Epidemiology and Etiology

Legionellae live in unsalty water and are widely distributed in nature. They thrive particularly well at or slightly above human body temperature, and thus are commonly found on the hot water side of potable water systems and in the recirculating water in cooling towers and other heat exchange devices.

The various ways in which Legionellae can go from their watery environment to infect humans are not all worked out, but one method is clear. Aerosols created by some mechanical disturbance of contaminated water, such as in the operation of a cooling tower, can on occasion infect people downwind. It is likely that, after the aerosol is generated, the droplets of water evaporate, leaving the bacteria airborne. Once airborne, they can travel a considerable distance, be inhaled, and then be deposited in the lungs. Potable water systems also can act as the source of legionellosis outbreaks, but it is not known whether this occurs through aerosols (as might be generated by sinks, showers, or toilets), by colonization of the throat and then aspiration into the lungs, by direct inoculation (as into the eye or a wound), or by ingestion.

Many outbreaks have been recognized, but most cases occur individually (or sporadically). The risk of Legionella pneumonia (including Legionnaires' disease) increases with age and is two to four times higher in men than in women. No racial predisposition has been seen. Cigarette smokers are more susceptible, as are people whose cellular immune system is compromised — for example, by medication

(such as corticosteroids) or underlying illness. Nosocomial (hospital-acquired) legionellosis is an important problem, probably because particularly susceptible people are gathered in a building with water systems that Legionella can contaminate.

Few children have been shown to have legionellosis, but one prospective study showed that one-half developed serum antibodies to L. pneumophila before 4 years of age, indicating that inapparent infection may be common, at least at that age. In contrast, studies of adults during Legionnaires' disease outbreaks have generally shown that fewer than one-half of the Legionella infections are inapparent.

Legionellosis can occur throughout the year but is most common from June through October in the Northern Hemisphere. The summer season predominates even in outbreaks unrelated to air-conditioning systems, but whether warmer weather causes the bacteria to flourish or, rather, humans to increase contact with their environment is not known. The fact that most bacterial and viral pneumonias are most common in the winter makes the seasonality of legionellosis one clue in diagnosis. Legionellosis does not seem to spread from one person to another.

Travel and certain occupations put people at increased risk of legionellosis. Several outbreaks have occurred in tourist hotels, and a disproportionate number of sporadic cases are in people who recently have traveled overnight. Disease caused by Legionella and serologic evidence of previous Legionella infection are more common in power plant workers who have close exposure to cooling towers. Other outbreaks have occurred in people who used compressed air to clean river debris out of a steam turbine condenser, and in workers exposed to aerosols of contaminated grinding fluid in an engine assembly plant.

Legionellosis outbreaks can be stopped by turning off the machinery that is making infectious aerosols, disinfecting that machinery, and heating or rechlor-inating contaminated drinking water. Several chemical additives that are used in the routine maintenance of cooling towers and evaporative condensers can limit the growth of Legionella if used in combination with regular cleaning. Certain rubber materials used in plumbing washers seem to provide nutrients for Legionella; using different components may help prevent legionellosis associated with drinking water. Maintaining a temperature of 60°C on the hot water side of institutional plumbing systems has stopped some legionellosis outbreaks by lowering the concentration of Legionella sharply.

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