What is Legionella?

Legionnaires' disease is a lung infection (pneumonia) caused by a bacterium named Legionella pneumophila. The name Legionella pneumophila was derived from the original outbreak at the 1976 American Legion Convention in Philadelphia. Pneumophila means lung-loving in Greek.

What organism causes Legionnaires' disease?

Legionnaires' disease is caused by bacteria that belong to the family Legionellaceae. This family now includes 48 species and over 70 serogroups. Approximately half of these species have been implicated in human disease. Legionella pneumophila is responsible for approximately 90% of infections. Most cases are caused by L. pneumophila, serogroup 1. Legionella species are small (0.3 to 0.9 ¼m in width and approximately 2 ¼m in length) faintly staining Gram-negative rods with polar flagella (except L. oakridgensis). They generally appear as small coccobacilli in infected tissue or secretions. They are distinguished from other saccharolytic bacteria by their requirement for L-cysteine and iron salts for primary isolation on solid media and by their unique cellular fatty acids and ubiquinones.

What is the natural habitat of Legionella bacteria?

Legionella organisms are readily found in natural aquatic bodies and some species have been recovered from soil. The organisms can survive in a wide range of conditions, including temperatures of 0 to 63o C, pH of 5.0 to 8.5, and dissolved oxygen concentrations of 0.2 to 15 ppm in water. Temperature is a critical determinant for Legionella proliferation. Colonization of hot water tanks is more likely if tank temperatures are between 40 and 50oC (104 to 122o F). Legionella and other microorganisms become attached to surfaces in an aquatic environment forming a biofilm. Legionella has been shown to attach to and colonize various materials found in water systems including plastics, rubber, and wood. Organic sediments, scale, and inorganic precipitates provide Legionella with a surface for attachment and a protective barrier. Interestingly, the growth of other environmental organisms is stimulated by organic sediment, which in turn leads to the formation of by-products that stimulate the growth of Legionella.

What are the symptoms of Legionnaires' disease?

The incubation period of Legionnaires' disease is from two to ten days; this is the time it takes before symptoms of the illness appear after being exposed to the bacteria. For several days, the patient may feel tired and weak. Most patients who are admitted to the hospital develop high fever often greater than 39.5°C ( 103°F). Cough can be the first sign of a lung infection. The cough may be sufficiently severe to cause sputum production (coughed up mucous). Gastrointestinal stomach symptoms are common with diarrhea being the most distinctive symptom. Many patients have nausea, vomiting, and stomach discomfort. Other common symptoms include headaches, muscle aches, chest pain, and shortness of breath.

Is Legionnaires' disease contagious?

Legionnaires' disease is not contagious. No special precautions are necessary. The disease is transmitted by breathing contaminated water droplets or aerosols, not by infected persons. (So it differs from SARS and influenza where masks must be worn). Likewise, women who are pregnant and their fetuses have nothing to fear from patients with Legionnaires' disease.