Use of a case–control study and control bank to investigate an outbreak of locally acquired cyclosporiasis in Canada, 2016

Abstract

Background: Cyclospora is an intestinal parasite that is not endemic in Canada. However, national outbreaks of locally acquired cases have been reported since 2013. These outbreaks were suspected to be associated with consumption of produce imported from countries where Cyclospora is endemic. Identification of the source can be challenging because of reporting delays and limited traceability of produce. Objective: To report on a national outbreak of locally acquired cyclosporiasis, highlight the challenges of investigating these outbreaks and document the first time use of a control bank to recruit controls for a national outbreak case-control study in Canada. Methods: Cases of cyclosporiasis were identified through provincial laboratory testing and reported through provinces to the national level. Cases were interviewed about food exposures using a questionnaire and food exposures reported by cases were compared to Foodbook reference values. To narrow down the food items of interest, a matched case-control study was conducted. Controls for the study were recruited primarily from a control bank, that is, a list of individuals who had previously agreed to participate in public health-related surveys. Results: In total, 87 cases of locally acquired cyclosporiasis with onset or report dates between May 19, 2016 and August 10, 2016 were reported by four provinces. Comparing case exposures to Foodbook reference values identified several food items of interest, including blackberries, other berries, herbs and leafy greens. The case-control study identified only blackberries and mesclun greens as significantly more frequently consumed by cases than controls. Due to lack of product details for blackberries and mesclun greens, the source of the outbreak was not conclusively identified. Conclusion: Blackberries were the primary food item of interest, but could not be identified as the conclusive source due to lack of traceability. The control bank was found to be a useful tool for control recruitment.

An outbreak of hepatitis A in Canada: The use of a control bank to conduct a case-control study

“…In this study, the response rate for the control bank was almost 70 times greater than that of the landline or cell phone samples. Similarly, in a recent case-control study related to an outbreak of cyclosporiasis, the response rate for the control bank was 60% compared to only 24% via random digit dialling [20].…”

“…Response rates for random digit dialling methods have seen a dramatic decline in recent years, due in part to the popularity of caller identification [15, 18, 19]. The use of a pre-established control bank of consenting participants could result in a higher response rate than that of random-digit dialling or calling listed numbers within the general population, potentially reducing time and cost to complete a case-control study [20]. A further limitation for random digit dialling methods is the lack of ability to make targeted calls, adding an additional challenge to recruiting controls in the context of a matched study.…”

“…A control bank, complete with demographic information for each individual, is one solution to this issue. Several case-control studies have used the control bank approach to recruit controls, including studies in Australia for outbreaks of salmonella , campylobacter and Escherichia coli [21, 22, 23], and a Canadian study examining an outbreak of cyclospora [20].…”

Cyclospora cayetanensis: A Perspective (2020–2023) with Emphasis on Epidemiology and Detection Methods

“…In a case–control study of an outbreak in Canada, a total of 87 (1 hospitalized) locally acquired cases in four provinces from May to August 2016 were examined [ 43 ]. Although the study was not able to identify the exact source of the outbreak, case exposure identified some possible foods, with imported blackberries from Mexico being the most likely source of infection.…”

Genotyping Canadian Cyclospora cayetanensis Isolates to Supplement Cyclosporiasis Outbreak Investigations

“…In Canada, the first cyclosporiasis outbreak was reported in 1996 [2] prior to becoming a nationally notifiable disease in 2000. Multijurisdictional foodborne outbreaks of locally acquired cases have been declared yearly during the spring or summer months since 2013 [3]. Previous Canadian foodborne cyclosporiasis outbreaks have been associated with a variety of food vehicles imported from endemic regions, including fresh berries (e.g., raspberries, blackberries) [3][4][5][6], fresh herbs (e.g., basil, cilantro) [7,8], leafy greens [9], and other fresh produce (e.g., sugar snap peas) [10].…”