Legionnaires' disease outbreak investigation toolbox

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Environmental effects on Legionella

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.


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.

Reference List

  1. BERENDT R. F. (1980) Survival of Legionella pneumophila in aerosols: effect of relative humidity The Journal of Infectious Diseases 141, pp.689 http
  2. 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
  3. EHRLICH R., MILLER S. & WALKER R. L. (1970) Relationship between atmospheric temperature and survival of airborne bacteria Applied Microbiology 19, pp.245 - 249 http pdf
  4. MURACA P., STOUT J. E. & YU V. L. (1987) Comparative assessment of chlorine, heat, ozone, and UV light for killing Legionella pneumophila within a model plumbing system Applied and Environmental microbiology 53, pp.447 - 453 http
  5. KATZ S. M. & HAMMEL J. M. (1987) The effect of drying, heat and pH on the survival of Legionella pneumophila Annals of Clinical and Laboratory Science 17, pp.150 - 156 http
  6. MAUCHLINE W. S., JAMES B. W., FITZGEORGE R. B., DENNIS P. J. & KEEVIL C. W. (1994) Growth temperature reversibly modulates the virulence of Legionella pneumophila Infection and Immunity 62, pp.2995 - 2997 http
  7. 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
  8. HICKS L. A., ROSE JR., C. E., FIELDS B. S., DREES M. L., ENGEL J. P., JENKINS P. R., ROUSE 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
  9. RICKETTS K. D., CHARLETT A., GELB D., LANE C., LEE J. V. & JOSEPH C. A. (2008) Weather patterns and Legionnaires' disease: a meteorological study Epidemiology and Infection 19, pp.1 -10 http
  10. 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
  11. 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
  12. 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
  13. LIGHTHART B. (1997) The ecology of bacteria in the alfresco atmosphere Federation of European Microbiological Societies 23, pp.263 - 274 http pdf
  14. TONG Y. & LIGHTHART B. (1996) Solar radiation has a lethal effect on natural populations of culturable outdoor atmospheric bacteria Atmospheric Enviroment 31, pp.897 - 900 http
  15. ANTOPOL S. C. & ELLNER P. D. (1979) Susceptibility of Legionella pneumophila to ultraviolet radiation Applied and Environmental Microbiology 38, pp.347 - 348 http pdf
  16. KNUDSON G. B. (1985) Photoreactivation of UV-irradiated Legionella pneumophila and other Legionella species Applied and Environmental Microbiology 49(4), pp. 975 - 980 http pdf
  17. 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
  18. GILPIN R.W. (1984) Legionella: Proceedings of the 2nd International Symposium Thornsberry (American Society for Microbiology, Washington) ISBN:0914826581, 9780914826583
  19. LIGHTHART B. & MOHR A. J. (1987) Estimating downwind concentrations of viable airborne microorganisms in dynamic atmospheric conditions Applied and Environmental Microbiology 53, 1580 - 1583 http pdf