LEGIONELLA - World Health Organization

Box . An outbreak of leg onellos s at the Melbourne Aquar um, Apr l 000
Between 11 and 27 April 2000, the Melbourne Aquarium, Australia, was linked to 125
confirmed cases of legionellosis. The cases were caused by Legionella pneumophila
serogroup 1. Two case–control studies confirmed the source of the outbreak and investigated
risk factors for infection. The aquarium cooling towers were found to be poorly disinfected
and contaminated with L. pneumophila, and visiting the aquarium was significantly associated
with disease. The case–control studies indicated that current smoking was a dose-dependent
risk; in contrast, chronic illness and duration of exposure at the site were not significant
risks (Greig et al., 2004).
The number of cooling towers in existence globally is not known, but about 30 000 are registered
in the United Kingdom alone. To date, large-scale natural updraft towers, such as those commonly
associated with electricity generation, have not been implicated in outbreaks of legionellosis,
although the potential for their involvement cannot be dismissed.
The air exhausting from cooling towers and evaporative condensers carries two types of water:
• water vapour that has evaporated within the device, which may recondense and appear as
steam
• water droplets that have been generated within the device and carried in the airflow; if
carried over without initial evaporation, these droplets are termed “drift”.
The water droplets in drift will contain any dissolved salts or suspended particles, including
organisms that were in the original water. It is these droplets that can create an infectious aerosol
when the water evaporates in the open air outside the tower, unless appropriate controls are
in place.
5.2 Water safety plan overview
A WSP needs to be comprehensive; however, an overview of such a plan is shown in Table 5.1,
as an example of the type of information a plan might contain. As explained in Chapter 3, a
WSP is part of a framework for safe water quality that also includes health-based targets and
surveillance.
Most cooling towers and evaporative condensers are likely to become contaminated with
Legionella at some point in their serviceable life (Koide et al., 1993; Bentham, 2000). It is
unrealistic to try to prevent entry of the organism into the cooling tower or to create an
environment that entirely precludes its growth and multiplication, although this is desirable.
In the case of the sample WSP shown in Table 5.1, a health-based operational target might
be to control microbial growth through modification of the environment and the use of
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS