Legionella - General Information

Legionnaires' disease was first identified following a large outbreak among people who attended an American Legion Convention in Philadelphia in 1976. A previously unrecognised bacterium was isolated from lung tissue samples which was subsequently named Legionella pneumophila.

It is normally contracted by inhaling Legionella bacteria, either in tiny droplets (aerosols) or in droplet nuclei (the particles left after the water has evaporated) contaminated with Legionella, deep into the lungs. There is evidence that the disease may also be contracted by inhaling Legionella bacteria following ingestion of contaminated water by susceptible individuals. Person to person spread of the disease has not been documented. Initial symptoms of Legionnaires' disease include high fever, chills, headache, and muscle pain. Patients may develop a dry cough and most suffer difficulty with breathing. About one third of patients infected also develop diarrhoea or vomiting and about half become confused and delirious. Legionnaires' disease can be treated with appropriate antibiotics.

The incubation is between 2-10 days (usually 3-6). Not everyone exposed will develop symptoms of the disease and those that do not develop the "full blown" disease may only present with a mild flu-like infection.

Infection with Legionella bacteria can be fatal in approximately 12% of reported cases. This rate can be higher in a more susceptible population; for example, immunosuppressed patients or those with underlying disease. Certain groups of people are known to be at higher risk of contracting Legionnaires' disease; for example, men appear more susceptible than women, as do those over 45 years of age, smokers, alcoholics, diabetics and those with cancer or chronic respiratory or kidney disease.

To date, approximately 40 species of the Legionella bacterium have been identified. L. pneumophila causes about 90% of cases. Sixteen different serogroups of L. pneumophila have been described; however L. pneumophila serogroup 1 is most commonly associated with cases of Legionnaires' disease in the UK.

On average there are approximately 200-250 reported cases of Legionnaires' disease each year in the United Kingdom. It is thought, however that the total number of cases of the disease may be generally underestimated. About half of cases are associated with travel abroad.