Infectiousness of aerosolised organisms
When associated with outbreaks, the locations of infection for Legionnaires' disease cases
have, on occasion, been epidemiologically-linked to sources up to several kilometres from the
point of release of aerosolised organisms [1-5]. As a plume becomes dispersed over a wider
geographic area beyond the point of release, it should naturally lead to a reduction in the
concentration of Legionella organisms in the air with distance from source, even if the cloud
is advected strongly by wind. This reduction in aerosol concentration, combined with the
effects of cell death caused by UV irradiation [6; 7] and low humidity in the atmosphere [8],
would suggest that the doses of inhaled organisms required to induce symptoms in humans
can be low, particularly considering the long ranges from sources at which infection can be
contracted. However, given the likely geographic extent of the exposed population, not only
may the infectious dose be low but also the probability of infection (i.e. the bacteria are
efficiently cleared from the host following inhalation or that the lungs of certain individuals
are not conducive to bacterial replication). It is notable that no evidence has been found to
indicate that cases infected nearer a source, and therefore more likely to have received a
higher dose, had shorter incubation periods than those at some distance from it. This is not
necessarily evidence against such an effect, just that it has not been not been observed to date
in studies of legionellosis outbreaks.
Exposure to higher atmospheric concentrations of Legionella might reasonably be expected
to increase the probability of infection [3; 4]. However, once infected, the disease appears to
progress in individuals in a similar time-dependent way, regardless of the doses that have
likely been more routinely inhaled in outbreaks. This supposition has been corroborated in
guinea pig studies.
Reference List
1. ADDISS D.G., DAVIS J.P., LAVENTURE M., WAND P.J., HUTCHINSON M.A.,
& MCKINNEY R.M. (1989) Community-acquired Legionnaires' disease associated
with a cooling tower: evidence for longer-distance transport of Legionella
pneumophila American Journal of Epidemiology 130(3), pp.557 - 568 http
2. BHOPAL R. S., FALLON R. J., BUIST E. C., BLACK R. J. & URQUHART J. D.
(1991) Proximity of the home to a cooling tower and risk of non-outbreak
Legionnaires' disease British Medical Journal 302, pp.378 - 382 http pdf
3. BROWN C. M., NUORTI P. J., BREIMAN R. F., HATHCOCK A. L., FIELDS B. S.,
LIPMAN H. B., LLEWELLYN G. C., HOFMANN J. & CETRON M. (1999) A
community outbreak of Legionnaires' disease linked to hospital cooling towers: An
epidemiological method to calculate dose of exposure International Journal of
Epidemiology 28, pp.353 - 359 http pdf
4. NGUYEN T. M. N., ILEF D., JARRAUD S., ROUIL L., CAMPESE C., CHE D.,
HAEGHEBAERT S., GANIAYRE F., MARCEL F., ETIENNE J. & DESENCLOS
J.C. (2006) A community-wide outbreak of Legionnaires' disease linked to industrial
cooling towers: How far can contaminated aerosols spread? Journal of Infectious
Diseases 193, pp.102-111 http
5. NYGARD K., WERNER-JOHANSEN O., RONSEN S., CAUGANT D. A.,
SIMONSEN O., KANESTROM A., ASK E., RINGSTAD J., ODEGARD R.,
JENSEN T., KROGH T., HOIBY E. A., RAGNHILDSTVEIT E., AABERGE I. S. &
AAVITSLAND P. (2008) An outbreak of Legionnaires' disease caused by long-
distance spread from an industrial air scrubber in Sarpsborg, Norway Clinical
Infectious Diseases 46(1), pp.61-69 http
6. ANTOPOL S. C. & ELLNER P. D. (1979) Susceptibility of Legionella pneumophila
to ultraviolet radiation Applied and Environmental Microbiology 38, pp.347 - 348 http
pdf
7. GILPIN R. W., DILLON S. B., KEYSER P., ANDROKITES A., BERUBE M.,
CARPENDALE N., SKORINA J., HURLEY J. & KAPLAN A. M. (1985)
Disinfection of circulating water systems by ultraviolet light and halogenation Water
research 19, 839 - 848 http
8. HAMBLETON P., BROSTER M. G., DENNIS P. J., HENSTRIDGE R.,
FITZGEORGE R. & CONLAN J. W. (1983) Survival of virulent Legionella
pneumophila in aerosols Journal of Hygiene, Cambridge 90, pp.451 - 460 http