Epidemiology of Foodborne Norovirus Outbreaks, United States, 2001–2008

Aron J. Hall

, Valerie G. Eisenbart, Amy Lehman Etingüe, L. Hannah Gould, Ben A. Lopman, and Umesh D. Parashar

Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (A.J. Hall, V.G. Eisenbart, A. Lehman Etingüe, L.H. Gould, B.A. Lopman, U.D. Parashar); University of Illinois, Urbana, Illinois, USA (V.G. Eisenbart); and North Carolina State University, Raleigh, North Carolina, USA (A. Lehman Etingüe)

Abstract

Noroviruses are the leading cause of foodborne illness in the United States. To better guide interventions, we analyzed 2,922 foodborne disease outbreaks for which norovirus was the suspected or confirmed cause, which had been reported to the Foodborne Disease Outbreak Surveillance System of the Centers for Disease Control and Prevention during 2001–2008. On average, 365 foodborne norovirus outbreaks were reported annually, resulting in an estimated 10,324 illnesses, 1,247 health care provider visits, 156 hospitalizations, and 1 death. In 364 outbreaks attributed to a single commodity, leafy vegetables (33%), fruits/nuts (16%), and mollusks (13%) were implicated most commonly. Infected food handlers were the source of 53% of outbreaks and may have contributed to 82% of outbreaks. Most foods were likely contaminated during preparation and service, except for mollusks, and occasionally, produce was contaminated during production and processing. Interventions to reduce the frequency of foodborne norovirus outbreaks should focus on food workers and production of produce and shellfish.

Noroviruses, the leading cause of foodborne illness in the United States, are responsible for an estimated 58% of all domestically acquired foodborne illness from known agents (1). The estimated 5.5 million annual foodborne norovirus illnesses in the United States, which constitute those associated with recognized outbreaks and those considered sporadic, result annually in 15,000 hospitalizations and 150 deaths and cost ≈$2 billion in health care expenses and lost productivity (1,2). Noroviruses are also the leading cause of foodborne disease outbreaks reported in the United States, accounting for about half of all foodborne outbreaks in which an etiologic agent is identified (3–5). Classified into the genus Norovirus within the family Caliciviridae, noroviruses are a genetically diverse group of nonenveloped, single-stranded RNA viruses, comprising at least 5 genogroups (GI–GV) and >35 genotypes (6). Since 2001, noroviruses within the GII.4 genotype have caused most viral gastroenteritis outbreaks worldwide (7).
Noroviruses can be spread through a variety of means, including direct person-to-person transmission through the fecal–oral route; ingestion of aerosolized vomitus; and indirect transmission through contaminated surfaces, food, or water. Norovirus has a low infectious inoculum (>18 viral particles) and is shed copiously by ill persons (10⁵–10¹¹ viral copies per gram of feces), which enables its rapid and efficient spread (8–10). Noroviruses also remain infectious on surfaces for as long as 2 weeks and in water for >2 months (11,12) and are resistant to many common disinfectants (13,14). Foods can be contaminated with noroviruses at any point along the farm-to-fork continuum, although the most frequent pathways are thought to be through an infected food handler or exposure to water contaminated with fecal matter (e.g., surface water used for produce irrigation or water containing sewage discharge where shellfish grow) (15–17). Bivalve mollusks, such as oysters, bioaccumulate noroviruses in their body through filtration and selective binding mechanisms and therefore are readily contaminated when they are grown in harvesting areas contaminated with human feces (18).
Attribution of norovirus disease to specific foods and increasing understanding of the various contamination pathways that result in disease can help identify potential targets for interventions. Although most foodborne norovirus disease in the United States is not outbreak associated, outbreaks provide the most robust information about the foods that cause illness and the factors contributing to their contamination. The last published description of foodborne norovirus outbreaks in the United States was based on surveillance data from 1991 through 2000, before molecular diagnostic tools were widely available (19). Since that time, norovirus diagnostics have become incorporated more routinely into public health outbreak investigations, resulting in markedly increased recognition of norovirus illnesses and outbreaks. Using more recent surveillance data, we therefore sought to robustly describe foodborne norovirus outbreaks, including temporal, geographic, and demographic trends, and attribution to specific foods, settings, and contamination factors.