For consumers and food safety professionals alike, noroviruses (family: Caliciviridae) are a serious issue. Although there are multiple routes for infection, foodborne disease is a major cause of outbreaks worldwide. According to a recent paper from the Centers for Disease Control (CDC) authored by Verhoef et al. (2015), almost 1 in 7 cases of norovirus disease come from eating contaminated food1. The authors came to this conclusion after tracking genotype against outbreak data gathered from worldwide databases.
There are six norovirus genogroups, I to VI, with approximately 38 different genotypes spread among them. Classification is according to molecular characterization looking at polymerase (P-type) or capsid (C-type) attributes. Verhoef et al. examined various disease surveillance databases in the United States, New Zealand and Europe, then cross referenced information from literature searches in peer reviewed journals. Using previous studies that associated norovirus genotype with routes of transmission, environmental stability, and other traits, they constructed population groups that could predict source of outbreak.
Although defining source of a norovirus outbreak is desirable, in practice it is difficult due to the multiple routes of transmission that can occur within a single outbreak. Food may indeed be a primary initiator of the disease, but person-to-person and environmental contamination can both occur thereafter. Furthermore, food contamination can occur at harvesting by environmental pollution (water, feces), during processing due to equipment sterilization failure and also by direct handling by infected people. Pinpointing the exact source is difficult.
However, previous studies have shown an association between specific norovirus genotypes and environmental sensitivity/stability2, showing that outbreaks in the United States implicate Norovirus genotypes GI.3, GI.6, GI.7, GII.3, GII.6 and G.12 most often in foodborne disease. Verhoef et al. sought to further define genotypic groupings to predict source of contamination by norovirus genotyping, gathering source data from various surveillance databases worldwide. They postulated that by knowing the genotype involved it would be easier for food safety surveillance to more correctly implicate foodborne as the route of infection.
|Database||Location||Years Searched||Total Cases Reviewed|
|FBVE/Noronet||European Union||1999-2012||5,583 (1,715)|
|CaliciNet||CDC – United States||2009-2012||3,094 (3,094)|
|EpiSurv||New Zealand||2008-2012||818 (584)|
|Literature Review||Global: peer-reviewed||1993-2012||966 (8)|
Table 1: Databases reviewed showing total cases for which genotyping and source data was available (final case number for analysis shown in brackets)
The researchers consulted three surveillance databases, Noronet, Calicinet and EpiSurv (Table 1), in addition to updating the systematic review of literature published in peer-reviewed journals between 1993 and 2011 made by Matthews et al. (2012)3. From these, they constructed genotype profiles, tested the global data and then estimated the probabilities of an undefined outbreak having foodborne infection as its source.
From the database searches, the scientists pulled out a total of 9,495 norovirus cases with an additional 966 meeting search criteria in the literature review (Table 1). The team exclude waterborne and environmentally caused outbreaks from the analysis. They also synchronized the time frames, examining only cases arising between 2009-2012 for the final analysis (see Table 1 for final numbers). Following statistical interrogation, Verhoef et al. grouped the outbreaks and genotype clusters, finding statistical significance for onward data analysis with the following categories (see Table 2):
|FBVE/Noronet||ESR/EpiSurv||CaliciNet||Global Incidence – Foodborne Norovirus|
|GII.4||7% (6-9%)||9% (6-11%)||12% (10-14%)||10% (9-11%)|
|All other single genotypes||31% (25-37%)||25% (17-33%)||26% (23-30%)||27% (25-30%)|
|Mixed genotypes including GII.4||75% (48-94%)||50% (20-75%)||16% (5-33%)||37% (24-52%)|
|Global incidence – foodborne Norovirus||12%||13%||16%||N/A|
Table 2: Outbreaks per category (2009-2012) according to genotype and database
Once established, the researchers grouped the global data from the three databases to find the percentage outbreaks caused by foodborne transmission (Table 2). Once the researchers established the global profiles, they analyzed 1,332 outbreaks during 2009-2012 where no known route of transmission was determined, using the genotype groupings to predict cause. The analysis shows that of these unknowns, 193 (14.5%) were probably due to a foodborne infection. This estimation agrees with findings in the individual databases.
Acknowledging the limitations of the study methods, the researchers suggest that genotyping can indeed provide important information for investigating norovirus outbreaks worldwide. The work also confirms previous data suggesting that GII.4 is more likely to be found in person-to-person spread than with primary foodborne outbreaks. Verhoef et al. advise that more work is required to complete the data maps for foodborne norovirus infections in order to develop improved safety strategies for the food industry.
2. Verhoef, L. et al. (2010) “Use of norovirus genotype profiles to differentiate origins of foodborne outbreaks“, Emerging Infectious Diseases 16 (pp.617–24) http://dx.doi.org/10.3201/eid1604.090723
3. Matthews, J.E. et al. (2012) “The epidemiology of published norovirus outbreaks: a review of risk factors associated with attack rate and genogroup“, Epidemiology Infect. 140 (pp.1161–72) http://dx.doi.org/10.1017/S0950268812000234