LEGIONELLA - World Health Organization

The WSP should be prepared in conjunction with, and made available to, all concerned parties
(e.g. health authorities, water suppliers, building managers and water treatment providers).
The plan should be reviewed on a regular basis to reflect changes and ongoing improvements
in the system, the available evidence base and the surrounding environment (WHO, 2004).
Finally, the plan should be amended if control is not maintained.
3.3.1 System assessment
Assessment of the system supports subsequent steps in the plan, allowing effective strategies for
controlling hazards to be developed and implemented. The steps involved — shown in
Figure 3.2, and discussed below — are to:
• assemble a team
• document and describe the system
• assess hazards and prioritize risks
• assess the system.
Assemble a team
As shown in Figure 3.2, a preliminary stage in developing a WSP is to form a team of experts
with a thorough understanding of the particular water system. A thorough knowledge and
understanding of the system’s operation is also critical. This should incorporate knowledge of
design strengths and weaknesses and operating characteristics, so that informed decisions can
be made about system maintenance and monitoring.
The training and experience of assessors are important factors in the quality of system assessments.
Ideally, assessors should be independent of those who supply water treatment services, to
avoid conflicts of interest. Assessors must be aware of the ecological factors that encourage
Legionella growth within a system (see Chapter 2), and have some understanding of the
design and engineering of the system, and of any modifications or alterations to the system,
particularly if the system is large and complex. Assessment of complex systems will generally
require a broad knowledge base and is best conducted by a multidisciplinary team that can
address all aspects of system operation and management, including microbiological aspects.
However, a team-based approach might not be feasible in some cases, for example where
resources are limited. Therefore, a system assessment should establish the type and level of
control that can realistically be imposed.
Document and describe the system
Large water systems, such as building water systems, are those most commonly associated
with widespread human exposure to Legionella. Identifying the layout and design of such
water systems is therefore an important step in controlling colonization, although the task
can be time consuming and difficult. Due to the high level of technical capability required,
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS