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Pathology of Mycobacterium Avium Intracellulare

Dr Sampurna Roy MD





Mycobacterium avium complex (MAC) include Mycobacterium avium and Mycobacterium intracellulare. 

Mycobacterium avium was identified in 1890 in chickens  and Mycobacterium intracellulare in 1969.  

Mycobacterium avium intracellulare (MAI) is found in the majority of acquired immunodeficiency syndrome (AIDS) patients at necropsy but is only isolated during life in 5-15% of patients with diarrhoea.

Disseminated infection is a common pre-terminal event in AIDS patients.

Mycobacterium avium intracellulare infection tends to occur late in the course of the disease and in most cases the cause of death is attributable to another infection or process.

There are some cases where malabsorption or colitis have been associated with heavy infection with Mycobacterium avium intracellulare as the only isolated potential pathogen.

Site: The commonest sites are large bowel, small bowel, liver, oesophagus and associated intra-abdominal lymph nodes but almost no site is exempt from infection.

There is evidence to suggest that gastrointestinal infection  is the likely early site of colonisation and precedes dissemination.

Recent evidence suggests that pulmonary Mycobacterium avium intracellulare may also precede dissemination.

Endoscopic appearance:

May be entirely normal or reveal thickened folds, macular colour changes, plaques or shallow ulceration.

Mycobacterium avium intracellulare can be isolated from stool but diagnosis can usually be made on examination of duodenal or colonoscopy biopsy specimens.

Microscopic features:

Microscopically, Mycobacterium avium intracellulare is characterised by accumulation of abundant foamy macrophages in the lamina propria.

These are slender, curvilinear organisms that are sometimes beaded and measure 4-6 micrometer in length. 

Little or no cellular reaction is noted around the macrophages.

The bacilli can also be seen lying free in the lamina propria, presumably as a consequence of disruption to the macrophages.

Granuloma formation accompanied by foci of necrosis is seen in those who are immunocompetent whilst those who are immunodeficient, such as AIDS patients, may show subtle pathological changes.

In lymph nodes there may be extensive infiltration of the paracortical regions by infected histiocytes.

Special stains:

The use of the Ziehl-Neelsen stain will reveal infected histiocytes containing numerous intracellular parallel arrays of acid-fast bacilli supporting the routine use of this stain  on all biopsies from immunodeficient patients.

The organisms stain well with silver impregnation methods.

The Mycobacteria are also periodic acid-Schiff (PAS)-positive and diastase-resistant and therefore may be confused with Whipple's disease. 

Ziehl - Neelsen stain will help to distinguish the two entities. However, culture is also recommended, as histology may not detect small numbers of Mycobacteria.

Visit: Atypical Mycobacterial Infection ; Mycobacterium Marinum Infection.  ; Mycobacterium ulcerans Infection ; Mycobacterium tuberculosis ; Mycobacterium Leprae Inf ;Mycobacterium Kansasii;

Further  reading:

Clinical and Molecular Analysis of Macrolide Resistance in Mycobacterium avium Complex Lung Disease.

Disseminated Mycobacterium avium infection in an immunocompetent aged patient.

A case of Mycobacterium intracellulare infection with chronic empyema.

Inhibition of maturation of human monocyte-derived dendritic cells in a patient with mycobacterium avium infection.

Multiple, severe lung infiltrates due to Mycobacterium avium-intracellulare in a patient with decompensated liver cirrhosis: Spontaneous resolution after a two-year follow-up.

Four cases of pulmonary Mycobacterium avium intracellulare complex presenting as a solitary pulmonary nodule and a review of other cases in Japan.

Pulmonary infection of Mycobacterium avium-intracellulare complex with simultaneous organizing pneumonia.





Dr Sampurna Roy  MD

Consultant  Histopathologist (Kolkata - India)






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