Clinical Pathology: Microbiology, Virology

755) A 13-year-old female bone marrow transplant patient presents with nasal congestion and a severe headache. A needle aspirate from the sinus reveals hyphae by light microscopy (see figure). Which one of the following infections is highest on the differential diagnosis?

• Zygomycetes are a fungal class that are mostly nonspetate. Most fungi in this class that cause infection belong to the order Mucorales, which includes Rhizopus, Mucor, Rhizomucor, and Absidia.

• The hyphae of the Mucorales fungi are nonseptate and branching is nondichotomous, irregular, and sometimes at right angles.

• Mucormycosis can invade the lung, nasal, sinus, brain, and mucous membranes.

• Fungi causing mucormycosis can be differentiated from aspergillosis by the morphology of the hyphae and by the presence of chlamydospores in the hyphae.

• Differential characteristics of commonly encountered pathogens causing mucormycosis include rhizoid production and sporangiophore pigment (see table).

Characteristics of Representatives of Mucorales
Fungus Rhizoids Sporangiophores Optimal Growth Temp (°C) Pathogenicity
Rhizopus Present Unbranched, brown 45-50, inhibited by cycloheximide Most common pathogen

Absent Branched, hyaline ≤37 Opportunistic, occasional
Rhizomucor Few Branched, dark brown 38-58 Opportunistic, occasional
Absidia (Lichtheimia) Present Fine branches, hyaline 45-f50 Uncommon pathogen

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