Emergence of a new norovirus GII.4 variant and changes in the historical biennial pattern of norovirus outbreak activity in Alberta, Canada, from 2008 to 2013

Abstract

The public health impact of the emergence of new norovirus (NoV) strains is uncertain. A biennial pattern of alternating quiescent and epidemic levels of NoV outbreak activity associated with the emergence of new GII.4 variants was observed in Alberta, Canada, between July 2000 and June 2008. In this study, NoV genogroup I (GI) and GII strains isolated from 710 outbreak specimens in Alberta between July 2008 and January 2013 were characterized to update historical data. The seasonality and annual variation in NoV outbreak burden were analyzed over a 10-year period (July 2002 to June 2012). We found that GII.4-2006b had persisted as the predominant variant over three observation periods (July 2006 to June 2009) during which the biennial NoV outbreak pattern continued. The emergence of GII.4-2010 (winter 2009) was not associated with increased outbreak activity, and outbreak activity between July 2009 and June 2012 when GII.4-2010 predominated (67.5 to 97.7%) did not follow a biennial pattern. GII.4-2012 first emerged in Alberta in September 2011 and became predominant in observation period July 2012 to June 2013. NoV GI, relatively rare in past years, had a higher activity level (37.3%) as represented by GI.6 and GI.7 in the winter of 2012 to 2013. A higher proportion of GI outbreaks occurred in non-health care facility settings compared to GII. Our study suggests that factors other than new variants emergence contribute to the levels of NoV outbreak activity in Alberta.

Fig 1

Monthly distribution of norovirus outbreaks by genogroups in Alberta, Canada, 1 July 2008 to 31 January 2013.

Fig 2

GII.4 variants identified in norovirus outbreaks in Alberta, Canada, 1 July 2002 to 31 January 2013.

Fig 3

Phylogenetic analysis of complete VP1 amino acid sequences of GII.4 variants identified in Alberta. The names used by the Norovirus Genotyping Tool are shown in parentheses. The scale bars represent the number of substitutions per site.

Fig 4

Neighbor-joining tree of NoV GI sequences (region C) from samples identified in Alberta. The phylogenetic analysis was performed using the Kimura two-parameter model with gamma distribution of rate variation among sites and 1,000 bootstrap replicates. The scale bars represent the number of substitutions per site.

Fig 5

Numbers of norovirus-positive and -negative outbreaks in Alberta, Canada, by observation period: 1 July 2002 to 31 January 2013.