1. Detect a Possible Outbreak

A waterborne disease outbreak occurs when two or more people become ill after exposure to a common contaminated water source. Multiple data sources can aid in detecting waterborne disease outbreaks. Types of data used to identify an outbreak include epidemiologic data, clinical/laboratory data, and environmental monitoring data.

Collecting Data to Detect Outbreaks

Epidemiologic Data
  • Interviews with patients who are ill with potential waterborne diseases provide important clues regarding the source of infection. Thorough interviews, utilizing both standardized risk factor questionnaires (e.g. CDC’s National Hypothesis Generating Questionnaireexternal icon) and unstructured questions about exposures, can give clues to develop hypotheses regarding the outbreak source.
  • Regular reviews of the data identify patterns in the geographic distribution of illnesses and the time periods when people became ill, facilitating identification of outbreaks. States mandate that diagnostic laboratories and healthcare providers report certain diseases, including many illnesses caused by pathogens that cause waterborne disease (e.g., Cryptosporidium, E. coli).
    • Sources of data to consider
      • Active/passive surveillance data
      • Available epidemiologic data
  • Illness complaint systems provide a means for reporting and triaging public health concerns from providers or citizens. These can be used to report illnesses associated with water exposures or food consumption, assisting states in quickly identifying clusters of illness among people with exposure to a particular water source and initiating an investigation.
Woman testing soil

  • Syndromic Surveillance systems at Emergency Departments, hospitals, or clinics can provide early warning of illness (e.g., Biosense, ESSENCEexternal icon). Predefined statistical algorithms may alert the health department when there is an unusual increase in reports of illness in space and time. Review of the data can identify a pattern of illness suggestive of an outbreak.
  • Increased sales of anti-diarrheal medications can indicate a community-wide outbreak. Some states regularly monitor their over-the-counter medication sales as part of surveillance.
  • Increased absentee reports from schools and major employers can point to development of an outbreak. For example, some states mandate that schools report absentee rates above a certain threshold.
  • Increased calls to Poison Control Centers can also be suggestive of an outbreak. Poison Control Centers have been instrumental in identifying potential public health threats via real-time public health surveillance through their National Poison Data System (NPDS). These data can be accessible through their data-request process, AAPCC data request processexternal icon.
  • Healthcare providers can notify local and state health departments regarding unusual or increased cases of illnesses within their hospitals or clinics; these reports might include additional clues from patient histories.
Clinical/Laboratory Data
  • Patient illness information can determine if a patient is potentially associated with an outbreak.
    • Medical record abstraction can identify additional cases who meet the specified case definition for follow-up. Medical records are helpful if a patient is identified as part of an outbreak investigation, as they often contain a brief exposure history.
  • Clinical laboratory data are used to determine if there are additional cases pending confirmation in the area. Helpful information from clinical labs includes baseline information (e.g., the average number of specimens they receive to test for a specific disease and how often that disease is positively identified) to understand if there is an elevation in confirmed cases.
  • Public Health labs can identify an increase in clinical specimens of the same molecular pattern by using techniques such as pulse-field gel electrophoresis (PFGE) pattern or whole genome sequence (WGS) and indicate that they might have a common exposure source.
Environmental Monitoring Programs
  • Environmental testing of water systems can identify risks to water quality and potential sources of contamination that can cause infections. Many states mandate public reporting (i.e., drinking water advisories) of decreased water quality in drinking water systems.
  • Beach monitoring programs can identify decreased water quality (i.e., fecal contamination) or the presence of harmful algal toxins at swimming beaches.
  • Events that increase risk for waterborne disease, including floods, sewer system overflows, and water system outages, are also important to monitor.
  • National data on water quality includes:
Other Data Sources
  • Situational awareness is an important component for outbreak detection.  Investigators should be conscious of events or large conventions happening in the area that can be potential sites of outbreaks. News or media alerts can assist public health in identifying additional ill persons for follow-up.
    • Monitor traditional and social media alerts and complaint hotlines to track illness complaints, follow up with the complaints to determine if they meet a case definition, or spread prevention messages to the affected community.
Investigation Tools
  • Line List
    • Line lists are utilized to summarize information provided during case investigation. Line lists should include demographic information and clinical information such as signs and symptoms (type, duration), onset dates and times, case status, and exposure information.
    • CDC Line List template pdf icon[PDF – 4 pages]
  • Epidemic Curve
    • To help keep track of the number of illnesses over time, create an epidemic (epi) curve. The pattern of the epi curve can help to determine if the exposure is a point course or if exposure occurred over a longer time period.
    • CDC Quick-Learn Lesson, Create an Epi Curve
  • Maps and spatial analysis tools
    • Maps and spatial analysis can be useful investigation tools for visualizing the location of cases, facilitating recognition of any spatial relationships, giving clues to outbreak source, and tracking geographic spread over time. Some tools include ArcGIS, or R studio to build data visualization maps and SatScan for spatial analysis.