Epidemiological and spatial study of a community-wide waterborne outbreak

Failures in the drinking water distribution system often cause gastrointestinal outbreaks associated with multiple pathogens. We investigated a community-wide waterborne outbreak using a polyphasic approach combining advanced epidemiological, statistical, spatial and microbiological methods. A water pipeline breakage due to construction works occurred in the water distribution line in Vuorela, Eastern Finland on July 4th 2012. Two weeks later, gastrointestinal illness in the community increased and immediate control measures were implemented. Of 2931 inhabitants of the defined outbreak area, a total of 473 (16 %) responded to the web-based questionnaire. Samples from patients and water were analyzed for multiple microbial targets, subjected to appropriate molecular typing and microbial community analysis. We developed a method that enabled us to calculate the distance between the water pipe line breakage point and inhabitant locations. We used the responses obtained from the questionnaires in the univariate and multivariate analysis as explanatory variables. In addition, we used spatial logistic regression model to further analyze the data. The main symptoms in the cohort were stomach ache, nausea and diarrhoea. The clinical picture was mild and the length of the illness had a median of three days. Several pathogens and/or faecal indicators were detected by from the patient faecal and/or water samples, including sapovirus, single Campylobacter jejuni, arcobacters and various E. coli types (EHEC, EPEC, EAEC and EHEC). A case definition was created based on the clinical symptoms, which was used as a response variable in the statistical models. Drinking untreated tap water from the defined outbreak area had a risk ratio (proportion of those exposed among ill to those exposed among healthy) of 5.6 (95% CI 1.9-16.4) increasing in a dose response manner. We were able to calculate the distance between the water breakage point and the inhabitant position by the path of the water distribution network with the method developed for this study. The closer a person lived to the water distribution breakage point, the higher the risk of becoming ill. Children were more likely to fall ill. In the multiple log and logistic regression models, age (inversely), distance from the breakage point (inversely) and drinking the tap water were significant. In addition, a spatial term (describing the spread of the infection between close contacts, inaccuracy in the distance variable and nonlinear fluctuation of the water in the distribution network) was significant. Transmission between persons is common among children and with viral infections. The novel methods used in this study improved the characterization of the source of the infections, and aided to define the extent and magnitude of this outbreak. These methods may be applied to wide range of future outbreaks.