66
Legionnaires disease (LD) was recognized in 1976 after an outbreak
of pneumonia at an American Legion convention in Philadelphia. Soon
after, the etiologic agent was identified as a gram-negative bacillus named
Legionella pneumophila. Although several other species of the genus
Legionellawere subsequently identified, Lpneumophila is themost frequent
cause of human legionellosis and a relatively common cause of community-
acquired and nosocomial pneumonia in adults.
Legionellosis refers to 2 distinct clinical syndromes: Legionnaires
disease, which most often manifests as severe pneumonia accompanied by
multisystemic disease, and Pontiac fever, which is an acute, febrile, self-
limited, viral-like illness.
The organism has been isolated in natural aquatic habitats (freshwater
streams and lakes, water reservoirs) and artificial sources (cooling towers,
potable water distribution systems).
Transmission occurs by means of aerosolization or aspiration of water
contaminatedwithLegionellaorganisms.The following systems are linked to
transmissionofLegionellaorganisms:coolingtowers,humidifiers,respiratory
therapy equipment,whirlpool spas[.evaporative condensers,potable water
distribution systems (eg, showers)
Most nosocomial infections and hospital outbreaks have been linked
to contaminated hot water supply. However, contamination of cold-water
supply has also been reported. Nosocomial Legionnaires disease associated
with water birth is reported in a few neonates, but the risk appears to be
low. Person-to-person transmission has not been demonstrated.
Mucociliary action clears Legionella organisms are cleared from
the upper respiratory tract. Any process that compromises mucociliary
