Infection: Legionella pneumonia; Legionnaire’s disease; Legionella infection

Brief description:
  • Common cause of community acquired pneumonia (CAP), especially in spring/summer in NZ.
  • Does not respond to beta-lactam antibiotics e.g. amoxicillin.
  • Clinically and radiologically indistinguishable from other forms of CAP.
  • Increased suspicion based on: time of year, potting mix exposure, lack of response to beta-lactams.
Did you know?

Does not cause chronic cough or chronic respiratory symptoms. Causes acute CAP.

Who should I test?
  • Testing generally reserved for patients with severe CAP in hospital. Testing outside this should be discussed with the microbiologist.
  • If there is suspicion of Legionella pneumonia in community patients, in most situations empiric treatment should be considered, rather than testing.
Test of choice:

Request Legionella PCR

PCR of sputum or other deep respiratory sample

  • Excellent specificity – positive result confirms the diagnosis
  • Good sensitivity – negative result (if good sample) makes diagnosis unlikely
Tests to avoid/specialist tests:

Urinary antigen – not recommended

  • Poor sensitivity in the NZ context, as does not detect L.longbeachae, which is the commonest cause of Legionnaire’s disease in NZ.

Serology – not recommended

  • Difficult to interpret, only used as part of a Public Health investigation.
  • Requires acute and convalescent samples several weeks apart. Cannot confirm infection in a timeframe that is useful for informing treatment.
Other considerations:

Legionella pneumonia is a notifiable disease