Christopher Bunch and Derrick WM Crook, Nuffield Department of Medicine, John Radcliffe Hospital, Oxford, UK
Opportunistic infections occur when host defenses are compromised by disease or its treatment. The pattern of infection that may occur in a compromised host can be predicted to some extent from an understanding of the nature of the underlying defect and the normal mechanisms that protect against infection with particular microorganisms. Table 1 illustrates the effect of various disease states on host defenses, and the kinds of infection typically encountered. By far the commonest conditions associated with opportunistic infection are malignant disease, organ transplantation, and human immunodeficiency virus (HIV) infection. Immune defects in these patients may be selective, but frequently affect more than one aspect of the immune response. In general, the risk of infection is influenced more by the duration of immune suppression than its degree.
Infections occur readily with organisms that are recognized pathogens, but are typically caused by commensal or other normally nonpathogenic agents. Thus community-acquired infections may be caused by ubiquitous saprophytic microorganisms such as
Table 1 Pattern of infection in disease states associated with impaired host defenses
Hospitalization, various conditions
Acute leukemias, aplastic anemia
Primary hypogammaglobulinemia, common variable immunodeficiency
B cell lymphomas, chronic lymphocytic leukemia, myeloma
Transplantation, immunosuppressive and cytotoxic therapy
HIV infection (AIDS)
Main host immune defects
Damage to mechanical barriers from i.v. cannulation, malignancy or cytotoxic therapy, obstruction or perforation Neutropenia, phagocytic deficiency
Impaired antibody production
Impaired T cell responses; neutropenia and phagocytic impairment
CD4 and monocyte/macrophage impairment
Typical pattern of infection
Bacteremias, phlebitis, pneumonias, empyemas, peritonitis, etc.
Bacterial infections (bacteremias, pneumonias, soft-tissue infections); invasive candidal and fungal infections
Bacterial infections, esp. pneumonias with H. influenzae, S. pneumoniae, S. aureus
As primary hypogammaglobulinemia
Bacteremias; pneumonitis; herpesvirus and mycotic infections; EBV-induced lymphoma
Pneumocystis, toxoplasma, Cryptosporidium, atypical mycobacteria, CMV, HSV, candidiasis; Kaposi's sarcoma, EBV-induced lymphoma
Aspergillus spp., Nocardia and Cryptococcus, or by exposure to geographically localized mycoses such as histoplasmosis, as well as by more common pathogens such as the pneumococcus, respiratory viruses and - in some communities - tuberculosis and common parasitic infections such as strongyloidiasis. Immunosuppressed patients are also prone to more general public health hazards such as the presence of Cryptosporidium in water supplies, Listeria monocytogenes in unpasteurized milk products, and salmonella food-poisoning.
Hospital-acquired (nosocomial) infections are particularly significant, as hospitals may harbor organisms that are resistant to standard antimicrobial therapy, as well as more serious pathogens such as Pseudomonas spp. Hospitalized patients are more likely to undergo invasive procedures, and may acquire infection from blood transfusion. Furthermore, some hospital buildings are potential sources of Aspergillus and Legionella spp.
The successful management of infection in the compromised host requires a high degree of suspicion, a knowledge of the underlying disorder, and a willingness to investigate rapidly and aggressively. The usual signs of infection may be absent: for example, fever may be reduced or suppressed in patients taking corticosteroids, and local inflammation and abscess formation may be impaired in patients with neutropenia. Diagnostic tests may also require different interpretation: antibody responses may be impaired in patients with humoral immunodeficiency, and skin tests may be similarly unreliable in those with cellular immunodeficiency. Standard microbiological culture techniques may require modification to detect opportunistic invaders, and tissue biopsy may be required to characterize some invasive infections.
The pattern of infection in the compromised host has been altered somewhat by the availability of effective prophylaxis for some of the commoner opportunistic infections. Cotrimoxazole effectively prevents Pneumocystis pneumonia in patients with hematologic malignancy and in transplant patients. It is less useful in HIV infection as hypersensitivity reactions to the sulfonamide component are common. Inhaled pentamidine is a useful alternative. Herpes simplex virus (HSV) reactivation can be prevented by acyclovir; some protection may also extend to other herpesviruses including cytomegalovirus (CMV). Some of the newer quinolone antibiotics such as ciprofloxacin and norfloxacin can reduce the incidence of bacteremia in neutropenic patients.
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