Mayo Clinic Health Manager
Get free personalized health guidance for you and your family.
Get Started
About our e-newsletters
- Free e-newsletters
- Mayo Clinic expertise
- We do not share your e-mail address
Housecall, our weekly general-interest e-newsletter, keeps you up to date on a wide variety of health topics with timely, reliable, practical information, recipes, blogs, questions and answers with Mayo Clinic experts and more. Our biweekly topic-specific e-newsletters also include blogs, questions and answers with Mayo Clinic experts, and other useful information that will help you manage your health.
Housecall Archives
Mayo Clinic Housecall
Stay up to date on the latest health information.
What you get
- Free weekly e-newsletter
- Mayo Clinic expertise
- Recipes, tools and other helpful information
- We do not share your e-mail address
Causes
By Mayo Clinic staffThe bacterium Legionella pneumophila is responsible for most cases of Legionnaire's disease, although several other species of legionella also can cause infection. Legionella bacteria thrive in warm, damp environments. In nature, they can survive for months in lakes, rivers, hot springs and soil. But the levels of bacteria in the natural environment are so low that they rarely cause problems. The real threat is indoors, where legionella bacteria can multiply in water systems, whirlpool spas, air conditioning systems and even the misters in grocery store produce departments.
Like many microorganisms, legionella bacteria have the ability to anchor themselves to the interiors of pipes, faucets and shower heads. Once attached, the bacteria replicate in a sticky substance called a biofilm. As water flows past, it dislodges some of the biofilm, dispersing bacteria throughout the water system. Although it's possible to contract Legionnaires' disease from home plumbing systems, most outbreaks have occurred in large buildings, perhaps because complex systems allow the bacteria to grow and spread more easily.
How the infection spreads
Most people become infected when they inhale microscopic water droplets containing legionella bacteria. This might be the spray from a shower, faucet or whirlpool, or water dispersed through the ventilation system in a large building. Outbreaks have been linked to a range of sources, including:
- Whirlpool spas on cruise ships
- Cooling towers in air conditioning systems
- Decorative fountains
- Swimming pools
- Physical therapy equipment
- Water systems in hotels, hospitals and nursing homes
Scientists aren't certain how much exposure to the bacteria is needed to cause disease, but some people have developed infections after inhaling contaminated droplets for just a few minutes. And unlike many bacteria, which spread within a small radius, legionella bacteria may be capable of traveling as far as four miles through the air.
Although legionella bacteria primarily spread through aerosolized water droplets, the infection can be transmitted in other ways, including:
- Aspiration. This occurs when liquids accidentally enter your lungs, usually because you cough or choke while drinking. If you aspirate water containing legionella bacteria, you may develop the disease.
- Soil. A few people have contracted Legionnaire's disease after working in the garden or using contaminated potting soil. It's also possible that the disease may spread when earth containing the bacteria is stirred up at large construction sites.
Beating the immune system
Once legionella bacteria enter your lungs, they're surrounded by immune system cells (alveolar macrophages) that normally attack and destroy foreign organisms. But rather than being destroyed by these cells, the legionella bacteria turn them to their advantage. The legionella bacteria enter the macrophages, use them to grow and replicate, and eventually kill them. When the macrophages die, thousands of new bacteria are released into your lungs, worsening symptoms during the first week of the infection and perpetuating the cycle of the disease.
- Pedro-Botet LM, et al. Epidemiology and pathogenesis of legionella infection. http://www.uptodate.com/home/index.html. Accessed Nov. 5, 2008.
- Patient facts: Learn more about Legionnaires' disease. Centers for Disease Control and Prevention. http://www.cdc.gov/legionella/patient_facts.htm. Accessed Nov. 21, 2008.
- Tossa P, et al. Pontiac fever: An operational definition for epidemiological studies. BMC Public Health. 2006;6:112.
- Pedro-Botet LM, et al. Treatment and prevention of legionella infection. http://www.uptodate.com/home/index.html. Accessed Nov. 5, 2008.
- Top 10 things every clinician needs to know about legionellosis. Centers for Disease Control and Prevention. http://www.cdc.gov/legionella/top10.htm. Accessed Nov. 21, 2008.
- Pedro-Botet LM, et al. Clinical manifestations and diagnosis of legionella infection. http://www.uptodate.com/home/index.html. Accessed Nov. 5, 2008.
- Neil K, et al. Increasing incidence of legionellosis in the United States, 1990-2005: Changing epidemiologic trends. Clinical Infectious Diseases. 2008;47:591.
- Nhu Nguyen TM, et al. A community-wide outbreak of Legionnaires disease linked to industrial cooling towers - How far can contaminated aerosols spread? Journal of Infectious Diseases. 2006;193:102.
- Legionnaires' disease associated with potting soil - California, Oregon, and Washington. Centers for Disease Control and Prevention. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm4934a1.htm. Accessed Nov. 4, 2008.
- Legionellosis. World Health Organization. http://www.who.int/mediacentre/factsheets/fs285/en/index.html. Accessed Nov. 24, 2008.
