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Legionnaire's disease is predominantly caused by the inhalation, by a susceptible person,
of aerosolised Legionella bacteria. It is important, therefore, to quantitatively
understand the extent to which this organism can survive in an aerosolised suspension, in order
to better inform airborne dispersion modelling and the development of outbreak investigation
tools. The better the aerosol survival of an organism with time and distance from a source, the
higher the chance that a person might receive a sufficient dose to cause infection at a given
distance from that source. For a number of airborne pathogens, including Legionella, it
is relatively well understood that the viability, and thus the ability to cause infection, of
aerosolised organisms decreases over time [1; 2; 3], and that this rate varies depending on
environmental factors (particularly UV irradiation and humidity).
There is a considerable amount known about the growth and survival of Legionella
bacteria in bulk aqueous environments, that is relevant to understanding their ability to
persist in water systems, and therefore their ability to pose a risk to public health [4]. For
example, in bulk aqueous environments, temperature is known to play an important role in
Legionella growth: the bacterium multiplies at temperatures in the range 25 - 42ºC,
with an optimal temperature for growth of 35ºC [5; 6]. Katz & Hammel [5] also found
that for lower pH values in the range 4 to 7, Legionella could survive well with only a
2 log drop in viable count over a period of a month in tap water. However, higher pH values (pH
8) significantly reduced Legionella survival, resulting in a 6 log drop in viable count
over the same time period. However, it should be noted that cooling towers naturally run at
high pH levels and still allow viable legionellae. Legionella bacteria are also
killed when exposed to higher temperatures, in the range 55 to
65ºC. The extent to which such
information regarding the survival of Legionella in bulk aqueous environments can be
extrapolated to aerosol survival is, however, uncertain and debatable.
In order to survive and remain infectious, bacteria in aerosols have to simultaneously
overcome several adverse environmental conditions. These are predominantly the effects of
desiccation, increasing concentrations of solutes originating from the bulk liquid from which
they were aerosolised, chemicals that might be absorbed from ambient air, and exposure to
harmful irradiation (predominantly UV), as well as the impact of biocides.
As for many pathogens that infect humans and animals by the respiratory route, there is
much less data on the survival of Legionella in aerosols (as opposed to other
environments), partly because it is much less straightforward to conduct relevant experiments
with relatively fragile aerosolised organisms, and largely because specialist equipment is
required in order to maintain micro-organisms in an airborne state and then representatively
sample them.
Using specialist techniques, however, it has been possible to demonstrate that for bacteria
aerosolised from less extreme environments (with respect to pH, temperature etc. as in the
experiments in the bulk aqueous phase considered above,) and within more naturally-occurring
environmental ranges, humidity is more important than temperature when affecting the survival
of airborne bacteria.
Humidity
Berendt [1] was the first to investigate the effects of humidity on Legionella
bacteria and found that survival was improved when the relative humidity reached 60%. This
finding was later confirmed by Hambleton et al. [7] and Dennis & Lee [2], with an
optimum relative humidity for survival of 65% and 60%, respectively. Dennis & Lee [2],
however, also found a very high survival rate at 90% relative humidity, in apparent
contradiction to the results of [7]. They noted however, that, for the spore being used to
provide a baseline for the calibration of Legionella survival, death occurs at 90%
relative humidity and state that this "…may have influenced the results giving
unrealistically good survival at this RH.". These experiments were undertaken in the dark and
so do not include the potentially deleterious effects of ambient light and other open air
factors on the bacteria.
Using sporadic case data, Hicks [8] found a positive association with increased rainfall
and the number of monthly cases. A positive relationship between increasing humidity and the
number of sporadic cases of Legionnaires' disease, that was greater than that for temperature,
was recently identified by Ricketts et al. [9] and Karagiannis [10], which confirmied a
similar result found by Fisman [11].
