Mycobacterium avium intracellulare
Friday 20 July 2007
Mycobacterium avium (which includes three subspecies) and Mycobacterium intracellulare are separate species, but the infections they cause are so similar that they are simply referred to as Mycobacterium avium-intracellulare complex, or MAC.
MAC is common in soil, water, dust, and domestic animals.
Clinically significant infection with MAC is uncommon except among people with AIDS and low levels of CD4+ lymphocytes (<60 cells/mm3).
In AIDS patients, MAC causes widely disseminated infections, and organisms proliferate abundantly in many organs, commonly including the lungs and gastrointestinal system.
Unchecked by the immune response, the organisms reach very high levels: up to 104 organisms/mL of blood and 106 organisms/gm in tissue. Patients are feverish, with drenching night sweats and weight loss.
In the rare case of MAC in a patient without HIV, the organisms primarily infect the lung, causing a productive cough and sometimes fever and weight loss.
The hallmark of MAC infections in patients with HIV is abundant acid-fast bacilli within macrophages. MAC infections are usually widely disseminated throughout the mononuclear systems, causing enlargement of involved lymph nodes, liver, and spleen.
There may be a yellowish pigmentation to these organs secondary to the large number of organisms present in swollen macrophages.
Granulomas, lymphocytes, and tissue destruction are rare.
The histologic findings associated with MAC vary considerably and range from granulomas to nodular foam cell lesions to purulent and necrotizing inflammations.
In 1994, Torriani et al studied a retrospective cohort of 44 AIDS patients with MAC bacteremia and complete autopsies over a period of 4 years.
They found that 30% had no histologic evidence of MAC. In the remaining 70%, reticuloendothelial and gastrointestinal involvement was most common.
However, the number and distribution of involved sites was variable.
Derived from this study’s findings, MAC bacteremia may precede widespread tissue disease, and the risk of development of detectable histologic involvement was related to the duration of bacteremia.
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