Epidemics of GI.2 sapovirus in gastroenteritis outbreaks during 2012-2013 in Osaka City, Japan.

Sapovirus (SaV) is a causative agent of gastroenteritis in humans in both sporadic cases and outbreaks. During the period from January 2005 to August 2014, SaV was detected in 30 (5.9%) of 510 gastroenteritis outbreaks in Osaka City, Japan using real-time RT-PCR. Seasonal distribution of SaV-associated outbreaks revealed an increase during the 2011-2012 season and the highest frequency of outbreaks during the 2012-2013 season. Genotyping analysis based on the capsid region demonstrated that the most common genotype was GI.2 (36.7%), in which the strains were closely related. The comparison of complete capsid gene sequences with 18 GI.2 strains (7 strains in this study and 11 from GenBank) between 1990 and 2013 showed that GI.2 strains were classified into at least three genetic clusters (1990-2000, 2004-2007, and 2008-2013) with chronologically unique amino acid residues and accumulation of mutations in the predicted P domain, suggesting the one of the causes of emergence and spread of GI.2 strains. This study will also be helpful for understanding the evolutionary mechanism of the SaV genome.