Implantation subcutaneous phaeohyphomycosis caused by Rhytidhysteron rufulum: A case report

Abstract

Subcutaneous phaeohyphomycosis is caused by traumatic implantation of melanized environmental fungi. The majority of cases occur in tropical areas of the world or are associated with travel from these regions. Herein, we describe a rare case of subcutaneous phaeohyphomycosis caused by Rhytidhysteron rufulum in an immunocompetent Somalia-born patient. The use of molecular diagnostics as an essential tool for identification of rare fungal pathogens is highlighted.

1. Introduction

Phaeohyphomycosis englobe several clinical manifestations, including superficial, cutaneous, subcutaneous and systemic infections, caused by melanized fungi, observed in tissue as hyphae, yeast cells, vesicular elements or a combination of fungal elements. The fungi causing phaeohyphomycosis are generally not highly virulent, save for a few species, and cause infection in both immunocompetent and immunocompromised hosts. Most infections are acquired by traumatic implantation, as in the case report described here. Herein, we describe a case of phaeohyphomycosis caused by the fungus Rhytidhysteron rufulum in a foreign-born Canadian patient. While this organism is found readily in the environment in tropical regions of the world, it is a rare cause of subcutaneous phaeohyphomycosis though becoming more frequently described in the literature.

2. Case presentation

A 38-year-old woman presented with a chronic nodule on her left leg. The lesion had been present for approximately 2 years, and there was no clear antecedent trauma. The lesion fluctuated in size, and was non painful.

She was originally from Somalia, and had lived there until approximately a decade prior to presentation. She had also traveled to Kenya. The patient had a history of diabetes mellitus for approximately 7 years, managed with insulin. She had recently completed a 9-month course of treatment for culture-proven tuberculosis involving lungs and cervical lymph nodes.

On examination (Day 0), there was a fluctuant, non-tender mass at the anterior left shin. There was no fistula or drainage. An ultrasound on Day 0 demonstrated a complex cystic lesion with vascularity and calcification, which was intimately associated with the fascia of the tibialis anterior muscle. An MRI at day +28 showed a lobulated mass within the subcutaneous fat on the anterior aspect of the left shin. The lesion measured 2.2 cm craniocaudal, 2.2 cm transverse and 1.1 cm AP. It was peripherally enhancing, and T2 hyperintense. There mass did not invade into muscle. The lesion was followed clinically, and another MRI was performed 16 months later (Day +512) (Fig. 1). At that time, the lesion had increased in size to 2.7 cm craniocaudal, 2.4 cm transverse, and 1.2 cm AP. Given the increase in size, it was considered concerning for sarcoma.

Fig. 1.

Coronal (left) and axial (right) MRI of the left lower extremity. There is a non-specific T1 hypointense and T2 hyperintense oval mass in the subcutaneous fat left lower extremity along the anterolateral shin with broad base contact of the tibialis anterior muscle.

An excisional biopsy was performed on Day +547. Tissue was sent to pathology and to the microbiology laboratory for fungal and mycobacterial culture. On pathology review, necrotizing granulomatous inflammation was observed with fungal elements on both the PAS and GMS staining (Fig. 2, lower right and left, respectively). Some areas of hyphal thickening were visible and no muriform cells were seen. The features seen were considered consistent with a deep fungal infection.

Fig. 2.

Upper Left, Rhytidhysteron rufulum growth on a phytone plate. Upper right, Rhytidhysteron rufulum tape mount, 200X magnification. Lower left, PAS stain of lower leg lesion biopsy section, 200X magnification. Lower right, GMS stain of lower leg lesion biopsy section, 200X magnification.

The microbiology laboratory set up bacterial, fungal and mycobacterial culture of the lesion biopsy. All culture media grew the same fungus. The isolate was slow-growing (>2 weeks) on phytone agar. The fungal culture grew a furry, dark brown colony with dark reverse suggesting a dematiaceous fungus (Fig. 2, upper left). On microscopy, hyphae were primarily observed with some areas of thickening (Fig. 2, upper right). Few loose oval conidia were also observed but were not enough to allow for microscopic identification. The isolate was subsequently sent for ITS sequencing [1] and identified as Rhytidhysteron rufulum.

Susceptibility testing was performed on the isolate per CLSI M38 guidelines [2]. Minimum inhibitory/effective concentrations were as follows: Amphotericin B 0.5 μg/mL, voriconazole 1.0 μg/mL, posaconazole 0.06 μg/mL, itraconazole 0.12 μg/mL and micafungin 0.03 μg/mL. There are currently no clinical breakpoints or epidemiologic cutoff values available to interpret susceptibility results for this species of fungus.

