Atypical Mycobacteria



Atypical Mycobacteria


Joseph Misdraji, MD





Key Facts


Terminology



  • Infection by mycobacteria other than tuberculosis or leprosy


Etiology/Pathogenesis



  • Exposure is almost inevitable due to ubiquitous presence of atypical mycobacteria as environmental saprophytes


  • M. avium and M. intracellulare, known together as M. avium-intracellulare complex (MAC), are most common atypical mycobacteria to cause hepatic disease


Clinical Issues



  • Common opportunistic infection in AIDS patients, particularly those that have CD4 counts less than 50 cells/µL


  • Presents with fever, hepatomegaly, and elevated alkaline phosphatase


  • Median survival is 6 months


Microscopic Pathology



  • Poorly formed granulomas, loose aggregates of histiocytes or of foamy histiocytes with numerous acid-fast organisms on acid-fast bacterial stain


  • Areas of necrosis with nuclear debris surrounded by loose collections of histiocytes and liver abscesses


  • Fibrin ring granulomas are rare


Ancillary Tests



  • Histochemical stain for acid-fast bacteria


  • PCR can be performed on paraffin-embedded tissue


  • Blood mycobacterial culture






Paraffin section of liver in MAC infection shows several loose aggregates of histiocytes and poorly formed granulomas image.






Acid-fast stain shows innumerable acid-fast bacteria within clustered histiocytes in a patient with MAC infection.


TERMINOLOGY


Abbreviations



  • Mycobacterium avium-intracellulare complex (MAC)


Definitions



  • Infection by any one of many species of mycobacteria that does not cause tuberculosis or leprosy


ETIOLOGY/PATHOGENESIS


Infectious Agents



  • Atypical mycobacteria are potentially pathogenic environmental mycobacteria (a.k.a. mycobacteria other than tuberculosis)



    • Includes M. avium, M. intracellulare, M. kansasii, M. marinum, M. gordonae, M. chelonae, M. scrofulaceum, M. szulgai, M. malmoense, M. xenopi, M. abscessus, and M. fortuitum


    • Classified according to growth rate, presence or absence, and type of pigment


    • M. avium and M. intracellulare, known together as M. avium-intracellulare complex (MAC), are nonchromogens and the most common of atypical mycobacteria to cause hepatic disease


    • Next to MAC, M. kansasii is most common cause of nontuberculous mycobacterial infection in HIV patients


CLINICAL ISSUES


Epidemiology



  • Incidence



    • Up to 35% of all AIDS patients develop disseminated MAC eventually



      • 1-year incidence is 3% among patients with CD4 counts between 100 and 199 cells/µL and 39% for patients with CD4 counts less than 10 cells/µL


      • Frequency is decreasing with widespread use of highly active antiretroviral therapy (HAART)


    • Worldwide distribution


  • Gender



    • More common in males, mirroring HIV prevalence


Site



  • Most commonly causes pulmonary disease, lymphadenitis, skin and soft tissue infection


Presentation



  • Fever


  • Hepatomegaly


  • Elevated alkaline phosphatase


  • Patients treated with HAART may develop clinical manifestations of disseminated MAC due to immune reconstitution inflammatory syndrome (IRIS)


Laboratory Tests

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Jul 7, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Atypical Mycobacteria

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