Burden of norovirus gastroenteritis in the ambulatory setting--United States, 2001-2009.

BACKGROUND: Gastroenteritis remains an important cause of morbidity in the United States. The burden of norovirus gastroenteritis in ambulatory US patients is not well understood. METHODS: Cause-specified and cause-unspecified gastroenteritis emergency department (ED) and outpatient visits during July 2001-June 2009 were extracted from MarketScan insurance claim databases. By using cause-specified encounters, time-series regression models were fitted to predict the number of unspecified gastroenteritis visits due to specific pathogens other than norovirus. Model residuals were used to estimate norovirus visits. MarketScan rates were extrapolated to the US population to estimate national ambulatory visits. RESULTS: During 2001-2009, the estimated annual mean rates of norovirus-associated ED and outpatient visits were 14 and 57 cases per 10 000 persons, respectively, across all ages. Rates for ages 0-4, 5-17, 18-64, and ≥65 years were 38, 10, 12, and 15 ED visits per 10 000 persons, respectively, and 233, 85, 35, and 54 outpatient visits per 10 000 persons, respectively. Norovirus was estimated to cause 13% of all gastroenteritis-associated ambulatory visits, with ~50% of such visits occurring during November-February. Nationally, norovirus contributed to approximately 400 000 ED visits and 1.7 million office visits annually, resulting in $284 million in healthcare charges. CONCLUSIONS: Norovirus is a substantial cause of gastroenteritis in the ambulatory setting.