Investigation of a waterborne outbreak

Who are the guidelines for?

These guidelines are intended for the municipal outbreak investigation team. The investigation of waterborne outbreaks is regulated by the Finnish Government Decree (1365/2011). This concise guideline provides recommended actions for situations where an authority suspects a waterborne outbreak and initiates an investigation.

Roles in investigating waterborne outbreaks

Under legislation, the municipal outbreak investigation team is responsible for:

• preparing in advance for foodborne and waterborne outbreaks
• agreeing on the organization of outbreak investigations.

The municipal health protection authority, as part of the investigation team, is responsible for monitoring the quality of drinking water and, when necessary, issuing instructions and orders to prevent health hazards caused by the use of drinking water. The quality and monitoring of drinking water are regulated by the Ministry of Social Affairs and Health Decrees (1352/2015) and (401/2001).

The Finnish Institute for Health and Welfare (THL) is responsible for detailed analysis, typing, and surveillance of pathogens isolated from humans or water. THL can provide expert support for investigating waterborne outbreaks.

Investigation Phases

Investigating an outbreak involves several stages, many of which progress partly in parallel during the process. The investigation team defines the necessary studies on a case-by-case basis.

1. Detection of the outbreak

Consider unusual cases of gastrointestinal illness. Information may come through healthcare channels.

Also take into account complaints from customers or the water utility regarding abnormal changes in water quality or reports of symptoms.

2. Communication and Containment Measures

Ensure that, already in the early stages of investigating a suspected outbreak, communication and actions are taken to contain the outbreak and prevent health hazards:

• Most important: Inform drinking water users about contamination. Communicate the contamination as early as possible and advise boiling drinking water for five minutes.
• Issue an order to the water utility to start chlorination and/or flush the distribution network.
• Provide instructions for arranging alternative water supply (e.g., tanks, bottled water).

3. Notification of Suspected Outbreak  

Submit a report of the suspected drinking water outbreak as soon as possible to the notification system for food- and waterborne outbreaks, i.e., the RYMY system (Finnish Food Authority)
RYMY-system (in Finnish and Swedish, Finnish Food Authority)

4. Inspection of the Drinking Water System  

Conduct an on-site inspection at the water utility and review raw water sources, including the location and condition of wells at groundwater plants.

Assess the functionality of water treatment processes: filtration, disinfection, automation.

Check the technical condition of the facility and any potential disturbances, such as surface runoff risks, well location and structure, and operational failures.

5. Drinking Water Testing

Collect water samples before chlorination or flushing to verify the original contamination. The required sample volume for analyzing various pathogens and indicator bacteria is approximately 15 liters per sampling point. Use sterile containers whenever possible. If non-sterile containers are used, rinse them thoroughly with sample water before filling. Contact the laboratory performing the analyses as early as possible.

Take samples from:

• Raw water
• Water leaving the treatment plant
• Distribution network water from multiple points, including endpoints where water exchange is minimal.

Request the following analyses:

• Indicator bacteria (E. coli, enterococci, Clostridium perfringens)
• Pathogens (e.g., norovirus, Campylobacter).

Monitor changes in drinking water quality during remediation. The goal is to obtain three consecutive clean sampling rounds for both raw and network water regarding indicator microbes.

Utilize patient sample results and, if necessary, request testing of drinking water for suspected epidemic pathogens. In Finland, noroviruses and Campylobacter have most frequently caused waterborne epidemics.

Typing and comparison of relevant microbes isolated from water and patients are carried out at THL through collaboration between bacteriology, virology, and water microbiology laboratories. THL’s water microbiology laboratory receives microbial isolates, genome samples, and sequences from other laboratories.

When sending isolates, complete the accompanying submission form:
Submission form for microbial isolates from water

6. Epidemiological Studies

Ensure that all necessary epidemiological investigations are conducted to clarify the outbreak.

7. Actions Related to Drinking Water Contamination

Determine the cause of drinking water contamination in cooperation with the water utility. The utility must correct the source of contamination, if possible, and secure the quality of drinking water.

8. After the outbreak

Ensure the safety of drinking water through water testing before lifting usage restrictions and the boil-water advisory.

Once the outbreak has ended, remove any remaining restrictions on drinking water use and inform users about the return to normal conditions.

Prepare an investigation report on the outbreak and upload it to the RYMY system no later than three months after the outbreak has concluded.

Update the drinking water monitoring plan as needed based on findings from the outbreak investigation.