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New Norovirus Strains Account for Epidemics

Summary and Comment |
February 20, 2008

New Norovirus Strains Account for Epidemics

  1. Neil M. Ampel, MD

Two new strains were responsible for 73% of norovirus-associated gastroenteritis in Australia and New Zealand during the first half of 2006.

  1. Neil M. Ampel, MD

Norovirus (NoV) is the most important cause of acute gastroenteritis worldwide. Outbreaks occur frequently in closed environments such as nursing homes and cruise ships, and transmission is via several routes, including person-to-person spread and consumption of contaminated food. NoV consists of five genogroups; three of them (GI, GII, and GIV) are associated with human disease, and one (GII) has accounted for all global NoV gastroenteritis epidemics since the 1990s.

Early in 2006, the number of NoV gastroenteritis outbreaks increased in Australia, New Zealand, and Europe. To examine this situation, researchers determined the genotype of 186 of 231 NoV isolates obtained from patients in Australia and New Zealand who had acute gastroenteritis during this period.

Six GII genotypes were identified, with GII.4 accounting for 86% of sequenced viruses. Further genetic analysis identified four clusters of GII.4. Two clusters involved previously identified strains and accounted for 15% of the GII.4 sequences. The remaining GII.4 sequences belonged to two novel clusters. One of these clusters (115 sequences derived from patients in all 3 Australian states studied and across New Zealand; 62% of the samples analyzed) was most closely related to a European variant called 2006a. The other (21 sequences, all from New South Wales; 11% of the samples analyzed) was genetically similar to variant 2006b, which was originally identified in the U.K. Although each of these new strains was related to an earlier epidemic strain of NoV GII.4, the two were not closely related to each other.

Comment

Global epidemics of NoV gastroenteritis are increasing in frequency. As the authors point out, the evolution of NoV appears to be similar to that of influenza A virus, in which new strains emerge every 2 to 3 years. Of note, the 2006a and 2006b strains of GII.4 were each associated with a large proportion of the norovirus outbreaks in the U.S. during 2006 and 2007 (MMWR Morb Mortal Wkly Rep 2007; 56:842).

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