Which histologic findings are characteristic of Mycobacterium avium-intracellulare (MAI) infection?

Updated: May 27, 2021
  • Author: Jaggi Rao, MD, FRCPC; Chief Editor: Dirk M Elston, MD  more...
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A spectrum of histologic findings is present in skin infection with Mycobacterium avium-intracellulare (MAI, or MAC). Tissue pathology varies with the stage of the disease and the host's immune status.

Granulomas are often present in tissue infected with MAI. Tuberculid, palisading, and sarcoidlike granulomas can be found in the context of a diffuse infiltrate of foamy histiocytic cells. Other nonspecific findings include panniculitis, chronic inflammation, cutaneous abscesses, necrotizing folliculitis, and suppurative granulomas.

Spindle cell pseudotumors and MAI

Liou et al noted that spindle cell pseudotumors due to mycobacterial infection may occur in immunocompromised hosts. [30] The investigators reported a case of spindle cell pseudotumor in a 37-year-old man with acquired immunodeficiency syndrome (AIDS), in which the tumor manifested as a firm nodule on the right arm. Histologically, the tumor was composed of spindle cells interspersed with histiocytes and inflammatory cells. Ziehl-Neelsen stain revealed many acid-fast bacilli, which were identified as MAI. Culture and sequencing of the polymerase chain reaction (PCR) product of the mycobacterial 65-kd heat-shock protein gene helped to establish the diagnosis. Immunohistochemically, the spindle cells stained with CD68, a marker found on active tissue macrophages. [30]

Similarly, Shiomi et al reported the case of a 58-year-old Japanese woman with systemic lupus erythematosus (SLE) and interstitial pneumonia for 17 years who was diagnosed with mycobacterial spindle cell pseudotumor. [31] The patient had a nodule in the deep dermis with extension into the subcutaneous fat tissue. Under histologic examination, the nodule was composed of spindle cells focally showing a vaguely storiform pattern, with focal foam cells, epithelioid histiocytes, and multinucleated giant cells. Ziehl-Neelsen staining showed numerous acid-fast bacilli within the spindle cells and epithelioid histiocytes, which were determined to be MAI after culture and PCR testing. [31]

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