Epidemiology

Pneumocystispneumonia (PCP) was historically one of the most common AIDS-defining illnesses among children, adolescents, and adults in high-income countries.​[1]

In the era of combination antiretroviral therapy (ART), the incidence of opportunistic infections (including PCP) has declined.[2][3][4][5][6]​​​​​ The EuroSIDA study followed a cohort of more than 8500 HIV-positive patients in Europe, finding that incidence of PCP fell from 4.9 cases per 100 person-years before March 1995 to 0.3 cases per 100 person-years after March 1998.[7] Similarly, the NA-ACCORD study, which followed 16 cohorts of more than 80,000 HIV-positive patients from the US and Canada, observed a decrease in the incidence of PCP from 0.92 cases per 100 person-years in 2000 to 2003 to 0.39 in 2008 to 2010.[8] However, PCP infection remains a common diagnosis in patients with HIV infection admitted to intensive care.[9] In an acute tertiary hospital in New York City (with a comprehensive outpatient HIV care programme), 4.8% of AIDS-related deaths between 2004 and 2008 were due to PCP.[10] In HIV-positive adults, the greatest risk factor for developing PCP is a CD4 cell count <200 cells/microlitre, with risk increasing the lower the CD4 cell count falls below this level.[11][12]​ The risk of PCP is decreased if patients sustain increases in their CD4 cell count to more than 200 cells/microlitre with the use of ART. Thus, nowadays most cases of PCP occur in patients who are unaware of their HIV infection, those who do not seek medical care for HIV or do not use ART or PCP prophylaxis owing to nonadherence or intolerance, and those with advanced immunosuppression (i.e., CD4 counts <100 cells/microlitrr).[2][5][13][14][15]​​​​ Between 2000 and 2013, a European study reported an increase in the age and proportion of patients in which an episode of PCP preceded HIV diagnosis (from 34 to 44 years, and from 48% to 67%, respectively).[16]​​

In the US, the prevalence of HIV infection may be under-estimated and, therefore, the incidence of the first episode of PCP may also be under-estimated, particularly in groups with poor access to health care.[17] Another group of patients at risk for PCP are HIV-positive refugees from low- and middle-income countries in whom it may be difficult to differentiate PCP from pulmonary tuberculosis infection or dual infection.[18]

In children, the overall incidence of PCP has declined since the beginning of the HIV era, partly owing to improved antenatal HIV testing and use of HIV treatment to prevent vertical transmission of the virus, but also due to ART.[2][19]​ In the Perinatal AIDS Collaborative Transmission Study, the incidence of PCP in HIV-positive children dropped from 5.2 cases per 100 person-years during the pre-ART era to 0.3 cases per 100 person-years in the post-ART era.[20] In HIV-positive children, the highest rates of PCP occur in infants 3 to 6 months old. The risk of PCP is not related to CD4 cell counts in those under 6 years of age but rather relates to CD4 percentage. In children over 6 years old, CD4 cell count is related to PCP risk, similar to adults.[3][21][22]

PCP is an important opportunistic infection in HIV-positive children and adults in low- and middle-income countries although definitive pathogen identification may be challenging due to limited resources.[23]

In HIV-negative patients, the overall incidence of PCP is low and occurs almost exclusively in patients who have other causes of immunocompromise.[24][25][26][27][28]​​​ While the incidence of PCP in HIV-positive patients decreased in England between 2000 and 2010 and in France between 2005 and 2013, PCP in HIV-negative patients increased over the same period.[29] In this population, PCP occurs mainly in organ transplant recipients (7% to 43%), people with haematological malignancies (23% to 39%), people with solid malignancies (18% to 27%), and people with inflammatory conditions (11% to 27%).[26][27]​​​ A retrospective longitudinal population-based study in Germany found that the overall incidence of PCP increased between 2014 and 2019 from 2.3 cases per 100,000 people to 2.6 cases per 100,000 people. More than 80% of PCP cases occurred in HIV-negative patients. Although the number of cases among patients with HIV, haematologic malignancies, and transplants decreased over time, patients with solid malignancies experienced an increase in the annual number of cases, indicating that the distribution of underlying immunosuppressive diseases among incident PCP cases changed over time. Similar trends were identified for PCP-related deaths.[6]​ PCP has been reported in patients using tumour necrosis factor-alpha antagonists or the anti-CD20 monoclonal antibody, rituximab.[30][31]​​​ PCP in the HIV-negative population is a more fulminant process and is associated with greater morbidity and mortality than in HIV-positive patients.[6][27]​​​​

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