Epidemiology

Legionnaires' disease was not recognised until 1976 when an outbreak of pneumonic illness occurred among American legionnaires attending a convention in a hotel in Philadelphia. It was not until early the following year that the causative organism was isolated and identified. Using stored sera it was subsequently possible to determine that a number of previously unexplained outbreaks of pneumonia had also been caused by Legionnella infections. Although the precise source of the eponymous outbreak was never identified, many other outbreaks have since been recognised and their source precisely identified.

The term ‘legionellosis’ encompasses any illness due to Legionella infection. ‘Legionnaires’ disease' is reserved for the pneumonic form of legionellosis whereas ‘Pontiac fever’ is generally used to describe the non-pneumonic form of the disease. In practice Legionella infections may cause a spectrum of illness from an asymptomatic infection through influenza-like illness, to fulminant pneumonia. Cases of Legionnaires' disease have now been identified in patients in most industrialised countries although reports of cases from developing countries are uncommon due, at least in part, to limited diagnostic facilities.

More than 30 species of Legionella have been identified but only a small number have been associated with disease in man. Over three-quarters of infections are caused by Legionella pneumophila species most usually serogroup 1. Legionella micdadei is the second commonest pathogen, more often affecting immunocompromised individuals, followed by Legionella bozemanii and Legionella dumoffii.