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The Canary Database
Center for One Health Research
University of Washington

Study methodologies: Case control

The Canary database curators determine, for each included study, the type of study methodology employed by the researchers (using this classification protocol). The possible categories are:

Fox has outlined criteria for objectively evaluating the relationship between an environmental hazard and an observed health effect in an observational study of animals (Fox 1991). These include probability, time order, strength of association, specificity, and consistency on replication, predictive performance, and coherence. The choice of study design can have a major effect on the ability of a study to fulfill such criteria.

Our preliminary review of the animal sentinel literature has found that some potentially useful study designs, such as case-control and cohort, are under-utilized in animal sentinel research.

Case Control Studies

In a case control study, individuals with a certain condition (cases) are compared with individuals without the condition (controls) in terms of history of previous exposures, thereby moving backward in time. Like cohort studies, this is a rarely used methodology in animal sentinel research. A study of leptospirosis compared a sample of sick dogs with controls who were not noticeably sick, and found greater evidence of previous leptospirosis infection in the sick dogs (Weekes, Everard et al. 1997). Hayes et al used service records to compare historical toxin exposures between military dogs with and without lymphoma (Hayes, Tarone et al. 1995).

Case control studies, where exposures are retrospectively assessed between cases and control subject, provide some advantages over cross-sectional studies in terms of time order. There are obvious difficulties in reconstructing historical exposures for a wild animal, although tissue samples, otolith analysis, and exposure records for an area etc. could provide some clues to previous exposures. As in cross-sectional studies sampling on the basis of outcome, case-control studies can examine several different risk factors at once, although only one disease outcome at a time can be studied. This allows the investigator to adjust for possible confounding variables, and economically explore several hypotheses simultaneously. Candidate hypotheses generated from such studies can then be subjected to experimental confirmation.

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