IX. Pneumocystis: the ‘carrrier state’: epidemiology and transmission of human pneumocystosis

Pneumocystis carinii pneumonia (PCP) is a common opportunistic infection in immunocompromised patients, especially in patients with acquired immune deficiency syndrome (AIDS), and in those receiving immunosuppressive and/or antineoplastic chemotherapy. An important, unresolved question in the epidemiology of PCP concerns the modes of infection and transmission. PCP was first described in premature or debilitated infants [1], and in immunosuppressed children [2]. Seroepidemiological surveys have shown that the great majority of healthy children develop antibodies against Pneumocystis carinii (Pc) during the first decade of life and suggest that asymptomatic infection occurs. According to Wakefield et al., the acquisition of immunoglobulin response to Pc starts at 6 months of age and peaks in prevalence (80%) at about 8 years old [3]. Even so, it had been demonstrated that human immunodeficiency virus (HIV)-infected patients without PCP can mount a specific IgG-mediated antibody response to Pc [4]. It is generally thought that the PCP that develops in AIDS patients with severe destruction of CD4+ cells or in other patients with immunosuppressive agents represents the reactivation of latent endogenous Pc in the lung alveoli. The hypothesis of reactivation of latent Pc sp. f. hominis in the lung had long been presumed but demonstration of latent Pc in the lungs and bronchoalveolar lavage fluids of immunocompetent patients or autopsy lung tissue from immunocompetent human subjects was rarely possible with monoclonal antibody and even when recent sensitive techniques such as DNA amplification by polymerase chain reaction (PCR) were used [5]. These results suggest that asymptomatic carriage of Pc in the lungs of immunocompetent individuals is rare. Therefore, it has been admitted that the occurrence of PCP in immunocompromised patients might be from a new infection resulting from exposure to exogenous sources of Pc through contact with environmental sources or infected persons rather than …