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Legionella
bacteria were first identified during investigation of an
explosive, common-source outbreak of pneumonia among attendees of
an American Legion convention in Philadelphia in 1976 (1, 2). The highly
publicized original reports of the epidemic identified 182 cases of disease,
29 of which were fatal. Legionellae are now associated with two forms of
respiratory illness, collectively referred to as legionellosis (3) Legionnaires’
disease is the pneumonic and more severe form of illness. The other form
of respiratory illness is named Pontiac fever after the first documented
outbreak, which occurred at a health department in Pontiac, Michigan (4).
The two different presentations of legionellosis have not yet been explained
but theories include differences in innate bacterial virulence, restricted host
range, decreased viability, and/or host susceptibility (4-6). A few limited
studies suggest Pontiac fever may be associated with exposure to
Legionella
endotoxin, rather than intracellular replication (7, 8).
Ecology of Legionellae
Legionella
bacteria are facultative intracellular parasites of free living
protozoa and are ubiquitous in the natural, freshwater environment.
Legionellae have been found in ponds, rivers, soil, and even as far afield as
the Mir space station and Antarctic lakes (4, 9, 10). They have an extremely
broad host range, demonstrated to multiply in 14 species of amoebae, 2
species of ciliated protozoa, and one species of slime mold while
Legionella
growth in the absence of protozoa has only been documented on laboratory
media (11, 12). Biofilms in building water systems are reservoirs for
the transmission of
Legionella
to humans (13). The complex association
of heterogeneous bacteria, protozoa, and fungi in the biofilm provides
nutrients and protection from adverse environmental conditions where
