Why “Winter” Vomiting Disease? Seasonality, Hydrology, and Norovirus Epidemiology in Toronto, Canada

Norovirus is a common cause of gastroenteritis, and is thought to be the causative agent in 68–90% of all gastroenteritis outbreaks. The seasonality of disease occurrence is sufficiently stereotyped to result in this disease being dubbed “winter vomiting disease.” The genesis of this seasonality has been obscure. We sought to identify environmental factors associated with Norovirus outbreaks in Toronto, Canada. We evaluated 253 outbreaks of gastroenteritis linked to Norovirus between November 2005 and March 2008. Poisson regression models were constructed to evaluate associations between average environmental exposures and case counts. A case-crossover approach was used to evaluate associations between acute changes in environment and outbreak risk. Case-crossover analysis indicated an association between low Lake Ontario temperature (≤4°C) (hazard ratio [HR], 5.61 [95% CI, 2.81–11.12]) and high flow (>2.5 m 3 /s) in the Don River (HR, 3.17 [95% CI, 2.30–4.36]), 1–7 days prior to case occurrence. For both exposure variables, the highest hazard ratios were found 24–48 h prior to case onset. Regression models provided further support for these patterns. The association between local watershed conditions and Norovirus outbreak risk suggest a source-water reservoir for this pathogen. We hypothesize that the reservoir may be maintained through the discharge of wastewater containing virus particles; wintertime seasonality may be explained by enhanced viral persistence at low temperatures.