LEGIONELLA - World Health Organization

Chapter 3 discusses risk management of Legionella. The public health risk posed by legionellosis
can be addressed by preventive measures — although the source of infection cannot be
completely eradicated, risks can be substantially reduced. The preferred approach to health
risk assessment in evaluating specific risks of exposure to Legionella from water systems is to
develop a water safety plan (WSP), which provides a detailed and systematic assessment and
prioritization of hazards, and operational monitoring of barriers and control measures.
Chapter 3 outlines the process involved in developing a WSP to minimise proliferation of
Legionella and exposure to the organism.
Chapters 4–8 are structured around the concept of a WSP. They are not intended to comprehensively
address the WSP approach outlined in Chapter 3; rather, these chapters summarise the general
principles and factors one would need to focus on in developing a WSP for the control of
Legionella in the different environments and operating scenarios covered.
• Potable water distribution systems — Chapter 4 covers factors affecting microbial growth
in potable water systems and in-building distribution systems. Distributed water is likely
to contain some microorganisms, including legionellae. It is therefore reasonable to assume
that all systems that use water could be seeded with microorganisms during construction,
repair and maintenance, even if the water is treated. Risk factors that can promote the
proliferation of legionellae include temperature, water quality, design, material used in
construction and the presence of biofilms. The focus of attention in managing legionellae
risks should be on preventing both proliferation and exposure. Therefore, Chapter 4 suggests
control measures ranging from source water quality and treatment of source water to
design of systems to prevent stagnation and control of temperature to minimise proliferation.
• Cooling towers and evaporative condensers — Cooling towers and evaporative condensers
have historically been implicated in numerous outbreaks of Legionnaires’ disease. Chapter 5
discusses the risk factors and management of cooling towers and evaporative condensers.
Globally, the primary legionellae associated with outbreaks of disease from these systems
appear to be L. pneumophila serogroup 1 MAb2 reactive strains. The major risk factor for
legionellae proliferation appears to be neglect or insufficient maintenance. A significant
proportion of outbreaks of Legionnaires’ disease in these systems have been attributable
to the start-up of stagnant systems without adequate chemical treatment. Cooling towers
and evaporative condensers are generally designed to maximize operational performance
of a thermal system; however, Chapter 5 spells out the importance of an effective water
treatment programme in controlling legionellae proliferation. Such a programme has multiple
benefits, in that it provides for more efficient operation from reduced fouling and a longer
system life from reduced corrosion, while ensuring safer operation of the system due to
reduced risk of legionellosis. Maintenance of properly treated cooling systems is also an
essential element in reducing legionellae risks in these environments.
xx LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS