Legionella

Legionella are gram negative bacilli. They are intracellular organisms.

Legionella bacteria cause 2 distinct clinical conditions: 

1. Legionnaires disease, a severe multisystem disease which includes pneumonia 
2. Pontiac fever, a self-limiting flu-like illness without pneumonia 

The natural environment for legionella is water, and they multiple at temperatures between 25 and 42C, with 35C being their optimal temperature meaning that hot water systems, whirlpools etc are ideal breeding grounds. Many outbreaks have also been associated with air-conditioning systems or with travel, especially cruise ships. 

Symptoms of Legionnaire disease include 

• Fever 
• Dry cough 
• Dyspneoa 
• Nausea, vomiting and abdominal pain 
• Confusion 

The incubation period is 2 to 18 days.

Legionnaire disease is also associated with 

• Hyponatraemia 
• Deranged LFTs 
• Thrombocytopeaenia 

Diagnosis may be by 

• Urinary antigen 
• Sputum culture 
• Serology 

Most patients have abnormal CXR at presentation, most commonly patchy basal consolidation. Up to 30% have pleural effusions. 

Treatment is with antibiotics with a high intracellular concenration, such as azithromycin, clarithromycin, fluroquinolone or rifampacin.  Note that Legionnaires disease is a notifiable disease.

References:

Darby, J. and Buising, K. Could it be Legionella? Aust Fam Physician. 2008; 37(10): 812-815. 

Fields, B., Benson, R. and Besser, R. Legionella and Legionnaires’ disease: 25 years of investigation. Clin Microbiol Rev. 2002; 15(3): 506-526.