Epidemiology

Atypical bacterial pathogens are a relatively common cause of lower respiratory diseases, including community-acquired pneumonia.[1] The incidence of atypical pathogens in community-acquired pneumonia is approximately 22% globally, but this varies with location.[2]

Mycoplasma pneumoniae causes up to 20% of cases of community-acquired pneumonia and has been implicated in some hospital-based epidemics. Infection is common in children and young adults, and is often seen in close community settings such as boarding schools, universities, and military bases.[3] There is a relative increase in incidence during the late summer or autumn. Epidemics occur at 3- to 6-year intervals.[1] Previous exposure is protective while smoking poses a risk for disease.[4][5][6][7]

Chlamydophila pneumoniae causes 3.5% to 10.0% of cases of community-acquired pneumonia. Like Mycoplasma pneumoniae, infection occurs mainly in children and young adults and is associated with close community settings.[1][3][5][8][9] A Dutch study identified Chlamydia psittaci by polymerase chain reaction (PCR) of sputum (when available) as a cause of community-acquired pneumonia in 4.8% of cases, higher than that previously reported (2.1%).[10]

Legionella pneumophila is responsible for a low percentage of community-acquired pneumonia cases (around 2.7%),[1] but it is responsible for up to 16% of cases that require hospitalisation. It is generally associated with exposure to a new source of aerosolised water in showers or from cooling systems. Smoking, chronic lung disease, immunosuppression, and immunomodulatory drugs are known risk factors.[11] It may be associated with recent travel and may have a male predilection.[5][12][13][14]

Viruses (including influenza, adenovirus, respiratory syncytial virus, as well as others) may cause pneumonia that can fit atypical bacterial pneumonia features in up to 25% of the community-acquired pneumonia patients.[15][16][17][18] Influenza was the most frequently identified virus in adults with community-acquired pneumonia in one systematic review, accounting for 9% of cases.[19]

Coxiella burnetii accounts for approximately 1% of pneumonia cases; however, higher incidence occurs in regions where there is high exposure to aerosols originated from livestock. It is a more common cause of pneumonia in Europe and certain regions of Canada.[20]

The specific cause of community-acquired pneumonia can vary by season, location, and age.[3][21][22][23]

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