Tissue diagnosis of selected AIDS-related opportunistic infections.

Opportunistic infections are the most common initial manifestations of AIDS and, in many instances, are first encountered in surgical specimens. Pneumocystis carinii pneumonitis is by far the most frequent infection seen in biopsy specimens of AIDS patients. Most pathologists are familiar with its histopathologic presentations from previous experience. By contrast, many other opportunistic infections are either new or present clinically and pathologically in unfamiliar ways. Cytomegalovirus affects primarily the alimentary tract and lung. Colitis is the most common presentation. Penetrating ulcers may perforate. Most often, mesenchymal cells, endothelium in particular, show the typical intranuclear and intracytoplasmic inclusion bodies. The greater the number of inclusion bodies in tissues the shorter is the survival of the patient. Mycobacterium avium-intracellulare affects mainly small intestine and lymph nodes and produces a clinical and histologic picture similar to that of Whipple's disease. Diffuse infiltrates of histiocytes stuffed with acid-fast bacilli are characteristic. Cryptosporidiosis is the most ominous enteric opportunistic infection. Protozoa attach themselves to the epithelial surface and produce severe profuse, watery diarrhea. Cryptococcosis is seen in lung, lymph node, and brain biopsy specimens. Large numbers of organisms, sometimes with deficient mucinous capsules, and little or no inflammatory reaction, are notable. Toxoplasmosis is the most common cause of neurological complications in AIDS. Brain biopsy specimens show necrosis, microglial nodules, perivascular lymphocytic infiltrates, and, in 50% of cases, trophozoites.