3. Discussion

Phaeohyphomycosis is a fungal infection involving subcutaneous or occasionally systemic disease caused by environmental melanized moulds. The primary causes of subcutaneous phaeohyphomycosis include Fonsecaea pedrosoi, Phialophora verrucosa, Exophiala species and Cladophialophora carrionnii. Rarely, other environmental melanized fungi cause subcutaneous phaeohyphomycosis. Here we report a case of subcutaneous phaeohyphomycosis caused by Rhytidhysteron rufulum in an immunocompetent female with diabetes mellitus with no apparent trauma prior to development of the subcutaneous nodule two years prior.

Rhytidhysteron species are melanized saprophytic fungi in the Ascomycotes phylum associated with plant disease found primarily on the wood of citrus and mangrove plants [3]. Rhytidhysteron has a worldwide distribution but is primarily found in tropical and subtropical climates of the world [4]. Of the four known Rhytidhysteron species, only R. rufulum has been associated with human disease, albeit rarely [3]. There have been 8 reported cases of R. rufulum phaeohyphomycosis in the literature originating in India [][[3], [4], [5], [6], [7], [8], [9][]] [[3], [4], [5], [6], [7], [8], [9]] [[3], [4], [5], [6], [7], [8], [9]]. More recently, a single case of R. rufulum phaeohyphomycosis was reported in an Ethiopian-born patient in Tennessee, USA [10]. Of the 10 reported cases, including ours, 8 were male and two were female. It is unclear whether the unequal sex distribution represents differences in exposure risks or susceptibility to disease, or diagnostic bias.

R. rufulum infection is acquired by traumatic implantation of contaminated soil or plant material, and occupational exposure (farming, walking barefoot) is the most common place of acquisition. Subcutaneous phaeohyphomycosis is the most common clinical presentation though a single case of subcutaneous R. rufulum chromoblastomycosis has also been reported in a renal transplant patient [4]. Reported cases have occurred in patients with diabetes [5,8,10] and kidney transplantation [4,10], however cases in persons with no underlying comorbidities have also been described [3,6,7,9]. Patients present with painless, swollen, soft, slow-growing, ulcerative or non-ulcerative nodules, most commonly on the feet or legs. Many patients present years after initial nodule observation and do not recall a specific trauma that incited their lesion. Diagnosis can be made by pathologic examination and fungal culture on nodule biopsy or fine needle aspirate of the lesion.

Upon histopathologic investigation, biopsies typically show irregular branching, septate hyphae which may be pigmented or non-pigmented. On direct KOH exam (if performed), thick brown septate hyphae are observed [3]. In culture, R. rufulum takes 2–3 weeks to grow from a clinical specimen, but grows well on routine fungal culture media. Initially the colony is light grey and velvety, becoming dark and cottony over time, with a darkly pigmented reverse. By microscopy, R. rufulum has dark, septate, smooth-walled hyphae [4] with occasional chlamydospores. Discoidal ascomata and bitunicate asci may be observed [4]. Ascospores are thick-walled, darkly pigmented and elliptical or fusiform shaped. The ascospores have 3 horizontal or longitudinal septa [3,5]. Due to lack of sporulation in culture, isolates require molecular identification, typically by ITS sequencing of the 18S rRNA gene, to definitively identify the organism. MALDI-TOF mass spectrometry has also proven reliable when databases are augmented with in-house generated spectra [11].

Accurate identification of the causative fungus in subcutaneous phaeohyphomycosis is essential to ensure appropriate patient management as significant species-specific antifungal susceptibility differences have been described. Due to its rarity, Rhytidhysteron-specific treatment protocols do not exist. ECMM guidelines on the management of phaeohyphomycosis indicate that surgery alone may be effective for treatment of subcutaneous nodules caused by melanized fungi [12]. Cryotherapy and laser therapy have also been successful. Oral antifungal agents are recommended as co-adjunctive therapy in immunocompromised patients and to prevent disseminated disease. For patients with multiple subcutaneous nodules, systemic antifungal therapy is recommended [12]. Of the reported cases of R. rufulum phaeohyphomycosis, the majority were treated with a combination of surgical excision and itraconazole. Combination therapy with terbinafine has also been described [8]. Our isolate had minimum inhibitory/effective concentrations less than or equal to 1.0 μg/mL for itraconazole, voriconazole, posaconazole, micafungin and amphotericin B.

In summary, we present a case of phaeohyphomycosis caused by R. rufulum in a Somalia-born patient in Canada. The patient was successfully managed by surgical excision of her lesion. This report, along with the others reported in the literature, highlight that molecular identification of rare melanized fungi is an asset to diagnosis and management of patients with subcutaneous phaeohyphomycosis.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Declaration of competing interest

There are none.