Atypical bacterial pathogens are a relatively common cause of lower respiratory diseases, including community-acquired pneumonia. The incidence of atypical pathogens in community-acquired pneumonia is approximately 22% globally, but this varies with location.
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. There is a relative increase in incidence during the late summer or autumn. Epidemics occur at 3- to 6-year intervals. Previous exposure is protective while smoking poses a risk for disease.
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. 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%).
Legionella pneumophila is responsible for a low percentage of community-acquired pneumonia cases (around 2.7%), 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. It may be associated with recent travel and may have a male predilection.
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. Influenza was the most frequently identified virus in adults with community-acquired pneumonia in one systematic review, accounting for 9% of cases.
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.
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