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Prevalence and significance of Legionella pneumophila contamination of residential hot-tap water systems.

A culture survey of hot-tap water systems in 95 apartments and houses in one area of Chicago showed that 30 (32%) were contaminated by Legionella pneumophila, ranging in concentration from 1 to 10(4) organisms/liter. Culture-positive and -negative systems differed significantly only in hot-tap water temperature (P less than .005), which was less than 60 C for all positive specimens. A questionnaire and serosurvey of a subject at each residence showed no cases of pneumonia while he or she lived in the residence and no association of high titers of antibodies to L. pneumophila with positive tap water cultures. Virulence of selected tap water isolates of L. pneumophila in embryonated eggs was similar to that of clinical isolates. In the area studied, residential hot-tap water systems maintained at less than 60 C are frequently contaminated by L. pneumophila, but systems with low levels of contamination (less than or equal to 10(4) organisms/liter) appear not to be an important source of infection of healthy individuals.