International Foodborne Norovirus Outbreaks | CDC EID

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Volume 17, Number 3–March 2011
Research
An Integrated Approach to Identifying International Foodborne Norovirus Outbreaks1
Linda Verhoef, Roger D. Kouyos, Harry Vennema, Annelies Kroneman, Joukje Siebenga, Wilfrid van Pelt, and Marion Koopmans, on behalf of the Foodborne Viruses in Europe Network2

Author affiliations: National Institute for Public Health and the Environment (RIVM), Bilthoven, the Netherlands (L. Verhoef, H. Vennema, A. Kroneman, J. Siebenga, W. van Pelt, M. Koopmans); and Swiss Federal

Suggested citation for this article

Abstract
International foodborne norovirus outbreaks can be difficult to recognize when using standard outbreak investigation methods. In a novel approach, we provide step-wise selection criteria to identify clusters of outbreaks that may involve an internationally distributed common foodborne source. After computerized linking of epidemiologic data to aligned sequences, we retrospectively identified 100 individually reported outbreaks that potentially represented 14 international common source events in Europe during 1999–2008. Analysis of capsid sequences of outbreak strains (n = 1,456), showed that ≈7% of outbreaks reported to the Foodborne Viruses in Europe database were part of an international event (range 2%–9%), compared with 0.4% identified through standard epidemiologic investigations. Our findings point to a critical gap in surveillance and suggest that international collaboration could have increased the number of recognized international foodborne outbreaks. Real-time exchange of combined epidemiologic and molecular data is needed to validate our findings through timely trace-backs of clustered outbreaks.

Noroviruses are the most prevalent causative agents of acute viral gastroenteritis in the community (1–4). Currently, 5 norovirus genogroups have been described and subdivided into at least 40 genotypes (5,6), but in recent years, most clinical effects have been caused by viruses from a single genotype in genogroup II, GII.4 (7–10). The symptoms of norovirus disease are usually mild and self-limiting, but there is some evidence the disease can contribute to proportion of deaths (11,12). Infection occurs by way of the gastrointestinal tract after contact with infected persons, after ingestion of contaminated food or aerosols, or through environmental contamination (13,14).

Because the different modes of transmission call for quite distinct control measures, it is important to assess which proportion of disease can be attributed to which mode of transmission. However, this question is difficult to answer. Due to the high rate and rapidity of secondary spread of norovirus infection following a foodborne introduction, outbreaks initially linked to a food source may appear to be person-to-person (PTP) outbreaks by the time they are recognized. Even if a foodborne source is suspected, confirmation of the source is complicated. Virus detection in food commodities is possible but hampered by such factors as low levels of norovirus in food, food matrix complexity, genetic variability of norovirus (15), the absence of an efficient cell culture system to propagate human noroviruses (16), and the unavailability of leftover food for pathogen detection.

Given the globalization of the food market, diffuse international outbreaks are likely (17,18). For public health officials, these may seem to be regular PTP outbreaks because infection of 1 or a few persons with viruses through food consumption will go unnoticed unless secondary spread occurs or the contaminated food is consumed by multiple persons, which may trigger an investigation to identify a source. However, identification of international links is complicated. Viruses remain infectious in frozen ready-to-eat products over prolonged periods, and linked outbreaks are likely to be separated in time (19). Other problems are virus mutation rate, which results in nonidentical strains from a common source (20); sewage contamination with multiple nonsimilar strains during production of shellfish or crops (21); underreporting of cases (22,23); and incompleteness of outbreak reports (24,25). Other complicating factors include the unknown background level of viruses in foods, the environment, or asymptomatic shedders. Clearly, methods combining molecular and basic epidemiologic criteria are needed to assist public health efforts to identify international foodborne outbreaks.

For this reason, we performed a retrospective analysis of norovirus outbreak surveillance data collected since 1999 by Food-Borne Viruses in Europe (FBVE), a combined laboratory and epidemiology network (6). Although the name FBVE suggests a foodborne focus, the network actually investigates outbreaks of viral gastroenteritis with all modes of transmission. It seeks to obtain a comprehensive overview of viral activity in the community and to enable capture of foodborne norovirus outbreaks that have evaded recognition. Strain sequences from outbreaks linked to a common source are expected to be more similar than strains from outbreaks with a different source (26). We sought to quantify strain variability within and among molecular sequence clusters of multiple outbreaks to identify outbreaks with probable links to other outbreaks. Our goal was to retrospectively identify potential common-source events not detected by routine investigations and also to provide criteria that may assist in detecting such events.

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International Foodborne Norovirus Outbreaks | CDC EID


Suggested Citation for this Article
Verhoef L, Kouyos RD, Vennema H, Kroneman A, Siebanga J, van Pelt W, et al. An integrated approach to identifying international foodborne norovirus outbreaks. Emerg Infect Dis [serial on the Internet]. 2011 Mar [date cited].
http://www.cdc.gov/EID/content/17/3/412.htm


DOI: 10.3201/eid1703.100979



1Some of these data were presented as a poster during the 15th International Bioinformatics Workshop on Virus Evolution and Molecular Epidemiology, September 7–11, 2009, Rotterdam, the Netherlands.

2Members of the Foodborne Viruses in Europe Network who contributed to this study are listed at the end of this article.


Comments to the Authors
Please use the form below to submit correspondence to the authors or contact them at the following address:

Linda Verhoef, National Institute for Public Health and the Environment (RIVM), Postbak 22, PO Box 1, 3720 BA Bilthoven, the Netherlands; email: linda.verhoef@rivm.nl