Page 92 - AIDSBK23C

Page 92

MYCOBACTERIAL INFECTIONS

MYCOBACTERIUM AVIUM COMPLEX.-- Mycobacterium avium complex (MAC), also
known as Mycobacterium avium-intracellulare (MAI), is considered a non-pathogen in non-
immunocompromised persons. This complex can be further sub classified, but the clinical and
pathologic findings are similar in AIDS, so that it remains useful to refer to these organisms
collectively as MAC. Patients probably become colonized with MAC via the gastrointestinal or
respiratory tract. The MAC organisms can penetrate the gastrointestinal mucosa and are taken
up into submucosal macrophages. These macrophages are then transported to abdominal lymph
nodes and from there to the bloodstream. Cases of MAC in immunocompromised persons
probably represent reinfection rather than reactivation of prior infection.[422]
The risk for disseminated disease with MAC is increased with CD4 lymphocyte counts
below 100/µL. Infections with MAC are common in persons with AIDS. The organ distribution
of MAC is widespread, with lymph nodes, spleen and liver most frequent organs involved.
Involvement of the gastrointestinal tract, bone marrow, respiratory tract, adrenal, or
genitourinary tract is less frequent. Mycobacterium avium complex is rarely seen in the central
nervous system, skin, and heart (Table 5).[423,424]
Clinical manifestations are primarily the result of cytokine elaboration. Typical features
of MAC infection include persistent fever, night sweats, and anemia in about 80% of patients,
diarrhea in about half, and weight loss, lymphadenopathy, abdominal pain, and nausea/vomiting
in about one third of patients. Mycobacterium avium complex is unlikely to be an acute life-
threatening infection, is often not suspected pre mortem, and most patients have a protracted
course or die from another disease first. Though the number of AIDS patients with MAC is
increasing with longer survival, the number of deaths from MAC has decreased significantly
with the use of newer antimicrobial therapies.[417]
Blood culture is the best laboratory means of diagnosis of MAC, particularly when
disseminated MAC infection is suspected.[425] Bone marrow or lymph node tissues may be
cultured. The best tissue biopsy sites for histologic diagnosis of MAC are lymph node and liver.
The diffuse organ involvement of MAC helps to minimize the sampling error with
biopsy.[423,424]
Mycobacterium avium complex does not often produce typical grossly visible granulomas
with one exception--the spleen. A classic miliary pattern of granulomas is present in spleen in
about half of AIDS cases with MAC. Another distinctive gross pathologic finding with MAC is
a tan-yellow to lemon-yellow cut surface of involved lymph nodes in one fourth of cases. MAC
involvement of the gastrointestinal mucosa may produce diffuse or slightly raised plaque-like
areas of yellowish discoloration. Visceral organomegaly, especially of liver and spleen, may
result from MAC infection even though there are often no grossly visible lesions.[424]
Microscopically, MAC most often demonstrates a proliferation of small nests to
extensive sheets of large round to elliptical striated pale blue macrophages (histiocytes) on
hematoxylin-eosin stain. These macrophages can be up to 50 microns in size. The small, round
to oval nuclei of these cells are often obscured by the sheer numbers of mycobacteria. The cell
borders can also be indistinct because of many mycobacteria scattered in and around the cells.
The cytoplasm of these cells is teeming with mycobacteria that can not only be identified by acid
fast stain, but also by methenamine silver, PAS, Giemsa, or Brown-Hopps tissue gram stain. The