In 1976 an epidemic of pneumonia followed the Legionnaires' convention in Philadelphia, USA. Legionella pneumophila was identified as the causative agent. Legionnaires' disease, or legionellosis as it is also called, is a disease of significant medical and public interest. In the USA between 8,000 and 18,000 people are hospitalized with Legionnaires' disease. However the number of infections may be even higher because many infections are not diagnosed or reported.
Legionnaires' disease has symptoms similar to other forms of pneumonia, and for this reason it can be difficult to diagnose. Symptoms include a fever, chills, and a cough. There may also be muscle aches and headaches. Chest X-rays are required to identify the pneumonia and microbiological tests can be performed on sputum, blood or urine to find evidence of the bacterium. Symptoms usually occur 2 to 14 days after exposure to the bacteria.
A milder infection caused by the same type of Legionella bacteria is called Pontiac Fever. In this disease the symptoms last for 2 to 5 days and include fever, headaches, and muscle aches but not pneumonia.
Legionnaires' disease can be very serious and can cause death in up to 30% of cases. Normally however cases are treated successfully with antibiotics and a full recovery is made. Legionella is found naturally in the environment normally in aquatic habitats where it can survive and multiply within different protozoa. This ability enables the bacterium to be transmissible and pathogenic to humans. Legionella grosw best in warm water, for example in hot tubs, cooling towers, hot water tanks, large plumbing systems, or parts of the air-conditioning systems of large buildings. Legionella infection is caused by breathing in a mist or vapor that contains the Legionella bacteria. Those most at risk include older people, smokers, patients with a chronic lung disease or who have a weak immune system, those suffering from other diseases such as cancer, diabetes, or kidney failure, and immunocompromised people.
Scientists using modern molecular biology techniques are investigating the mechanisms used by L. pneumophila to grow in protozoa and in lung macrophages. The genome sequences of four L. pneumophila strains are being studied and evaluated. Research is also being conducted in the areas of clinical features, diagnosis, treatment and epidemiology. In the research laboratory Legionella is used as a model system to analyse the mechanisms by which bacteria live within macrophages and the lung.
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