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Legionnaires' Disease

What is Legionnaires’ disease?

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 more than 58 species approximately 25 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 μm in width and approximately 2 μm 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.

Where do Legionella bacteria come from?

Legionella are natural inhabitants of water and can be detected in rivers, lakes, and streams. One type of Legionella species (L. longbeachae) has been found in potting soil.

What have been the water sources for Legionnaires’ disease?

The major source is water distribution systems of large buildings including hotels and hospitals. Cooling towers have long been thought to be a major source for Legionella, but new data suggest that this is an overemphasized mode of transmission. Other sources include mist machines, humidifiers, whirlpool spas, and hot springs. Air conditioners are not a source for Legionnaires’ disease.

How do people contract Legionella?

The most popular theory is that the organism is aerosolized in water and people inhale the droplets containing the Legionella bacteria. However, new evidence suggests that another way of contracting Legionnaires’ disease is more common.

Aspiration is the most common way that bacteria enter into the lungs to cause pneumonia. Aspiration occurs when secretions in the mouth get past the gag reflex and instead of passing into the esophagus, mistakenly enter the lung. The protective mechanisms to prevent aspiration is defective in patients who smoke or have lung disease. Aspiration now appears to be the most common mode of transmission.

For more FAQs on Legionaires’s disease and Legionella, visit SPL’s educational website, legionella.org.