Infections in immunocompromised patients. I. Pathogenesis, etiology, and diagnosis.

Granulocytopenia is the major factor predisposing cancer patients to infection, chiefly by bacteria. Most of the infections are caused by gram-negative aerobic organisms (Escherichia coli, Pseudomonas aeruginosa, and Klebsiella sp) that arise from endogenous gastrointestinal, mucosal, or cutaneous flora (often modified by hospital-acquired pathogens). Some fungi (Candida sp and Aspergillus sp) are also likely to invade granulocytopenic patients. The next most important factor predisposing cancer patients to infection is alteration of anatomic barriers. Mucosal and skin protection is compromised by tumors, radiotherapy, chemotherapy, surgical diagnostic and therapeutic procedures, and intravenous lines and other devices. The pathogens found in cases of altered anatomical barriers are similar to those encountered in patients with granulocytopenia, which is a major cause of alteration of anatomical barriers. The third factor predisposing patients to serious bacterial and nonbacterial infections is immunosuppression, especially in patients with lymphomas, multiple myelomas, and chronic lymphatic leukemia. The pathogens isolated in these patients include Listeria monocytogenes, Salmonella sp, Brucella sp, Mycobacterium sp, and Nocardia asteroides, although P aeruginosa and staphylococci may be found as well. Viruses (herpes simplex, herpes zoster, and vaccinia), parasites, fungi, and Pneumocystis carinii and Toxoplasma gondii are involved in many infections in these patients, in whom cell-mediated immunity is impaired. In neutropenic patients, antimicrobial or antifungal therapy should be instituted before the microbiological diagnosis is made; in immunosuppressed patients, therapy is optimally guided by a specific microbiological diagnosis.