Dr  Sampurna Roy  MD

Mycobacterium avium complex include Mycobacterium avium and Mycobacterium intracellulare.  M.avium was identified in 1890 in chickens and M. intracellulare in 1969.  

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

Mycobacterium avium intracellulare (MAI) is found in the majority of 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.

MAI 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 MAI 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 MAI may also precede dissemination.

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

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

Microscopic features: Image Link

Microscopically, MAI 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. Image Link(Webpath)

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.