Syndromic surveillance for local outbreak detection and awareness: evaluating outbreak signals of acute gastroenteritis in telephone triage, web-based queries and over-the-counter pharmacy sales

Abstract

For the purpose of developing a national system for outbreak surveillance, local outbreak signals were compared in three sources of syndromic data--telephone triage of acute gastroenteritis, web queries about symptoms of gastrointestinal illness, and over-the-counter (OTC) pharmacy sales of antidiarrhoeal medication. The data sources were compared against nine known waterborne and foodborne outbreaks in Sweden in 2007-2011. Outbreak signals were identified for the four largest outbreaks in the telephone triage data and the two largest outbreaks in the data on OTC sales of antidiarrhoeal medication. No signals could be identified in the data on web queries. The signal magnitude for the fourth largest outbreak indicated a tenfold larger outbreak than officially reported, supporting the use of telephone triage data for situational awareness. For the two largest outbreaks, telephone triage data on adult diarrhoea provided outbreak signals at an early stage, weeks and months in advance, respectively, potentially serving the purpose of early event detection. In conclusion, telephone triage data provided the most promising source for surveillance of point-source outbreaks.

Fig. 1.

Number of 1177 calls relating to adult gastrointestinal symptoms during the outbreaks in (a) Östersund and (b) Skellefteå. The smoothed curve is based on a locally weighted polynomial regression performed with the R function ‘lowess', using a smoother span of 14 days. The solid triangles indicate the call count at the outbreak midpoint, i.e. the day when regional and local authorities issued official public information. The vertex indicates the signal count at the midpoint.

Fig. 2.

(a) Pharmacy over-the-counter sales of antidiarrhoeals and (b) daily sums of web queries on gastrointestinal symptoms during the outbreak in Östersund. The smoothed curve is based on a locally weighted polynomial regression performed with the R function ‘lowess’, using a smoother span of 14 days. The solid triangles indicate the unit and search counts at the outbreak midpoint, i.e. the day when regional and local authorities issued official public information on the outbreak. The vertex indicates the signal count at the midpoint.

Fig. 3.

Signal rates. Regression analysis of count data during the observed outbreak period of Östersund on municipality population size for (a) adult diarrhoea calls and (b) over-the-counter (OTC) sales. The analyses included municipalities from half, to twice the size of the targeted municipality (Östersund), excluding municipalities affected by outbreaks. The OTC plot extends beyond the range of the analysis.

Fig. 4.

Signal detection analysis. The stepped graphs represent daily counts of adult gastrointestinal (GI) calls during the outbreak periods in (a) Östersund and (b) Skellefteå, before the outbreak midpoints (27 November 2010 and 19 April 2011, respectively). The solid and open circles indicate strong and weak outbreak signals when the detection algorithm was applied to three streams of 1177 triage data: adult GI calls (upper circles), diarrhoea (middle circles), and stomach pain (lower circles).