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

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


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".)


The taxonomic classification of Pneumocystis as a genus of protozoan organisms was questioned for many years. Now these organisms are recognized as ascomycetous 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]. Several species have been described including P. carinii, which infects rats, and P. jirovecii, which infects humans [6]. P. carinii was formerly the species name attributed to infections in humans, but P. jirovecii has been designated as the species name used to describe 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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Literature review current through: Nov 2017. | This topic last updated: Oct 03, 2017.
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