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Legionellosis (also known as "Legionnaires' disease") is a bacterial infection that was first identified after a 1976 outbreak in Philadelphia among attendees of the American Legion's annual convention. In the United States, an estimated 8,000 to 18,000 cases occur each year. Legionella are widespread in the environment, including in soil, natural bodies of water, water distribution systems in homes and buildings, and building cooling towers. Legionella can also grow in swimming pools, hot tubs and saunas, decorative fountains, and other man made water sources.

Disease occurs when the organism is inhaled in aerosolized water droplets, causing pneumonia. It has also been associated with inhalation of aerosols generated when using potting soil. Persons at increased risk for legionellosis include the elderly as well as those with underlying lung and heart disease, cancer, organ transplants, and other immune system disorders. Legionellosis outbreaks have occurred in hospitals and long-term care facilities, where residents are at higher risk because of advanced age and other chronic conditions. It is not spread person-to-person.

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Purpose of surveillance:

  • To identify common source outbreaks and nosocomial cases
  • To identify and eliminate preventable sources of transmission

Legionella case data

Local epidemiology:

Twelve cases of legionellosis were reported in 2015, compared to a five-year average of 12 cases per year. All of the cases were typed as L. pneumophila. The cases ranged from 42 to 81 years of age; all of the cases were hospitalized, and three deaths were reported. Four of the cases reported domestic travel with hotel stays during their exposure period. Two cases routinely used medical equipment that aerosolizes water and could be a risk for infection. One case reported recent home installation of a hot tub. Two cases were homeless, one of whom was not able to be interviewed. At least 25% of the cases were current smokers.

Most cases of legionellosis are sporadic (not associated with an outbreak), with no source identified.

Each year in Washington state between 15 and 45 cases of legionellosis are reported.