UV radiation
Another factor that can hamper Legionella survival is UV radiation [12; 13; 14]. This was first investigated
by Antopol & Ellner [15] who found an inverse linear relationship between the strength of
UV radiation and
Legionella survival for bacteria held in a water suspension, with only 50% of the
bacteria surviving after a UV
dose of 380 μW-s/cm2. Following on from this, Knudson [16] used a 240
μW/cm2 UV
source and measured survival as a function of time for Legionella on agar plates. Even
taking into account photoreactivation, the Knudson survival curve shows that <10% of the
bacteria remain viable after a 10 second exposure. To put this into context, from 2002 - 2008
the six year average solar UV
strength (as measured in Manchester, UK) is 118.95 μW/cm2 (average range 6.00 -
448.00 μW/cm2), implying that UV radiation could have a significant impact on the atmospheric
survival of Legionella following its airborne release from aerosol-generating sources.
Kowalski et al. [17] developed an exponential model for bacterial decay in
the atmosphere due to UV
dose, exposure time and a bacteria specific decay constant. Kowalski et al. [17] provide
a value of this constant for Legionella of 0.0025 (derived from Antopol & Ellner
[15], in water) and 0.0020 (derived from Gilpin [18], in water). Experimental data has also
been used by Lightheart & Mohr [19] to develop a "composite viral" survival term dependent
on relative humidity, temperature, solar radiation and time.
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humidity The Journal of Infectious Diseases 141, pp.689 http
- DENNIS P. J. D. & LEE J. V. (1988) Differences in aerosol survival between pathogenic
and non-pathogenic strains of Legionella pneumophila serogroup 1 Journal of Applied
Bacteriology 65, pp.135 - 141 http
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temperature and survival of airborne bacteria Applied Microbiology 19, pp.245 - 249
http pdf
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Applied and Environmental microbiology 53, pp.447 - 453 http
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(1983) Survival of virulent Legionella pneumophila in aerosols Journal of Hygiene,
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B. S., BLYTHE D., KHALIFAH A. P., FEIKIN D. R. & WHITNEY C. G. (2007) Increased rainfall is
associated with increased risk for legionellosis Epidemiology and Infection 135, pp.811
- 817 http
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patterns and Legionnaires' disease: a meteorological study Epidemiology and Infection
19, pp.1 -10 http
- KARAGIANNIS I., BRANDSEMA P. & VAN DER SANDE M. (2009) Warm, wet weather associated
with increased Legionnaires' disease incidence in The Netherlands Epidemiology and
Infection 137, pp.181 - 187 http
- FISMAN D. N., LIM S., WELLENIUS G. A., JOHNSON C., BRITZ P., GASKINS M., MAHER J.,
MITTLEMAN A., SPAIN C. V., HAAS C. N. & NEWBERN C. (2005) It's not the heat, It's the
humidity: wet weather increases legionellosis risk in the Greater Philadelphia Metropolitan
area The Journal of Infectious Disease 192, pp.2066 - 2073 http pdf
- NEWSOME D.H. (2001) Legionella in the environment (the cause of Legionnaires' disease). A
review of current knowledge Foundation for water research FR/R0004 http pdf
- LIGHTHART B. (1997) The ecology of bacteria in the alfresco atmosphere Federation of
European Microbiological Societies 23, pp.263 - 274 http pdf
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populations of culturable outdoor atmospheric bacteria Atmospheric Enviroment 31, pp.897
- 900 http
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ultraviolet radiation Applied and Environmental Microbiology 38, pp.347 - 348 http pdf
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Legionella species Applied and Environmental Microbiology 49(4), pp. 975 - 980
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- KOWALSKI W. J., BAHNFLETH W. P., WITHAM D. L., SEVERIN B. F. & WITTAM T. S. (2000)
Mathematical modelling of ultraviolet germicidal irradiation for air disinfection
Quantitative Microbiology 2, pp.249 - 270 http
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Thornsberry (American Society for Microbiology, Washington) ISBN:0914826581,
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