Genome sequence based molecular epidemiology of unusual US Rotavirus A G9 strains isolated from Omaha, USA between 1997 and 2000.

After discovery in the early 1980s, Rotavirus A serotype G9 was detected infrequently for almost a decade. Since the mid-1990s, however, serotype G9 has emerged to become a globally common strain linked to the introduction of a single, new genetic variant of G9 VP7 gene. Studies have demonstrated that genetically divergent G9 strains co-circulated at low frequency with the emerging variants. Examples include unique U.S. G9 strains Om46/Hu/USA/1998 and Om67/Hu/USA/1998, isolated in Omaha during the 1997-1998 rotavirus season, that are more closely related phylogenetically to reference strains from the 1980s than to most emerging G9 strains from the U.S. and globally. Here, we sequenced the VP7 full open reading frame for all available G9 strains (n=12) identified in Omaha during 1996-2000 seasons to investigate their epidemiology and evolution. In addition, the full or partial length open reading frames of the remaining 10 genes for five divergent Om46-like strains and one modern G9 variant were sequenced to evaluate their potential origin. Our findings suggest that Om46-like G9 strains may have been introduced into humans recently, perhaps in 1997-1998 when it was first detected, and the presumed original host of this VP7 gene variant may have been an animal species based on the unexpected detection of porcine rotavirus related NSP2 gene in the genome. The relatively high fitness of Om46-like strains during the 1997-1998 rotavirus season, 1 year after the globally important G9 variant was documented to be already spreading in the study area and other sites of the United States, appears to parallel findings on seasonal replacement of various genetic and antigenic variants of other common human rotavirus antigen specificities.