Mycobacterium avium intracellulare (MAI)

Mycobacterium avium and M. intracellulare are very similar bacteria which are usually grouped together. In the United States, they are usually described as Mycobacterium avium complex, or MAC. They belong to the same genus as the organism which causes tuberculosis (TB; M. tuberculosis).

By the early 1990s, MAI was the most commonly reported bacterial infection among people with AIDS in the United States. One study found that 43% of people who survived two years after being diagnosed with AIDS had detectable MAI organisms in their blood.¹ Since the introduction of highly active antiretroviral therapy (HAART), the incidence of MAI has fallen significantly among HIV-infected people, and people who have been diagnosed with MAI live longer.

MAI organisms are found in the soil, including potted plant soil, and tap water throughout the world. They are generally thought to enter the body in food, water or inhaled dust. The organisms are probably not transmitted from one person to another.

In people with healthy immune systems, MAI rarely causes illness. However, in people with HIV who have CD4 counts less than 100 cells/mm³, and often below 20 cells/mm³, MAI can spread widely or 'disseminate' throughout the body, affecting almost any organ, but especially the liver, spleen and bone marrow. They mainly infect cells of the reticuloendothelial system, a group of related cells in the body that includes macrophages.