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Pneumocystis pneumonia (PCP) is a form of pneumonia caused by the yeast-like fungus, Pneumocystis jirovecii. This species of fungus is specific to humans. It has not been shown to infect other animals while other species of Pneumocystis that parasitize other animals (all of which are mammals) have not been shown to infect humans.[1] The causal agent was originally described as a protozoan and spelled P. jiroveci and prior to then was classified as a form of Pneumocystis carinii, a name still in common usage.[2][3] These names are discussed below. As a result, Pneumocystis pneumonia (PCP) has also been known as Pneumocystis jiroveci[i] pneumonia and as Pneumocystis carinii pneumonia, as is also explained below.[4][5][6] Confusion caused by the misapplication of the name P. carinii to all species of Pneumocystis may incorrectly suggest that the human pathogen could reside in other animals, including domesticated animals.
The disease condition (PCP) caused by P. jirovecii is relatively rare in people with normal immune systems but common among people with weakened immune systems, such as premature or severely malnourished children, the elderly, and especially AIDS patients, in whom it is most commonly observed today.[7][1] PCP can also develop in patients who are taking immunosuppressant medications (e.g. patients who have undergone solid organ transplantation) and in patients who have undergone bone marrow transplantation.
The organism is distributed worldwide[8][9]
Pneumocystis pneumonia has been described in all continents except Antarctica.[8] It was originally described as a rare cause of pneumonia in neonates. It is commonly believed to be a commensal organism (dependent upon its human host for survivial). The possibility of person-to-person transmission has recently gained credence, with supporting evidence coming from many different genotyping studies of Pneumocystis jirovecii isolates from human lung tissue. [10][11] (In one outbreak of 12 cases among transplant patients in Leiden it was suggested as likely, but not proven, that human-to-human spread may have occurred).[12] Greater than 75% of children are seropositive by the age of 4, which suggest a high background exposure to the organism.
Since the start of the AIDS epidemic, PCP has been closely associated with AIDS. Because it only occurs in an immunocompromised host, it may be the first clue to a new AIDS diagnosis if the patient has no other reason to be immunocompromised (e.g. taking immunosuppressive drugs for organ transplant). An unusual rise in the number of PCP cases in North America, noticed when physicians began requesting large quantities of the rarely used antibiotic pentamidine, was the first clue to the existence of AIDS in the early 1980s.[13][14]
Prior to the development of more effective treatments, PCP was a common and rapid cause of death in persons living with AIDS. Much of the incidence of PCP has been reduced by instituting a standard practice of using oral co-trimoxazole to prevent the disease in people with CD4 counts less than 200/mm³. In populations that do not have access to preventive treatment, PCP continues to be a major cause of death in AIDS.
In immunocompromised patients (e.g. cancer patients on chemotherapy, or persons living with AIDS with a CD4+ T-cell count below 200/µl), prophylaxis with regular pentamidine inhalations or sulfamethoxazole/trimethoprim (co-trimoxazole or TMP-SMX) may be necessary to prevent PCP.
Symptoms of PCP include fever, non-productive cough, shortness of breath (especially on exertion), weight loss and night sweats. There is usually not a large amount of sputum with PCP unless the patient has an additional bacterial infection. The fungus can invade other visceral organs, such as the liver, spleen and kidney, but only in a minority of cases.
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