Phaeomycotic cyst caused by exophiala xenobiotica in a patient with rheumatoid arthritis and lung cancer

Abstract

Here, we report a case of pheomycotic cyst caused by E. xenobiotica, which was well controlled via drainage and local thermotherapy. A 70-year-old man developed a cystic nodular lesion on the dorsum of his right thumb over the previous 3 months. He had been treated with prednisolone and methotrexate for 4 years for rheumatoid arthritis. The patient also had lung cancer with vertebral bone metastasis. Direct microscopic examination of the greenish pus aspirated from the cyst revealed mycelial elements. Culture of the pus on blood and Sabouraud dextrose agar yielded numerous black colonies multiple times. Histopathological examination of a biopsy specimen showed subcutaneous abscess formation surrounded by granulomatous tissues. Faintly pigmented pseudohyphae were seen within the abscess. The presence of melanin in the fungal cells was determined by Fontana-Masson staining. Initial microscopic examination of the isolate revealed annellidic conidiogenous cells, suggestive of E. jenselmei. This strain was further identified as E. xenobiotica by sequence analysis of the internal transcribed spacer（ITS）region of ribosomal RNA, showing a 100% sequence homology with the strain type. In black fungal infections, Exophiala species are frequently encountered as causative agents of human mycosis, particularly in immunocompromised patients. Among them, Exophiala jenselmei was previously reported as the most common etiological agent. Advances in molecular taxonomy proved this taxon to be heterogeneous, and led to newly introduced or redefined species. Exophiala xenobiotica is one of the novel species differentiated from E. jenselmei on the basis of molecular phylogeny. Pheomycotic cysts should be considered on identifying a slowly developing chronic subcutaneous abscess in immunocompromised patients. Sequencing is recommended for accurate species identification of causative pathogens.