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 ofnorovirus gastroenteritis in ambulatory US patients is not well understood.Methods. Cause-speciﬁed and cause-unspeciﬁed gastroenteritis emergency department (ED) and outpatientvisits 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 gastroenteritisvisitsdue tospeciﬁcpathogensother thannorovirus.Model residuals wereused toestimatenorovirusvisits.Market-Scan 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 visitswere 14 and 57 cases per 10000 persons, respectively, across all ages. Rates for ages 0–4, 5–17, 18–64, and ≥65years were 38, 10, 12, and 15 ED visits per 10000 persons, respectively, and 233, 85, 35, and 54 outpatientvisits per 10000 persons, respectively. Norovirus was estimated to cause 13% of all gastroenteritis-associated am-bulatory visits, with ∼50% of such visits occurring during November–February. Nationally, norovirus contributedto approximately 400000 ED visits and 1.7 million ofﬁce visits annually, resulting in $284 million in healthcarecharges.Conclusions. Norovirus is a substantial cause of gastroenteritis in the ambulatory setting.Keywords. norovirus; gastroenteritis; ambulatory care; United States.Noroviruses are the leading cause of acute gastroenter-itis (ie, diarrhea or vomiting) worldwide. The adventof molecular assays have shown that noroviruses arethe leading cause of foodborne disease [1], are respon-sible for approximately half of all gastroenteritis out-breaks [2], and are the leading cause of sporadicgastroenteritis across all ages [3,4].In the United States, noroviruses are estimated to cause21 million episodes of gastroenteritis every year [1 ]andare increasingly recognized as a cause of severe disease,leading to an estimated 71000 hospitalizations [ 5] and800 deaths annually [6].However, the absence of sensi-tive diagnostic tests in clinical settings has precluded acomprehensive description of the US norovirus diseaseburden. Notably, the contribution of norovirus to gas-troenteritis in the ambulatory setting is sparsely char-acterized. The few studies estimating the incidence ofnorovirus-associated outpatient visits across all ages inEurope and the United States have yielded rates of 40–640 cases per 10000 persons [3,7–9].An improved understanding of the role of norovirusamong healthcare encounters for gastroenteritis hasimportant implications for the practice and develop-ment of preventive strategies (eg, infection controlmeasures and vaccines) and could help guide clinicalmanagement (eg, antibiotics vs rehydration therapy).Our objective was to assess the contribution of norovi-rus to gastroenteritis-associated ambulatory visits inthe United States, including both ED and outpatientvisits, across all ages and to provide estimates of thenational burden.