One of the characteristics of advanced cancer, especially neoplasms of the immune system, is a decrease in the host resistance to various infectious agents. This phenomenon appears to have three characteristics: (1) a decrease in mature granulocytes, seen in several lymphomas and leukemias, which may result in impaired phagocytosis; (2) an impaired cell-mediated immune response, commonly seen in several lymphomas and leukemias, especially Hodgkin disease; and (3) a decrease or alteration in circulating γ-globulin of host origin, occurring in a number of lymphomas and leukemias and resulting in an increased susceptibility to bacterial infections (Armstrong et al., 1971). In addition to the natural loss of host immunity resulting from the host- tumor relationship, chemotherapy itself may result in a marked suppression of the host immune response, leading to a number of infectious complications. In hematological neoplasms, infections may cause as many as 63% of the deaths in patients with these diseases (Nosari et al., 1991).
Many of the infectious processes that occur in patients with advanced cancer or in patients undergoing chemotherapy are the result of agents that normally do not cause disease in the host. Table 19.18 lists organisms that may cause severe infection in patients with neoplastic disease. The reader will note some familiar infectious agents such as Salmonella (typhoid), tuberculosis, measles, and varicella (chickenpox). But most of the agents listed in the table are relatively rare causes of disease in the normal human. These latter organisms can usually attack only the indi- vidual whose immune defenses are compromised. This is especially true of fungi (cf. Gold,
1984; Bow, 1998), which appear to be ubiquitous in the human population but rarely cause dis- ease in the immunocompetent host. Earlier we saw that patients with progressive multifocal leu- koencephalopathy (PML) were found to have a papovavirus infection within the central nervous system. It is likely that the virus causes PML, but predominantly in humans who exhibit signifi- cant immunosuppression associated with lymphomas and other debilitating diseases. In normal individuals the endogenous microbial flora (enteric bacteria) exist in a balanced symbiotic rela- tion within the host. Nevertheless, 86% of the infections that occur in cancer patients arise from such endogenous flora (cf. Pizzo, 1981). Some 47% of the infecting organisms are acquired by patients during hospitalization. The relative malnutrition that accompanies cachexia may further decrease the immunocompetence of the host if the malnutrition is not treated aggressively (cf. Pizzo, 1981).
The extensive growth of tumors, predominantly but not exclusively those of the immune system, results in significant loss of immune resistance by the host, with the concomitant danger of an “opportunistic” infection by a variety of agents. On the other hand, heroic measures of therapy by chemicals and radiation may also leave the host immunosuppressed and vulnerable to infection. In many instances it is this infectious sequel of cancer or its therapy that leads to the ultimate demise of the patient.