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%)
- 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”).