LEGIONELLA - World Health Organization

In many cases, direct methods of molecular detection have shown Legionella to be present in
drinking-water leaving treatment facilities and in distribution systems. Detection of low numbers
of culturable Legionella is difficult; therefore, information about the presence and behaviour
of the organism in distribution systems is scarce. Nevertheless, Legionella found in plumbing
systems has frequently been shown to originate from drinking-water. Thus, it appears that
Legionella may be present in distribution systems (at least in temperate climate zones), but at levels
below the detection limit of culture techniques. There is no evidence that such low levels of
contamination pose a direct health threat to consumers.
In assessing piped water systems, it is important to investigate whether the combination of factors
present in the system is likely to lead to the proliferation of legionellae. Such factors are listed
in Box 4.3 and discussed below. These factors are strongly interrelated, and it is not currently
possible to rank them. The risk factors discussed below include not only those for growth of
Legionella, but also those — such as aerosol production — that are likely to increase the risk of
infection.
Box . R sk factors for growth of or exposure to Legionella n p ped water systems
Factors that can lead to proliferation of, or exposure to, Legionella in piped water systems include:
• poor water quality and treatment failures
• distribution system problems such as stagnation and low flow rate
• construction materials that contribute to microbial growth and biofilm formation
• inefficient or ineffective disinfection
• water temperature of 25–50 °C
• presence of biofilms
• aerosol production.
Water quality and treatment — risk factors
As discussed in Chapter 2 (Section 2.3), L. pneumophila growth can only be sustained in piped
water if nutrients are available, either from the source water or (directly or indirectly) from
other microorganisms (Anand et al., 1983; Stout, Yu & Best, 1985, 1992; Barbaree et al., 1986;
Vickers et al., 1987; Lück et al., 1991). Thus, poor quality water or water that has not been
effectively treated may allow legionellae to proliferate within the system.
Distribution system — risk factors
Proliferation of legionellae is promoted by stagnation, which occurs, for example, in the deadends
of distribution system pipework, and in storage tanks and systems that are not frequently used.
Another risk factor associated with potable water distribution systems is the potential dissemination
of legionellae through aerosols. In the home, inhalation of legionellae can occur from the aerosols
that are generated by showers and toilet flushing, and from devices such as nebulizers if they
are cleaned or filled with tap water.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS