LEGIONELLA - World Health Organization

in warmer regions appears to be lacking, but up to 10 8 cells/litre have been found in surface
waters in tropical regions, and Legionella has been cultured in high numbers from warm water
sources (Ortiz-Roque & Hazen, 1987).
Naturally occurring L. pneumophila can survive and multiply in water at temperatures of
25–45 °C, with an optimal range of 32–42 °C and the greatest increase in viable counts at
37–42 °C (Yee & Wadowsky, 1982). The multiplication rate decreases at temperatures below
37 °C, with no observable growth below 20 °C (HSE, 2004). In certain geographical regions,
temperatures may routinely be above 20 ºC and, in some cases, may reach optimal temperatures
for legionellae growth.
4.4 Monitoring
This section should be read in conjunction with Section 3.3.2 of Chapter 3. The steps involved
in monitoring, some of which are discussed below, are to:
• identify control measures (Section 4.4.1)
• monitor control measures (Section 4.4.2)
• validate effectiveness of the WSP.
4.4.1 Identify control measures
This section should be read in conjunction with Table 3.2 of Chapter 3, which provides information
on the advantages and disadvantages of alternative methods of controlling Legionella in piped
water systems.
The focus of attention in managing legionellae risks should be on preventing both proliferation
and exposure, in line with the multiple-barrier approach that forms part of a WSP. Systems
will need to be assessed individually, and any treatment will need to be validated by testing
for its effectiveness against legionellae and for the presence of legionellae in operating systems.
For example, in order to choose appropriate control measures, it will be necessary to know the
“normal” operating temperature of the water supply.
Water quality and treatment — control measures
Water from the supplier should meet the appropriate drinking-water standards or guidelines
of the jurisdiction (e.g. WHO, 2004), and should not contain high levels of nutrients.
Measures for reducing numbers of Legionella are not routinely applied in drinking-water
distribution systems because (as explained in Section 4.3) levels of Legionella are usually below
the detection limit of culture techniques. However, in most countries, surface water treatment
includes a series of barriers to eliminate or inactivate pathogenic microorganisms of faecal origin.
These physical techniques, such as coagulation–sedimentation, filtration and disinfection will
also reduce the number of legionellae (Kuchta et al., 1983).
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS