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clearance (eg, smoking tobacco) increases risk of infection. Virulence varies
between strains of L pneumophila. For example, some strains can adhere to
the respiratory epithelial cells via pili, whereas strains with a mutated gene
that encodes for the pili show reduced adherence in vitro.
Organisms that reach the alveoli undergo phagocytosis by the alveolar
macrophages but are not actively killed. Macrophages may actually support
the growth of Legionella organisms. The bacteria multiply intracellularly
until the cell ruptures. Liberated bacteria then infect other macrophages.
Additional virulence factors include genes that potentiate infection of
macrophages and inhibit phagosomal fusion, allowing intracellular growth.
Legionella organisms cause an acute fibrinopurulent pneumonia with
alveolitis and bronchiolitis. In addition to the lungs, Legionella organisms
may infect the lymphonodes, brain, kidney, liver, spleen, bone marrow, and
myocardium.
The mortality rate in patients with Legionnaires disease is 5-80%,
depending on certain risk factors. The factors associated with high
mortality rates include the following: age (especially those younger than 1
y and elderly patients),predisposing underlying conditions, such as chronic
lung disease, immunodeficiency, malignancies, end-stage renal disease,
and diabetes mellitus Nosocomial acquisition Delayed initiation of specific
antimicrobial therapy
After
an
incubation
period
of
2-10
days,
patients
typically develop the following nonspecific symptoms: fever,
weakness,fatigue,malaise,myalgia,chills. Fever is typically present (98%).
Temperatures exceeding 40°C occur in 20-60% of patients. Hypotension has
beenreportedin17%ofpatientswithcommunity-acquired pneumonia(CAP)
