Who are the instructions for?
The guideline is intended for the municipal investigation team responsible for foodborne, waterborne, and bathing water–related outbreaks. The investigation of bathing water–related outbreaks is regulated by the Government Decree (1365/2011) issued under the Food Act (297/2021) and the Health Protection Act (763/1994). This concise guideline provides recommended actions for situations in which the authority suspects a bathing water–related outbreak and initiates an investigation.
Roles in the Investigation of Bathing Water–Related Outbreaks
The municipal investigation team is responsible for investigating foodborne, waterborne, and bathing water–related outbreaks. Under the relevant legislation, the duties of the municipal food- and waterborne outbreak investigation team are to
- prepare in advance for bathing water–related outbreaks, and
- agree on the organisation of outbreak investigations.
The task of the municipal health protection authority, which is represented in the investigation team, is to monitor the quality of bathing water and, when necessary, issue instructions and orders to prevent health hazards to bathers. Bathing water quality and its monitoring are regulated by the Decrees of the Ministry of Social Affairs and Health (177/2008 and 354/2008).
The Finnish Institute for Health and Welfare (THL) guides and supports outbreak investigations carried out in municipalities and wellbeing services counties. THL also conducts outbreak investigations when an outbreak has spread across several wellbeing services counties, or when the disease is particularly severe or otherwise nationally significant. THL supports the municipal food- and waterborne outbreak investigation team in its work and, when necessary, coordinates the investigation, for example in exceptionally large or geographically widespread outbreaks. THL is responsible for the detailed examination, typing, and surveillance of pathogens isolated from humans or water during outbreaks.
Steps of the Investigation
Investigating an outbreak involves several overlapping phases that may progress simultaneously. The municipal outbreak investigation team determines the necessary investigations on a case-by-case basis.
1. Communication and Containment Measures
Ensure that communication and the measures required to contain the outbreak and prevent health hazards are addressed already in the early phase of the investigation.
Such measures may include, for example, a temporary bathing ban or a recommendation to avoid bathing. When needed, use the common European signs Bathing Prohibited and Bathing Not Recommended.
2. Notification of a Suspected Outbreak
Submit a notification of a suspected bathing water–related outbreak to the RYMY reporting system (the national food- and waterborne outbreak reporting system) as soon as possible.
Through the notification, information on the suspected outbreak becomes available to THL and the Finnish Supervisory Agency.
RYMY is intended for communication between authorities. Private individuals and operators should contact the environmental health inspector of their municipality.
Note: If electronic reporting to the RYMY system is not functioning, contact THL’s outbreak investigation team at ryhmazoo(at)thl.fi or an infectious disease physician at 029 524 8557. Outside office hours, contact THL’s infection duty officer at duty24(at)thl.fi.
3. Inspection Visit
Conduct an inspection visit to the bathing site to observe local conditions and visually assess the general quality of the bathing water. Determining the microbiological quality of the water always requires taking samples and laboratory testing.
Observe the presence and abundance of cyanobacteria (commonly known as blue-green algae) visually. Cyanobacteria may appear as greenish or yellowish flecks, colour the water green, or even form thick surface scums. Winds and currents may move cyanobacterial masses, so the amount present can vary rapidly even within the same day.
4. Bathing Water Monitoring
Monitor the levels of Escherichia coli bacteria and intestinal enterococci with additional water samples. These indicator bacteria reflect faecal contamination of the water: the higher their concentrations, the greater the likelihood that other faecal microbes are also present.
Make use of stool sample results from symptomatic individuals, and request laboratory testing of bathing water for the suspected causative agent when needed.
In Finland, noroviruses and Campylobacter bacteria have caused bathing water–related outbreaks.
Typing and comparing microbial strains isolated from water and patient samples are carried out at THL in cooperation between the bacteriology, virology, and water microbiology laboratories. THL’s water microbiology laboratory receives microbial isolates, genome samples and sequences from water laboratories in accordance with Valvira guideline 1/2013 (In Finnish). When submitting isolates, the accompanying form (In Finnish) for microbes isolated from water must be completed.
5. Bathing Water Profile
Utilise the bathing water profile, which compiles information on the characteristics of the bathing site and the factors that may impair water quality.
A bathing water profile has been prepared for all public bathing sites in cooperation between the site owner and the municipal health protection authority. The likelihood of cyanobacterial occurrence is also described in the profile. Profiles are typically available on the municipality’s website.
Common causes of bathing water contamination include:
- Faecal microbes from bathers
- Wastewater leaks reaching the bathing area
- Faeces from wild animals or grazing livestock near the site
- Pollutants carried by stormwater runoff from built-up areas
- Impurities washed from soil during heavy rainfall or flooding
6. Epidemiological Investigations
Carry out the epidemiological investigations related to the outbreak on a case-by-case basis. These investigations often progress in parallel with other investigative activities.
In the early phase of the outbreak, request stool samples from a sufficient number (3–5) of symptomatic individuals for viral and bacterial testing. Use the results to support the planning of water microbiological investigations.
7. Measures Related to Bathing Water Contamination
Identify the cause of the deterioration in bathing water quality. Where feasible, implement measures in cooperation with the site operator, relevant authorities, and other stakeholders to prevent contamination and reduce the risk of similar outbreaks recurring.
8. After the Outbreak
Once the outbreak has ended and bathing water quality has returned to safe levels based on indicator bacteria or other monitored parameters, lift any restrictions on bathing and inform the public.
Prepare an investigation report on the outbreak and record it in the RYMY system no later than three months after the outbreak has ended.
Update the bathing water profile if necessary, based on findings from the outbreak investigation.
9. Guidance for Bathers and the Bathing Site Operator
Provide guidance to bathers to encourage responsible behavior and help prevent bathing water–related outbreaks.
Bathers should:
- avoid swallowing bathing water while swimming
- maintain good personal hygiene and use the washing and toilet facilities provided at the bathing site
- change young children’s nappies somewhere other than in the water, for example in toilet facilities
- avoid swimming when experiencing diarrhoea or vomiting
- follow public notices at the bathing site and comply with instructions issued by authorities.
To help prevent bathing water–related outbreaks, advise the bathing site operator to:
- keep washing, changing and toilet facilities clean
- ensure that facilities are stocked with sufficient hand soap, hand towels, toilet paper and waste bins.
If cyanobacteria (blue-green algae) are present in the bathing water, advise swimmers as follows:
- do not swim in water containing cyanobacteria, and do not fill pools or hot tubs with such water
- do not allow children or pets to play or swim in shoreline water containing cyanobacteria
- do not use contaminated bathing water as drinking water for humans or animals, not even after boiling
- do not use such water for sauna, washing, dishwashing or for watering edible plants
Contact details
Microbiology Unit, Water Microbiology Team
vesimikrobiologia(at)thl.fi
(Bathing water quality and water microbiological analysis)
Unit for the Prevention of Health Threats
ryhmazoo(at)thl.fi
(Epidemiological investigations)