Atypical Mycobacteria

Ten strains of atypical mycobacteria cause clinical disease, but M. avium-intracellulare, also known as MAC (Mycobacterium avium complex) is a particularly common opportunistic organism in AIDS. The atypical mycobacteria are present in the environment worldwide. High rates of infection have been found in Australian AIDS patients, which is not surprising because a high prevalence has been noted there before the AIDS epidemic. In Brazil, atypical mycobacterial isolates from bone marrow aspirates are more than twice as common as Mycobacterium tuberculosis in AIDS patients with persistent fever, anemia, and leukopenia. In the United States, the atypical mycobacteria are one of the most common manifestations of AIDS, but clinical atypical mycobacterial infections are relatively rare in African AIDS patients. For example, in the autopsy study of Lucas and colleagues in the Ivory Coast, atypical this patient with advanced AIDS mycobacterial infections were only seen in 3% of HIV-positive patients, and were not primarily responsible for any death. As von Reyn et al. culture of environmental and water supply samples from the United States, Finland, Zaire, and Kenya showed, that African samples were less likely to be contaminated with atypical mycobacteria than those from the United States or Finland. Although one study found atypical mycobacteria in 13% of African AIDS patients, these infections did not appear to be clinically relevant. Similarly, atypical infections are not often encountered in most South America countries, other than Brazil.

In the developed world, most patients do not develop clinically relevant atypical mycobacterial infection until CD4+ counts fall below 100 cells/mm³, below the mean value at death in the tropics. However, this would not seem to completely explain the soft tissue density marked disparity in the numbers of cases encounered, so it has been theorized that prior infection with tuberculosis, very common in the African HIV population and elsewhere in the tropics, may produce some class resistance to the development of clinically significant atypical infections. This is contrary to the commonly accepted belief held before AIDS that there is no cross-resistance between M. tuberculosis and the atypical mycobacteria.

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