomas, a distinction determined by the pigmentation of the infecting agent’s hyphae.

Some hyaline septate molds can cause mycetomas; however, the disease is covered in this section because many of the etiologic agents are dematiaceous fungi. Etiologic agents of eumycotic mycetoma to be discussed include Pseudallescheria boydii and Acremonium spp., causative agents of white grain mycetomas, and Exophiala jeanselmei, Curvularia spp., and Madurella mycetomatis, causative agents of black grain mycetomas.

Most patients with mycetomas live in tropical regions, but infections can occur in temperate zones. The most common etiologic agent of white grain mycetoma in the United States is P. boydii, a member of the Ascomycota. The organism is commonly found in soil, standing water, and sewage; humans acquire the infection through traumatic implantation of the organism into the skin and subcutaneous tissues.

Chromoblastomycosis

Chromoblastomycosis is a chronic fungal infection acquired through traumatic inoculation of an organism, primarily into the skin and subcutaneous tissue. The infection is characterized by the development of a papule at the site of the traumatic insult that slowly spreads to form warty or tumorlike lesions characterized as resembling cauliflower. Secondary infection and ulceration may occur. The lesions usually are confined to the feet and legs but may involve the head, face, neck, and other body surfaces.

Histologic examination of the lesion reveals characteristic sclerotic bodies, which are copper-colored, septate cells that appear to be dividing by binary fission and are thought by some to resemble copper pennies. These infections cause hyperplasia of the epidermis of the skin, which may be mistaken for squamous cell carcinoma. Fungal brain abscess, known in the past as cerebral chromoblastomycosis, may be caused by the dematiaceous fungi; however, it is more appropriately considered a type of phaeohyphomycosis and is discussed with that disease. Chromoblastomycosis is widely distributed, but most cases occur in tropical and subtropical areas of the world. Occasional cases are reported from temperate zones, including the United States. The infection is seen most often in areas in which agricultural workers do not wear protective clothing and suffer thorn or splinter puncture wounds.

Pathogenesis and Spectrum of Disease

The spectrum of disease caused by the dematiaceous fungi ranges from superficial infections (e.g., skin and hair) to emergent, rapidly progressive, and often fatal disease (e.g., brain abscess). The following list, which is not comprehensive, provides the common etiologic agents of diseases that may be caused by dematiaceous fungi (Table 61-1).

TABLE 61-1

Dematiaceous Fungi

Organism Disease Site Tissue Form
Slow-Growing Species      
 Cladosporium spp. Chromoblastomycosis Subcutaneous Sclerotic bodies
Phaeohyphomycosis Brain, subcutaneous Septate hyphae
 Ochroconis gallopava Phaeohyphomycosis Brain, subcutaneous, lungs Septate hyphae
 Exophiala dermatitidis Phaeohyphomycosis Brain, eye, subcutaneous, and dissemination Hyphal fragments and budding yeast
Pneumonial Lungs  
 Hortaea jeanselmei Mycetoma phaeomycotic cyst Subcutaneous Hyphal fragments and budding yeasts
 Hortaea werneckii Tinea nigra Skin Hyphal fragments and budding yeast
 Fonsecaea spp. Chromoblastomycosis Subcutaneous Sclerotic bodies
Phaeohyphomycosis Brain Septate hyphae
Cavitary lung disease Lungs Septate hyphae
 Phialophora spp. Chromoblastomycosis Subcutaneous Sclerotic bodies
Phaeohyphomycosis Subcutaneous Septate hyphae
Septic arthritis Joints Septate hyphae
 Piedraia hortae Black piedra Hair Asci-containing nodules cemented to hair shafts
 Madurella mycetomatis Mycetoma Subcutaneous Hyphal fragments
Rapid-Growing Species      
 Alternaria spp. Phaeohyphomycosis Subcutaneous Septate hyphae
Sinusitis Sinuses Septate hyphae, possibly fungus ball
Nasal septal erosion Nasal septum Septate hyphae
Ulcers and onychomycosis Skin, nails Septate hyphae
 Bipolaris spp. Phaeohyphomycosis Subcutaneous, brain, eye, bones Septate hyphae
Sinusitis, fungus ball Sinuses Septate hyphae, possibly fungus ball
 Curvularia spp. Sinusitis Sinuses Septate hyphae; possibly fungus ball
Phaeohyphomycosis Subcutaneous, heart valves, eye, and lungs Septate hyphae
 Drechslera spp. Phaeohyphomycosis Subcutaneous and brain Septate hyphae
Sinusitis Sinuses Septate hyphae
 Exserohilum spp. Phaeohyphomycosis Subcutaneous Septate hyphae
 Pseudallescheria boydii Mycetoma Subcutaneous Granules of hyaline hyphae
Phaeohyphomycosis Subcutaneous, skin, joints, bones, brain, lungs Septate, hyaline hyphae
 Cladophialophora bantiana Phaeohyphomycosis Brain Septate hyphae

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