An outbreak of cyclosporiasis in 1996 associated with consumption of fresh berries: PATIENTS AND METHODS part 2
Case-control study: A matched case-control study was conducted to determine further whether consumption of berries was associated with cyclospora infection in Ontario. Sporadic cases were included if they were Ontario residents at least age 18 years and had an onset of symptoms by July 1, 1996. Sporadic cases were excluded if other enteric pathogens were identified on stool examination.
Potential neighbourhood and age-matched controls were interviewed by telephone. Inclusion criteria for controls were no gastrointestinal illness on or after May 1, 1996, no foreign travel during the four weeks before onset of illness of the matched case and no more than three nights spent outside of Ontario during the two weeks before onset of illness of the matched case.
A structured questionnaire gathered information about consumption of fresh berries and other fresh fruits and vegetables, domestic water supply and water consumption, and exposures to domestic animals and soil. Cases and their matched controls were questioned about exposures during the two weeks before the relevant case’s onset of illness. Cases and controls were interviewed from July 4 to July 27, 1996. Statistical analysis: For analyses of the clusters of cases, uni- variate relative risks were calculated for exposure variables, and two-tailed P values were computed with test or, if appropriate, Fisher’s exact test. For the case-control study univariate analyses were performed in EPI INFO 6.04 (CDC, Atlanta, Georgia). Continuous variables were dichotomized into high and low categories by the median. Mantel-Haenszel matched odds ratios and confidence intervals determined by the exact 95% percent mid-P limits. Conditional logistic regression was performed inLogXact (Cytel Corp, Cambridge, Massachusetts).
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