36 - Atypical pneumonia

Summary

Epidemiology

Atypical pneumonia is caused by bacteria rather than viruses, but it is included in this book because laboratory tests for evidence of infection with these organisms are normally carried out in virology laboratories. They are called ‘atypical’ because in general they produce less severe and more protracted symptoms than other bacterial pneumonias; however, atypical pneumonias can be severe and can be fatal.

Atypical pneumonia is caused mainly by five different bacteria. Prompt diagnosis of infection is important because these organisms can produce severe or fatal infection and they are all susceptible to antibiotic treatment. The five organisms are Mycoplasma pneumoniae, Chlamydophila psittaci, Chlamydophila pneumoniae, Coxiella burnetii and Legionella pneumophila.

Laboratory diagnosis

All of these infections can be diagnosed by testing clotted blood for specific antibodies. Antibody may be present at the time of presentation (especially M. pneumoniae) but may not be detectable until 10 days after the onset of symptoms (up to a month for L. pneumophila). Urine taken in the first 5 days (for antigen detection) after the onset of symptoms is the best way to diagnose L. pneumophila infection. To identify outbreaks of L. pneumophila infection, it is important to culture respiratory samples on specialized media so that strains can be characterized.