PHAEOHYPHOMYCOSIS

Phaeohyphomycosis refers to infections of skin, subcutaneous tissues, and internal organs caused by dematiaceous (phaeoid) fungi. These fungi produce pigmented hyphae and/or yeast-like cells in culture and frequently in the infected tissue. Species of several genera of dematiaceous fungi, e.g., Alternaria, Bipolaris, Curvularia, Cladophialophora, Cladosporium, Exophiala, Exserohilum, Phaeoacremonium, or Phialophora, are commonly reported as agents of phaeohyphomycosis.

Superficial phaeohyphomycosis:
Etiology: Previously called as tinea nigra, it is caused by Phaeoannellomyces werneckii.

Clinical presentation: Tinea nigra is the infection of stratum corneum and consists of brown-black macules. Infection occurs as a result of inoculation from source (soil, sewage, wood) following trauma in the affected area.  Incubation period is about 2-7 weeks. The fungus is tolerant to an environment with a high salt concentration and a low pH; hence thrives on human skin. Lesion is characterized by a sharply marginated, single lesion that increases centrifugally in size. The lesion begins as a brown macule that becomes darker as lesion grows. Dark-colored macule is due to the accumulation of a melanin like substance in the fungus. Palmar surface and fingers are commonly involved. Other areas such as plantar surface, thorax and neck may also be involved. Most patients are <19 years of age and females outnumber males 3:1.

Laboratory diagnosis: Skin scrapings are collected. KOH mount reveals yeast cells or thick septate, branching hyphae that contain a dark pigment in their walls. Blastospores or chlamydoconidia may also be present. Biopsy specimens stained with PAS reveals septate hyphae present in the stratum corneum. Culture on SDA, mycosel agar at 25-30�C yields growth in approximately 1 week. Initially, yeast-like colonies that are shiny, black, and mucoid are present. Subsequently, the colonies become olivaceous brown and develop aerial mycelia in 2-3 weeks. Microscopically, dematiaceous two-celled yeast that produce annelloconidia along with dematiaceous, septate hyphae with conidia.

Treatment: It readily responds to keratolytic agents such as Whitfield’s ointment. Topical agents are used that either aid in the removal of excessive keratin in hyperkeratotic skin disorders or increase epithelial cell turnover. Imidazoles such as miconazole and ketoconazole are also effective.

Subcutaneous phaeohyphomycosis:

Etiological agents: Curvularia spp, Bipolaris spp, Exserohilum spp, Exophiala jeanselmei, Scedosporium apiospermum, Phialophora parasitica, Wangiella dermatitidis etc. These are soil saprophytes or phytopathogens.

Clinical features: Consists of a group of mycotic infections characterized by the presence of dematiaceous septate hyphae, yeast or a combination of both in tissue. Infections occur at the site of traumatic implantation of the fungi. Often subcutaneous forms may progress to systemic infections due to spread in both compromised and noncompromised patients. The lesions usually remain localised as subcutaneous nodules and in many cases present as abscesses encapsulated in fibrous connective tissue. In some cases, the lesions may resemble the cauliflower-like lesions characteristic of chromoblastomycosis. Patient presents with solitary, discrete, well encapsulated nodule or cyst and lesions may occur on all body sites. Nodule usually becomes elevated, central area becomes necrotic and forms encapsulated abscess that may become 2 cm in diameter or larger. Necrotic area fills with yellowish, viscous, purulent fluid.

Laboratory diagnosis: KOH mount may also reveal fungal elements. Biopsy of the affected are is collected, tissue sections are stained and observed for phaeoid fungal elements.  Fungus in tissue samples appears as chains of yeast cells, moniliform hyphae, true hyphae, distorted hyphae or combinations of these but muriform (sclerotic) cells are not found. In some cases the dematiaceous pigment may not be apparent in which case the Fontana-Masson silver stain (which specifically stains hyphae containing melanin) can be used to confirm that the hyphae are dematiaceous. Specimen are inoculated on Sabouraud's dextrose agar and incubated at room temperature. Many of the etiologic agents of phaeohyphomycosis are sensitive to cycloheximide. The cultures are discarded as negative if no growth occurs in four weeks.

Other phaeohyphomycosis:

Infections of the eyes and skin by the black fungi could also be classified as phaeohyphomycosis. In nasal infections dark lesion on the septum is a common presentation. Sinusitis caused by species of Bipolaris, Exserohilum, Curvularia and Alternaria is increasingly being reported, especially in patients with a history of allergic rhinitis or immunosuppression. Infection of the periotoneal cavity is associated with peritoneal dialysis. Infection of bones usually follows a traumatic injury. Wangiella dermatitidis is a neurotropic fungus. Central nervous system infections have been reported. It may also cause keratitis, otitis, pneumonia, and endocarditis. Disseminated infections may develop particularly in immunocompromised patients. Cerebral infections caused by Cladophialophora bantiana (a neurotropic fungus) have been reported in a number of patients without any predisposing factors.

 Since these fungi exist as saprophytes in the environment and are common environmental airborne contaminants, cultures must be interpreted with care. In order to consider as significant, a positive culture from a non-sterile specimen, such as sputum or skin, needs to be supported by direct microscopic evidence.