5. Identify Source of Outbreak

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After forming and testing the hypothesis, the investigators should use evidence from epidemiologic and environmental investigation to identify the source of the outbreak. Results from the hypothesis testing can assist in focusing an investigation and should be integrated with other information gathered in the investigation to understand the exposure associated with illness. This involves understanding the epidemiology, environmental factors, microbiology, and transmission dynamics of the particular situation. For example:

  • If the outbreak is small, there might not be enough statistical power to use epidemiology alone to link the illnesses to a specific exposure.
  • If it is a large outbreak, several risk factors might be statistically significant. This might require a more complex analytic approach (e.g., regression modeling) to account for confounders.
  • For acute illnesses (e.g., chemical exposure at a pool), time between exposure and illness is so short that an epidemiological study might not be necessary.

Epidemiologic Investigation

Investigators review the collected and analyzed epidemiologic data to determine whether it implicates a particular water source or exposure.

  • Investigators review clinical data and clinical laboratory reports to assess whether illnesses and identified pathogens are consistent with contamination of the implicated water source and the suspected organism, chemical or toxin.
  • Multiple people with similar signs or symptoms linked to a common water source
  • Multiple people diagnosed with the same illness linked to a common water source
  • Attack rates from specific water exposures: Attack rates for water exposures that can account for most cases
  • Odds Ratios from case-control studies or risk ratios from cohort studies: Epidemiologic data provided about exposed and unexposed persons, with relative risk or odds ratio ≥2 or p-value ≤0.05, that links outbreak cases to the same water exposure.
  •  Molecular Epidemiology:
    • Molecular characterization of pathogens from clinical specimens matched for ≥2 cases who had the same water exposure
    • Molecular typing results matched to environmental sampling results

Environmental Investigation

Investigators review the results of the environmental investigation to assess supporting or confirmatory evidence of outbreak source.

  • Disinfectant residual: measure the disinfectant residual (e.g., chlorine) for treated drinking water and recreational water systems to see if is adequate to inactivate the implicated pathogen
  • Historical information about lapses in water treatment: review monitoring data on water systems to assess evidence for lack of effective treatment
  • Indicators of fecal contamination: test for fecal bacteria to assess whether it exceeds levels for safe drinking or untreated recreational water
  • Detection of contamination in a treated or untreated water source: test water for pathogen of interest