LEGIONELLA - World Health Organization

1.5.4 Host defence
The host defence against Legionella relies principally on cell-mediated immune mechanisms.
At least two proteins produced by L. pneumophila can induce protective cell-mediated immunity
without being virulence factors — the major secretory protein (MSP, 39 kDa) and the major
outer membrane protein (ompS, 28 kDa) (Blander & Horwitz, 1991). Circulating antibodies
are produced during L. pneumophila infections in humans, but do not seem to be protective.
1.5.5 Transmission
An infected source (e.g. a fountain) can disseminate sprays or droplets of water containing
legionellae, commonly referred to as aerosols. When this occurs, most or all of the water in
the droplet evaporates quickly, leaving airborne particulate matter that is small enough to be
inhaled. Particles of less than 5 µm in diameter can be deeply inhaled, and enter the respiratory
airways to cause legionellosis (Fitzgeorge et al., 1983).
Legionella infections have frequently been associated with sources at distances of up to 3.2 kilometres
(Addiss et al., 1989); recent evidence suggests that infection may be possible at even longer
distances (Tran Minh et al., 2004). There is evidence that virulence is an important factor in
the survival of Legionella in aerosols, with the most virulent strains surviving longer than
their less virulent counterparts (Dennis & Lee, 1988).
There is no evidence of person-to-person transmission of either Legionnaires’ disease or Pontiac
fever (WHO, 2004).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS