Syndromic surveillance of gastrointestinal illness using pharmacy over-the-counter sales. A retrospective study of waterborne outbreaks in Saskatchewan and Ontario.

Objective: To ascertain if monitoring over-the-counter (OTC) drug sales could provide a timely syndromic surveillance method of detecting outbreaks of gastrointestinal illness.

Method: This study evaluated the potential of a syndromic surveillance system by comparing retrospective pharmacy OTC sales of anti-nauseants and anti-diarrheals to emergency room visits and case numbers from two Canadian outbreaks related to water contamination by Cryptosporidium, and E.coli O157:H7 and Campylobacter.

Results: Local sales trends of weekly aggregate OTC products were comparable to the outbreak epidemic curves. Statistical control tests on the sales data indicated the start of the outbreak periods.

Conclusions: An automated monitoring tool based on spatial and temporal trend analyses of daily OTC sales would provide supplemental community health information for public health officials that is timelier than currently available laboratory-based surveillance systems.