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Legionella pneumophila was first recognized
as the etiology of an outbreak of pneumonia among attendees at a
1976 American Legion Convention.1,2 Pneumonia caused
by Legionella, known as Legionnaires disease or legionellosis,
is a common cause of community-acquired pneumonia in adults and
is a cause of health care-associated pneumonia and pneumonia in
immunocompromised adults and children. L pneumophila may
also cause Pontiac fever, an uncommon, short incubation, influenzalike
illness that primarily affects adults with symptoms of fever, malaise, myalgia,
chills, headache, and pleuritic pain.3-5
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Although at least 50 Legionella species have
been identified, only one half have been isolated from humans.6-8L
pneumophila, comprised of 14 serotypes, is the most virulent species,
accounting for the majority of human infections. L pneumophila serogroup
1 causes 50% to 90% of human infections, whereas L pneumophila serogroup
6, other L pneumophila serogroups, and Legionella
longbeachae, Legionella bozemanii, Legionella dumoffi, and Legionella
micdadei cause most of the remainder of human infections.8-10 These
bacilli are nutritionally fastidious, aerobic rods that, after recovery
on artificial media, stain as gram negative. L micdadei is
unique among Legionella species in that it can
be visualized in specimens using a modified acid-fast stain.
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Infection with Legionella results from inhalation
of contaminated aerosols from environmental or aquatic sources.11-15 Legionella are not
transmitted from person to person. Legionella spp
are ubiquitous in natural freshwater habitats such as lakes, rivers,
and groundwater. From these sources, they gain entry into water
systems of buildings, including hospitals. These bacteria thrive
at temperatures between 30°C (86°F) and 54°C (129.2°F) but are killed
at temperatures above 60°C (140°F). Legionella spp
are facultative intracellular pathogens and may replicate in nature
within various protozoa, including amoeba. Community outbreaks,
almost all due to L pneumophila,8 have
occurred and have been linked to aerosol-generating machinery such
as cooling towers and evaporative condensers; however, showers,
respiratory therapy devices, air conditioners, ultrasonic mist machines
for vegetables,13 whirlpool spas,15 and
humidifiers have also been associated with outbreaks. Health care-associated
infections and outbreaks also occur and are most commonly traced
to the water supply, particularly the hot water supply. Sporadic
cases have often been linked to home water heaters, particularly
electric water heaters.16
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Most cases of Legionnaires disease occur in susceptible elderly
or middle-age adults. Legionella are responsible
for 1% to 15% of community-acquired pneumonias
in adults that require hospitalization. The incubation period has
been estimated to range from 2 to 10 days, with an average of 7
days.1 The main risk factors in adults are chronic
lung disease; immunosuppression,17,18 especially
associated with corticosteroid treatment or organ transplantation;
and cigarette smoking.1,13
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Legionellosis is uncommon in the pediatric age group. Only 40
(1.4%) of 2766 cases of legionellosis reported to the Centers
for Disease Control and Prevention in 2006 were in persons 24 years
of ...