Answer: Mucormycosis caused by Rhizopus species. A feared opportunistic invasive fungal infection of the immunocompromised host, mucormycosis typically presents as rhinocerebral, pulmonary, or cutaneous disease in patients with diabetes or hematological malignancies. Rhizopus and Mucor are the most common etiologies and belong to the order Mucorales, for which the clinical syndrome is named (1, 2). The term “mucormycosis” is now preferred over “zygomycosis” because molecular analyses have recently shown the phylum Zygomycota to be polyphyletic (i.e., its members do not share an immediate common ancestor) (3).
Diagnosis can be onerous, requiring direct examination of tissue biopsy specimens and/or growth in culture (2). On histopathology, the Mucorales have broad, ribbon-like hyphae of variable diameters (6 to 15 μm) and are aseptate or hyposeptate (see Fig. 1A to C in the photo quiz), unlike the thin, relatively consistent diameters (2 to 3 μm) of the septate hyphae of Aspergillus or the pseudohyphae of certain Candida species. Irregularly spaced, orthogonal branching may also be observed. Grown in culture, the Mucorales have the gross appearance of cotton candy or steel wool, growing rapidly (within 1 to 7 days) and exuberantly, sometimes reaching the lid of the petri dish, earning the designation “lid lifter.”
The morphologic structures of the sporangiospore-producing Mucorales include the stolon, sporangiophore, sporangiospore-containing sporangium, and rhizoids. In this case, the well-developed, abundant, and “nodal” rhizoids (i.e., developing directly below the unbranched sporangiophores) led to the morphologic identification of Rhizopus (see Fig. 1D in the photo quiz). In contrast, Lichtheimia (formerly Absidia) and Rhizomucor have branched sporangiophores, with primitive-appearing, rare, and “intranodal” rhizoids (i.e., developing between rhizoidal tufts). Mucor has branched sporangiophores and no rhizoids (4).
The mortality of untreated mucormycosis approaches 100%, and mortality remains at 24 to 30% even when mucormycosis is treated with both surgery and antifungals (1, 2). Our patient was started on liposomal amphotericin B but declined definitive treatment with surgery, which would have been painful and disfiguring. He was discharged home with hospice and died 5 days later surrounded by family.
See https://doi.org/10.1128/JCM.00380-19 in this issue for photo quiz case presentation.
ACKNOWLEDGMENTS
We thank Rebecca Reed and Christine Carothers of the University of Maryland Medical Center Microbiology Laboratory for culturing the mold.
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