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Pneumocystis pneumonia (PCP) occurs almost exclusively in the severely immunocompromised host, especially patients with congenital immunodeficiency diseases, AIDS, and cancer, and those who have had organ transplantation. Once pneumonia is apparent, the fatality rate is near 100% if untreated. Effective therapeutic and prophylactic drugs are available. The causative agent is a protozoan-like fungus known for the past century as Pneumocystis carinii. Recently, the term Pneumocystis jirovecii has been proposed for the organism found in humans,1 but acceptance has not been universal.2

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Epidemiology

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Pneumocystis infection is recognized among humans and lower animals worldwide. The natural habitat and mode of transmission in man are unknown, but animal studies suggest animal-to-animal transmission occurs by the airborne route.3-5 Animal-to-human transmission has not been reported, and available evidence suggests that human-to-human transmission is possible.11 Symptomatic infection occurs sporadically in immunocompromised individuals.

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PCP was first recognized in humans by Van der Meer and Brug in 1942.6 During this time, epidemics of interstitial plasma cell pneumonitis were occurring in European infants. In the 1950s, several reports confirmed that Pneumocystis was the etiology of epidemics of interstitial plasma cell pneumonitis in European infants.7

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With the advent of immunosuppressive therapy for cancer and methods for diagnosing congenital immunodeficiency disorders, PCP became recognized as a potentially fatal infection in such individuals.8-10

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Intrauterine transmission of Pneumocystis has also been reported.12,13 Of eight pregnant women with AIDS and PCP, one infant had Pneumocystis infection.14,15 Considerable data show that at least 75% of normal children reaching 4 years of age have acquired antibody to Pneumocystis and that more than 90% of normal adults have detectable antibody.16-18,19 Furthermore, a prospective study of otherwise normal infants, ages 2 to 12 weeks, with pneumonia showed that 10 of 69 babies had detectable antibody to Pneumocystis and that one of the 10 infants had PCP.20 In serial observations of 107 normal infants, Pneumocystis DNA was detected in nasopharyngeal aspirates in 74 of the infants tested.21 In addition, Pneumocystis has been associated with sudden infant death syndrome (SIDS), but no cause and effect has been proven.22,23 The natural course of PCP can be delineated from studies done before the advent of chemoprophylaxis in 1977 in individuals at high risk for PCP. Data collected at the Centers for Disease Control (CDC) revealed 194 documented cases of PCP encountered in the United States during the 3-year period from 1967 to 1970. Twenty-five (12.9%) of these cases had primary immunodeficiency disorders; 91 (47%) had leukemia; 41 (21%) had other malignancies; 22 (11%) were organ transplant recipients, and 15 (8%) had other immunocompromising conditions.24-26From 1962 to 1971, the incidence of PCP was determined in 1251 children with malignancies and 379 patients with nonmalignant neoplasms or immunodeficiency diseases at St. Jude Children’s Research hospital; PCP was found in 4.1%, 0%, and 0% ...

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