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Epidemiology, clinical manifestations, and diagnosis of Pneumocystis pneumonia in HIV-uninfected patients

Authors
Charles F Thomas, Jr, MD
Andrew H Limper, MD
Section Editor
Kieren A Marr, MD
Deputy Editor
Anna R Thorner, MD

INTRODUCTION

Pneumocystis pneumonia (PCP) is a potentially life-threatening infection that occurs in immunocompromised individuals [1,2]. The nomenclature for the species of Pneumocystis that infects humans has been changed from Pneumocystis carinii to Pneumocystis jirovecii; this was done to distinguish it from the species that infects rats [3-7].

HIV-infected patients with a low CD4 count are at the highest risk of PCP. Others at substantial risk include hematopoietic cell and solid organ transplant recipients, those with cancer (particularly hematologic malignancies), and those receiving glucocorticoids, chemotherapeutic agents, and other immunosuppressive medications. The incidence of PCP is increasing as the number of people receiving immunosuppressive medications continues to grow [8].

The epidemiology, clinical manifestations, and diagnosis of PCP in patients without HIV infection will be reviewed here. PCP in HIV-infected patients and the treatment, outcome, and prophylaxis of PCP in HIV-uninfected patients are discussed separately. (See "Clinical presentation and diagnosis of Pneumocystis pulmonary infection in HIV-infected patients" and "Treatment and prevention of Pneumocystis infection in HIV-infected patients" and "Treatment and prevention of Pneumocystis pneumonia in HIV-uninfected patients".)

TAXONOMY

The taxonomic classification of Pneumocystis as a genus of protozoan organisms was questioned for several years. Now they are recognized as fungi based on ribosomal RNA and other gene sequence homologies, the composition of their cell walls, and the structure of key enzymes [9].

The nomenclature for the organism demonstrates the diversity of the Pneumocystis genus [3]. Four species have been described: two species that infect rats, including P. carinii, and one that infects humans, P. jirovecii [6]. P. jirovecii has been designated as the species name to use in future publications and references to human infections [6,10]. However, the abbreviation of "PCP" is still used to refer to the clinical entity of "PneumoCystis Pneumonia"; this allows for the retention of the familiar acronym amongst clinicians and maintains the accuracy of this abbreviation in older published papers.

                   

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