Detection of antibody‐coated Mucor in skin biopsy by direct immunofluorescence

Cutaneous mucormycosis may be caused by direct inoculation or hematogenous spread of mucormycetes in immunocompromised patients. Skin biopsy is characterized by a deep fungal infection with frequent angioinvasion. The fungal hyphae can usually be identified on H&E stain. We report a case of cutaneous angioinvasive mucormycosis in which the fungi were also visualized on direct immunofluorescence. A 57-year-old patient with relapsed myelodysplastic syndrome status-post allogeneic hematopoietic cell transplant, diabetes mellitus, and graft-versus-host disease presented with painful, palpable, dark-red to violaceous retiform purpuric plaques. Light microscopy of punch biopsy revealed numerous broad, ribbon-like, pauci-septate hyphae in the dermis with angioinvasion, consistent with mucormycosis. Direct immunofluorescence performed on a concurrent biopsy to exclude immune complex vasculitis showed smooth IgG, IgA (weak), IgM (faint), and C3 deposition on the hyphal structures, compatible with antibody-coated fungi. Tissue culture subsequently confirmed Mucor species. Although mucormycosis was readily diagnosable on routine light microscopy in this case, recognition of the unique phenomenon of antibody-coated fungi can be crucial when the invasive fungi are sparse or only present in the direct immunofluorescence specimen.