Dispersion of Legionella bacteria in atmosphere: A practical source location estimation method

Abstract

Legionnaires' disease, a form of pneumonia which can be fatal, is transmitted via the inhalation of water droplets containing Legionella bacteria. These droplets can be dispersed in the atmosphere several kilometers from their source. The most common such sources are contaminated water within cooling towers and other air-conditioning systems but other sources such as ornamental fountains and spa pools have also caused outbreaks of the disease in the past. There is an obvious need to locate and eliminate any such sources as quickly as possible. Here a maximum likelihood model estimating the source of an outbreak from case location data has been developed and implemented. Unlike previous models, the average dose exposure sub-model is formulated using a atmospheric dispersion model. How the uncertainty in inferred parameters can be estimated is discussed. The model is applied to the 2012 Edinburgh Legionnaires' disease outbreak.

Fig 1. Maximum log-likelihood heat map.

Heat map of maximum log-likelihood within search district EH11 with inferred source location postcode estimate outlined in orange. Postcodes within the approximate 95% confidence region calculated as described under Parameter Uncertainty are outlined in black.

Fig 2. Super-parameter α heat map.

Heat map of super-parameter α at maximum log-likelihood within search district EH11 with inferred source location postcode estimate outlined in orange. Postcodes within the approximate 95% confidence region calculated as described under Parameter Uncertainty are outlined in black.

Fig 3. Stability class map.

Map with postcodes coloured to represent the stability class at maximum log-likelihood within search district EH11 with inferred source location postcode estimate outlined in orange. Postcodes within the approximate 95% confidence region calculated as described under Parameter Uncertainty are outlined in black.