The Organism

Legionella pneumophila causes about 90% of cases of legionella infection. This species has now been subdivided into smaller groups known as serogroups, and although all of them can cause the disease, the commonest cause is serogroup 1.

Studies have now revealed a family of related organisms, the Legionellaceae, in which there are over 40 species, some of them not yet named. The named species have been isolated from environmental sources or patients, and their names reflect either the place where they were found or the person who discovered them, e.g. L Bozemanii and L. Longbeachae. Some of the environmental species have not yet been shown to cause human disease.

Legionellas are widely distributed in nature and their natural habitats are the water of rivers, warm springs, streams, lakes and river banks. In their normal environment legionellas do not cause infections, but transferred to the water supplies and air conditioning units of large buildings they can cause large outbreaks of disease.

In a survey carried out by the PHLS in England and Wales between 1981 and 1985, 53% of hotels, 70% of hospitals and 75% of business premises studies contained legionellae in at least one sample from their hot and cold water systems.

• Legionnaires Disease - The Facts
• Legionella in Water Supplies
  
• Legislation
• Outbreaks