4. Test Hypotheses Using Epidemiologic and Environmental Investigation

Once a hypothesis is generated, it should be tested to determine if the source has been correctly identified. Investigators use several methods to test their hypotheses.

Epidemiologic Investigation

Case-control studies and cohort studies are the most common type of analytic studies conducted to assist investigators in determining statistical association of exposures to ill persons. These types of studies compare information collected from ill persons with comparable well persons.

Cohort studies use well-defined groups and compare the risk of developing of illness among people who were exposed to a source with the risk of developing illness among the unexposed. In a cohort study, you are determining the risk of developing illness among the exposed.

Case-control studies compare the exposures between ill persons with exposures among well persons (called controls). Controls for a case-control study should have the same risk of exposure as the cases. In a case-control study, the comparison is the odds of illness among those exposed with those not exposed.

Using statistical tests, the investigators can determine the strength of the association to the implicated water source instead of how likely it is to have occurred by chance alone. Investigators look at many factors when interpreting results from these studies:

  • Frequencies of exposure
  • Strength of the statistical association
  • Dose-response relationships
  • Biologic /toxicological plausibility

For more information and examples on designing and conducting analytic studies in the field, please see The CDC Field Epidemiology Manual.

Information on the clinical course of illness and results of clinical laboratory testing are very important for outbreak investigations. Evaluating symptoms and sequelae across patients can guide formulation of a clinical diagnosis. Results of advance molecular diagnostics can be evaluated to compare isolates from patient and the outbreak sources (e.g., water).

Environmental Investigation

Investigating an implicated water source with an onsite environmental investigation is often important for determining the outbreak’s cause and for pinpointing which factors at the water source were responsible. This requires understanding the implicated water system, potential contamination sources, the environmental controls in effect (e.g., water disinfection), and the ways that people interact with the water source. The factors considered in this investigation will differ depending on the type of implicated water source (e.g., drinking water system, swimming pool). Environmental investigation tools for different settings and venues are available.

The investigation might include collecting water samples. Sampling strategy should include the goal of water testing and what information will be gained by evaluating water quality parameters including measurement of disinfection residuals, and/or possible detection of particular contaminants. The epidemiology of each situation will typically inform the sampling effort.