Legionnaire's Disease

Today we discussed a case of a pneumonia that did not initially respond to empiric therapy. Supplemental investigations were notable for a positive Legionella urinary antigen.

Legionnaire's Disease was first described after an outbreak in Philadelphia in 1976. It is now being recognized as a more common cause of respiratory infections. It is a difficult to culture organism requiring special growth media (talk to the micro lab if you are thinking about it). Testing can also be done by the rapid urinary antigen assay (only detects L. pneumophilia serotype 1). Interestingly, one study demonstrated persistent urinary antigen positivity months after exposure (in an immunocompromised host).

Overall, empiric therapy for CAP (respiratory fluoroquinolones or a macrolide) should cover Legionella species and there are no RCTs that show one class to be superior to the other.

Here is a recent review.