Prediction of gastrointestinal disease with over-the-counter diarrheal remedy sales records in the San Francisco Bay Area

Abstract

Background: Water utilities continue to be interested in implementing syndromic surveillance for the enhanced detection of waterborne disease outbreaks. The authors evaluated the ability of sales of over-the-counter diarrheal remedies available from the National Retail Data Monitor to predict endemic and epidemic gastrointestinal disease in the San Francisco Bay Area.

Methods: Time series models were fit to weekly diarrheal remedy sales and diarrheal illness case counts. Cross-correlations between the pre-whitened residual series were calculated. Diarrheal remedy sales model residuals were regressed on the number of weekly outbreaks and outbreak-associated cases. Diarrheal remedy sales models were used to auto-forecast one week-ahead sales. The sensitivity and specificity of signals, generated by observed diarrheal remedy sales exceeding the upper 95% forecast confidence interval, in predicting weekly outbreaks were calculated.

Results: No significant correlations were identified between weekly diarrheal remedy sales and diarrhea illness case counts, outbreak counts, or the number of outbreak-associated cases. Signals generated by forecasting with the diarrheal remedy sales model did not coincide with outbreak weeks more reliably than signals chosen randomly.

Conclusions: This work does not support the implementation of syndromic surveillance for gastrointestinal disease with data available though the National Retail Data Monitor.

Figure 1

Plots of Outbreak-Associated Gastrointestinal Cases, Individual Gastrointestinal Cases, Diarrheal Remedy Sales, and Differenced Diarrheal Remedy Sales. Standardized weekly counts of gastrointestinal outbreak-associated cases, diarrheal illness case reports, Diarrheal Remedy Sales and differenced Diarrheal Remedy Sales in three San Francisco Bay Area Counties from January 2004 to July 2005. All data aggregated to the first Sunday of week. Diarrheal Remedy Sales are aggregated by week of sale, cases by week of report to the health department and outbreak cases by week of onset of the first associated case. Vertical axes are measured in standard deviations.

Figure 2

Cross Correlations Between Diarrheal Remedy Sales and Diarrheal Illnesses. Diarrheal Remedy Sales and diarrheal illness case reports cross correlations at time lags from zero to 19 weeks. No significant correlations, bars exceeding the 95% confidence interval (shaded), were found.