The use of online surveys to collect responses from healthy people during outbreak investigations has been tested by researchers in Canada.

In foodborne outbreak investigations, case-control and cohort studies are used to test hypotheses and identify a source, but they are resource-intensive and recruiting appropriate controls, or non-ill people, is challenging, according to the study published in the journal Epidemiology and Infection.

Other methods include population-based food consumption surveys and use of case-case analysis to generate or test a hypothesis but such population data may not provide a representative control group, or include sufficient food product precision and can be outdated. Case–case analysis can only be done if comparable cases are accessible.

Rapid and representative
Researchers used online surveys to collect population control data for two foodborne outbreaks and compared it to cases and existing population exposure data. Findings demonstrate the surveys were a rapid and representative way to collect responses from healthy people during outbreaks to support the epidemiological investigation.

Online survey population controls were comparable to patients based on age and sex. Exposure data collected through the surveys were more precise than existing control data, represented the disease-specific exposure period and could be easily modified.

Online surveys for the outbreak investigations were developed. When people visited the British Columbia Centre for Disease Control website they were asked to participate to help solve an outbreak.

Participants were included if they were a resident of British Columbia and had not experienced symptoms during the exposure period of 14 days for Cyclospora and seven days for Salmonella. They were asked whether they had eaten two to three foods of interest, their gender, age and city of residence.

Tested on Cyclospora and Salmonella outbreaks
The purpose of the online survey during the 2018 Cyclospora outbreak was to test hypotheses from previous investigations. The survey was posted from early May to late August 2018, the onset of the outbreak. It initially asked about exposure to cilantro, blackberries and raspberries. Preliminary analyses of outbreak patient data suggested low exposure to raspberries, but high exposure to spinach. In late June, raspberry was removed and spinach added to the survey.

A total of 1,687 responses were received and 1,403 met inclusion criteria. Online controls were similar to the patients in age, gender and geographic distribution. For cilantro, raspberries and spinach, control populations showed similar exposure proportions to each other and to the cases.

Patients had slightly higher odds of having blackberries than the online controls. This epidemiological information led to the review of import data and to traceback of implicated blackberries based on case purchase data. While the source was not confirmed to a single supplier or source of blackberries, this fruit was the leading hypothesis.

The second survey was during a national outbreak of Salmonella Infantis with most infections, 47, in British Columbia. While English cucumbers had been hypothesized as a possible source, other exposures were also frequently reported. The survey objective was to test the hypothesis that English cucumbers were the source of illness. The online survey ran from mid-October to early November 2018.

A total of 286 responses were received and 253 met inclusion criteria. The online controls were less likely to have English cucumber exposure than cases. These results directed traceback activities which identified a common supplier and confirmed the hypothesis.

Online surveys can be prepared quickly with few resources and get a large number of responses. Accessing data could be done by investigators in real-time but the survey did not prevent the same person from contributing as a control multiple times. Researchers said based on this experience, the method will be employed in future outbreak investigations.

It has not yet been used to test a hypothesis where a specific brand is identified given there are potential confidentiality risks to disclosing that information if the hypothesis is incorrect. However, this could be overcome by asking about various brands of the same product.

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