Effects and Clinical Significance of GII.4 Sydney Norovirus, United States, 2012–2013

Eyal Leshem

, Mary Wikswo, Leslie Barclay, Eric Brandt, William Storm, Ellen Salehi, Traci DeSalvo, Tim Davis, Amy Saupe, Ginette Dobbins, Hillary A. Booth, Christianne Biggs, Katie Garman, Amy M. Woron, Umesh D. Parashar, Jan Vinjé, and Aron J. Hall

Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (E. Leshem, M. Wikswo, L. Barclay, U.D. Parashar, J. Vinjé, A.J. Hall); Ohio Department of Health, Columbus, Ohio, USA (E. Brandt, W. Storm, E. Salehi); Wisconsin Department of Health Services, Madison, Wisconsin, USA (T. DeSalvo); Wisconsin State Laboratory of Hygiene, Madison (T. Davis); Minnesota Department of Health, St. Paul, Minnesota, USA (A. Saupe, G. Dobbins); Oregon Public Health Division, Portland, Oregon, USA (H.A. Booth, C. Biggs); Tennessee Department of Health, Nashville, Tennessee, USA (K. Garman, A.M. Woron)

Abstract

During 2012, global detection of a new norovirus (NoV) strain, GII.4 Sydney, raised concerns about its potential effect in the United States. We analyzed data from NoV outbreaks in 5 states and emergency department visits for gastrointestinal illness in 1 state during the 2012–13 season and compared the data with those of previous seasons. During August 2012–April 2013, a total of 637 NoV outbreaks were reported compared with 536 and 432 in 2011–2012 and 2010–2011 during the same period. The proportion of outbreaks attributed to GII.4 Sydney increased from 8% in September 2012 to 82% in March 2013. The increase in emergency department visits for gastrointestinal illness during the 2012–13 season was similar to that of previous seasons. GII.4 Sydney has become the predominant US NoV outbreak strain during the 2012–13 season, but its emergence did not cause outbreak activity to substantially increase from that of previous seasons.

Noroviruses (NoVs) are the most common cause of epidemic gastroenteritis worldwide and the leading cause of foodborne outbreaks in the United States (1–3). In the United States, NoVs cause 19–21 million illnesses and lead to 56,000–70,000 hospitalizations and 570–800 deaths each year (4). Severe disease associated with NoV occurs most frequently among older adults, young children, and immunocompromised patients (4–7). NoV outbreaks occur year round, but activity increases in the United States during the winter months; 80% of reported outbreaks occur during November–April (8,9).
NoVs belong to the family Caliciviridae and can be grouped into at least 5 genogroups (GI–GV), which are further divided into at least 35 genotypes (2,10). Most NoV outbreaks are attributed to genotype GII.4, which evolve rapidly over time (11,12). During the past decade, new GII.4 strains have emerged every 2–3 years and replaced previously predominant GII.4 strains. The emergence of new NoV strains is believed to be related, in part, to the antigenic diversity of the novel strain that leads to at least partial escape from preexisting herd immunity acquired against the predominant circulating strain (12). These new NoV strains have often, but not always, led to increased outbreak activity (8,10,13,14).
In March 2012, a new GII.4 NoV strain was identified in Australia. Named GII.4 Sydney, this emergent strain has since caused acute gastroenteritis outbreaks in New Zealand, Japan, Western Europe, and Canada (15–17). Preliminary indicators of increased NoV outbreak activity, including an increase in the number of confirmed cases and hospital-related outbreaks in late 2012, were presumed to be associated with emergence of GII.4 Sydney in several of those countries (15,17,18). In the United States, GII.4 Sydney became the predominant NoV strain implicated in outbreaks during the last 4 months of 2012 (19).
To assess whether the emergence of GII.4 Sydney strain was associated with an increase in overall NoV disease activity in the United States, we examined data from NoV outbreaks in 5 states and emergency department visits for gastrointestinal illness in 1 state during the 2012–13 season and compared these data with those of the 2 previous seasons. Furthermore, we compared epidemiologic (e.g., setting and mode of transmission) and clinical features of patients in outbreaks attributed to GII.4 Sydney with those of outbreaks attributed to other strains during the 2012–13 season.