Case of the Month #4

Pete Hersey

A 67 year old man has been admitted overnight to the intensive care unit.  He presented with flu-like symptoms and a non-productive cough.  His wife reported that he had been ‘seeing things’.  Investigations revealed hypoxaemia, lobar consolidation, neutrophilia and a raised lactate dehydrogenase.

You suspect the patient may have Legionnaires’ disease.


Legionellosis describes disease caused by Legionella Sp. Over 50 species of Legionella have been described, although not all are known or suspected human pathogens.  Legionella Pneumophilia was the first described and is responsible for most cases of Legionnaires’ disease.

Legionella Sp. are found in water throughout the world, and infection is due to deep inhalation/aspiration of water droplets containing a pathogenic species.

Up to 5% of the general population who inhale Legionella Pneumophilia will develop Legionnaires disease (pneumonia).  Up to 95% will develop the less severe influenza-like illness, Pontiac fever.  People at increased risk of community acquired Legionnaires disease are older men, smokers and the immunosuppressed. 

There has only been one reported case of person-person transmission.


Although often described in ‘classical terms’, a diagnosis of Legionnaires disease cannot be made on clinical features alone.  The features that may promote a diagnosis however include:

  • Influenza like symptoms
  • Diarrhoea (25-50%)
  • Confusion/delirium/headache/Other neurological features (approx. 50%)
  • Hyponatraemia
  • Raised LDH
  • Unproductive cough (50%)
  • Pleural effusion at presentation
  • Associated myocarditis/endocarditis/pericarditis
  • Treatment failure with beta lactam therapy

Whilst approximately 20% of European cases are travel related, this means that 80% are not.  Droplets can also spread further than 3km, so whilst travel/exposure history is important a characteristic exposure may not be present.

The testing of urine for Legionella Ag (lipopolysaccharide from the bacterial cell wall) is the most rapid diagnostic test available.  The antigen test is specific to Legionella pneumophilia serogroup 1.  Whilst this organism is the cause of 70% of Legionnaires’ disease, 20-30% are due to other subgroups and 5-10% by other Legionella sp

Culture (of sputum or any other infected tissue/fluid) is the gold standard investigation.  Whilst only half of patients are reported to produce pus-containing sputum, Legionella can be grown from sputum that has been considered ‘poor quality’ (“not worth sending”).


A good outcome from Legionnaires disease relies on early appropriate treatment.  Whilst macrolides are generally first choice, fluroquinolones and Doxycycline are alternatives.

Further Reading