Dematiaceous fungi can cause a variety of cutaneous and systemic infections in humans. They contain melanin or melanin-like pigments in their cell walls. The pigmented infections known as phaeohyphomycoses can affect both immunocompetent and immunocompromised individuals. Phaeohyphomycosis was first defined by Ajello et al in 1974.1
Exophiala xenobiotica grows in environments which are moist and hot and contain an abundance of toxic hydrocarbons.2 In humans, E. xenobiotica is predominantly responsible for causing subcutaneous infections but has also been shown to infect blood, dialysis fluid, intra-ocular tissues and mucous membranes. Diabetes and ingestion of corticosteroids are recognised as important risk factors.3 Systemic infections are uncommon.
Formal diagnosis requires special techniques of microbiology and anatomical pathology for confirmation.4 For localized skin lesions treatment with a combination of antifungal drugs and surgical debridement has been recommended.4
The patient was a 56-year-old male with reportedly well-controlled type 2 diabetes (blood glucose values between 5 and 7 mmol/L) treated with metformin and gliclazide. There was no known deficiency of immune function and he was not taking corticosteroids. He presented with an irregular asymptomatic cystic lesion 2cm in length affecting the right lower thigh anteriorly. The lesion size had increased gradually over a year. No other lesions were present. He had an office-based job and denied any trauma or obvious environmental exposure. He was systemically well. Clinical differential diagnoses included epidermoid cyst, lipoma, pilomatricoma and an atypical fungal infection. Ultrasound appearance was non-specific but reported as suggesting a sebaceous cyst. The cyst was excised, at which point thick creamy yellow pus was noted. The majority of the lesion was sent for histopathology and a smaller portion was sent for fungal and bacterial culture including atypical mycobacteria. The patient was treated empirically with flucloxacillin. Histopathology showed suppurative granulomatous inflammation with accompanying pigmented (dematiaceous) deep fungal infection (Fig 1a). Tissue microscopy (Fig 1b) showed fungal hyphae and E. xenobiotica species was cultured (Fig.2). The sample was negative for other standard and atypical organisms. A swab from pus grew fungal hyphae but was negative for bacteria.
After two weeks the wound had healed well. The patient was referred to an infectious disease physician who advised monitoring the site in view of complete cyst excision and the quiescent wound, as the side-effects of the drug treatment of choice (voriconazole) potentially outweighed the clinical benefit. The scar continues to appear stable three months after surgery (Fig. 3).
Exophiala species are opportunistic black yeasts, widely distributed in the domestic environment in moist, hot, or extreme (micro) habitats such as Turkish steam baths, dishwashers, industrial bio-filters, or environments enriched by toxic hydrocarbons.5 This genus is a member of the family Herpotrichiellaceae which encompasses more than 30 species.6
The most prevalent species are E. jeanselmei and E. dermatitidis. E. xenobiotica, identified in 2006 as a segregant of E. jeanselmei, is uncommon and rare relative to the predominant species.1,7 These organisms are notoriously difficult to classify and identify.7 Literature suggests it has consistently been underdiagnosed in recent years, because of poor microbiological identification techniques and the discovery of multiple new species within the genus.2
E. dermatitidis manifests in four clinical forms: cutaneous, subcutaneous, systemic and cerebral phaeohyphomycosis. The subcutaneous form is the most common presentation. It develops after traumatic implantation of fungi, especially on the extremities. Clinically it resembles benign skin and soft tissue neoplasms like lipoma, sebaceous cyst or neurofibroma. The infection has been reported to occur in immunocompetent patients.3 Histopathology serves as a very useful tool in diagnosing these cysts by identifying the fungal elements.8,9 Surgical excision is the first choice for localized and well delineated Exophiala lesions. Most melanized fungi are susceptible to azoles, which makes itraconazole and voriconazole the main drugs used,
followed by amphotericin B. However, data on clinical management of E. xenobiotica infections is scarce and treatment is usually individualised.5,7
Subcutaneous phaeohyphomycosis caused by E. xenobiotica is rare and can mimic benign skin and soft tissue cysts and hence should be on the radar of the treating dermatologist. Histopathological evaluation and culture play a major role in diagnosis and thus assist in appropriate patient management.
Provenance: Internally reviewed
Ethical Approval: Not required
Corresponding Author: Dr Hina Ali Khan, St John of God Hospital, Barry Marshall Parade, Murdoch, WA 6150, Australia. Email: KhanH@ramsayhealth.com.au