Legionnaires’ Disease Information

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Legionellosis is an infectious disease caused by bacteria belonging to the genus Legionella.

An estimated 8,000 to 18,000 people get legionellosis in the United States each year. Some people can be infected with the Legionella bacterium and have only mild symptoms or no illness at all.

Outbreaks of Legionnaires’ disease receive significant media attention. However, this disease usually occurs as a single, isolated case not associated with any recognized outbreak. When outbreaks do occur, they are usually recognized in the summer and early autumn, though cases may occur at any time of year. The fatality rate of Legionnaires’ disease has ranged from 5 to 30% during various outbreaks.

Legionellosis infection normally occurs after inhaling an aerosol (suspension of fine particles in air) containing Legionella bacteria. Such particles could originate from any infected water source. When mechanical action breaks the surface of the water, small water droplets are formed, which evaporate very quickly. If these droplets contain bacteria, the bacteria cells remain suspended in the air, invisible to the naked eye but small enough to be inhaled into the lungs. This often occurs in poorly ventilated areas such as prisons where a condensating air conditioner can spread it throughout the entire room, infecting anyone not immune to the strand of bacteria. Potential sources of such contaminated water include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, evaporative coolers, hot water systems, showers, whirlpool spas, architectural fountains, room-air humidifiers, ice making machines, misting equipment, and similar disseminators that draw upon a public water supply. The disease may also be spread in a hot tub if the filtering system is defective. Freshwater ponds, creeks, and ornamental fountains are also potential sources of Legionella. The disease is particularly associated with hotels, cruise ships and hospitals with old, poorly maintained pipework and cooling systems.

Legionella will grow in water at temperatures from 20 °C to 50 °C (68 °F to 122 °F). However, the bacteria reproduce at the greatest rate in stagnant water at temperatures of 35 °C to 46 °C (95 °F to 115 °F).

Legionella longbeachae, an organism in the Legionella family, is found in soils and compost. Thus, the dust from purchased bags of soil, compost, or potting mix is also a potential source of Legionella.

Credits: Information contained in this article was derived in whole or in part from the following sources:

Wikipedia, Legionellosis Entry