Environmental sampling in an outbreak
Sampling is essential in an outbreak situation and an outbreak control team generally needs to
sample the environment for Legionella for two reasons. Firstly to identify the source,
and secondly to assess the effectiveness of remedial action. Sampling should be targeted
towards high risk processes or equipment first, followed by sampling of lower risk processes.
It is important to continually assess potential sources and redirect the
numbers and focus of the sampling as the investigation develops and results and information
become available. It is important to note that sampling staff will require training before an investigation.
Reliably detecting the presence of Legionella bacteria is technically difficult
and requires specialised and accredited laboratory facilities. The interpretation of results is
also difficult; a negative result is no guarantee that Legionella bacteria are not
present. Conversely, a positive result may not indicate a failure of controls, as
Legionella are present in almost all natural water sources. Interpretation would ideally
be made by an environmental microbiologist.
Choosing sampling points and number of samples
Generally, any water source that may produce aerosols should be considered a potential
source for the transmission of Legionellae
- Review epidemiological information to decide where to focus the initial environmental
investigations and control measures.
- The number and types of sites that should be tested to detect Legionellae must be
determined on an individual system basis. This requires detailed knowledge of the design of the
water system to be examined and all equipment that utilises water or generates aerosols. It is
also important to have a thorough understanding of the ecology of the organism prior to taking
- Examples of sampling sites with suggested number and volume of samples to be taken can be
found here. Although sites suggested are within a hospital, they can be
applied more generally to investigations in other establishments.
- In outbreak investigations there may be no information available on the design of the
system or of any previous risk assessments. Therefore one would need to
conduct a dynamic risk assessment to support the outbreak investigation
and the health and safety interests of sampling staff.
- Establish which outlets the patient(s) has been exposed to.
- Ensure traceability and continuity of evidence. Sampler should use tamper proof bags etc.
Particular areas might have larger numbers of certain equipment/machines that might cause
concern to the outbreak control team such as spa pools at hotels and
respiratory devices in hospitals. If resources permit, the domestic water
systems of the case should be sampled so as to eliminate such potential sources .
Click here for further guidance on methodologies.
Collecting environmental samples 
Two primary sample types - water samples and swabs of point-of-use devices or system
surfaces - should be collected when sampling for Legionellae.
Water samples: collection of at least 1 litre of water allows the sample to be
concentrated. If the water source has recently been treated with an oxidizing biocide, such
as chlorine or bromine, sodium thiosulfate must be added to each 1litre sample in
sufficient quantities to neutralise any disinfectant present. Depending on the reason for
sampling, the sample may be taken as a first flush (i.e. no disinfection). This is
appropriate for most occasions and will represent the worst case. After disinfection, the
sample will be taken from a running outlet representing the circulating system.
Swabs: during outbreak investigations, swabs should be taken in conjunction with water
samples from sites where biofilms are likely to form. These swabs can be taken from various
points within plumbing systems, from surfaces such as biofilms, and from areas that are
difficult to reach, such as within the jets of hot tubs, thermostatic
mixer valves or showers. The swabs can be submerged in a small volume of water taken at the
same time, or in Pages's saline, to prevent drying during transportation to the laboratory.
All samples should be transported to the laboratory in dark, within insulated containers to
protect them from extreme temperatures and from light.
Contextual information should be recorded to help interpret the results, usually summarised on
the laboratory sample request form below, but individual outbreaks might require additional
information for operational reasons.
Sample request forms - minimum information
Each laboratory will have its own sample request forms and these should be used to provide
continuity of evidence. As a minimum, the following information should be included on the
- the site and sample point
- the sample references and date
- the reason for sampling
the temperature of the sample source (e.g. the temperature of a hot-water system
at one minute after turning on tap, and at two minutes after turning on cold tap)
- any biocide used
- the timing of the dosage in relation to sampling
- the concentration detected at the time of sampling
any other risk factors of importance (e.g. closed system opened for
- risk of nutrient present, such as in plastics manufacturing plants
- any cases associated with the site.
- LEE J. V. & JOSEPH C. ON BEHALF OF THE PHLS ATYPICAL PNEUMONIA WORKING GROUP (2002)
Guidelines for investigating single cases of legionnaires' disease Communicable Disease and
Public Health 5(2), pp.157-62 http
- BARTRAM J. (2007) Legionella and the prevention of legionellosis WHO, Geneva ISBN 92
4 156297 http pdf