During a survey of human rotaviruses in Okayama Prefecture, Japan in the 2011-2012 rotavirus season (between September 2011 and August 2012), G3P[8] was found to be a predominant genotype overall. However, G1P[8] emerged in the latter half of the season. To clarify the genetic background of the G1P[8] strains, the VP7, VP4, VP6, NSP4, and NSP5/6 genes of the strains were sequenced and genotyped. As a result, it was demonstrated that the strains with two different genotype constellations (G1-P[8]-I1-E1-H1 and G1-P[8]-I2-E2-H2) prevailed in the season. The G1P[8] strains possessing the DS-1-like VP6, NSP4, and NSP5/6 genes (the DS-1-like G1P[8] strains), which should reveal a short electropherotype, were originated from possible intergenogroup reassortment events. The DS-1-like G1P[8] strains accounted for 74.1% of all G1P[8] strains and were detected continuously throughout the season but not in the preceding season, indicating the possibility of new introduction and rapid spreading of these strains in the 2011-2012 season. The results suggest that the intergenogroup reassortants, considered generally unstable, can spread rapidly and become relevant.

Fecal specimens from patients with acute diarrhea were collected from 10 prefectures in Japan over a 6-month period (November 1992 to April 1993), and the specimens that were negative for human group A rotaviruses were screened for the presence of human group C rotaviruses (CHRVs) by the reverse passive hemagglutination test. Of 784 specimens examined, 53 samples (6.8%) that were collected in 7 of 10 prefectures were positive for CHRV, indicating that CHRVs are widely distributed across Japan. Most of the CHRV isolates were detected in March and April, and CHRVs mainly prevailed in children ages 3 to 8 years. The genome electropherotypes of eight strains isolated in five individual prefectures were surprisingly similar to each other and were different from those of CHRV strains isolated to date. The outer capsid glycoprotein (VP7) gene homologies of the isolates retrieved in 1993 were subsequently analyzed by the dot blot hybridization method. As a result, the VP7 genes of the isolates revealed very high levels of homology not only with each other but also with the VP7 gene of the OK118 strain isolated in 1988. These results suggest that a large-scale outbreak of CHRV occurred during the winter of 1992 and 1993 in Japan.

An enzyme-linked immunosorbent assay using monoclonal antibodies was established for the detection of human group C rotaviruses. Seventeen clinical samples which were found to contain group C rotaviruses were all strongly positive, whereas 9 samples containing group A rotaviruses and 51 samples lacking rotaviruses were all negative with this test.

In May 2000, an outbreak of acute gastroenteritis caused by human group C rotavirus (CHRV) occurred in a youth educational center located in the southern area of Okayama Prefecture. A total of 172 schoolchildren and teachers, who consisted of 51 persons belonging to F school and 121 persons belonging to K school, joined in an educational program at the center from May 24 to 26. Eighty-seven individuals (50.6%) of them showed clinical symptoms of gastroenteritis from May 24 to 30, and the outbreak peaked on May 27. The major clinical symptoms were abdominal pain (87.4%), diarrhea (50.6%), nausea or vomiting (21.8%), fever > 37 degrees C (12.6%), and headache (14.9%). The clinical symptoms of the patients in F school were more severe than those in K school. Thirty-two fecal specimens were collected from the patients and examined for gastroenteritis viruses by electron microscopy, ELISA, reverse passive hemagglutination test, and reverse transcription (RT)-PCR. As a result, CHRVs were detected in 21 specimens (65.6%) by RT-PCR. The possible route of the CHRV infection was thought to be a person to person transmission by following reasons: (i), CHRVs were detected in stools from patients who became ill on the first day of the program; (ii), CHRVs were not detected in stools from cooks; (iii), no possible causal food was found by epidemiological analysis of the outbreak. Furthermore, phylogenetic analysis of the VP7 gene among CHRVs isolated in Okayama revealed that the virus detected in this study was more closely related to the virus isolated from a sporadic case of gastroenteritis in 1996 than that isolated from an outbreak occurred in 1999.

Nucleotide sequences of the VP7 gene of human group C rotavirus were determined for two strains isolated in Okayama, Japan, during a 1988-to-1990 epidemic. These isolates, OK118 and OK450, were selected as prototypes of two different electropherotypes, patterns I and II, respectively. The genes were identical in size (1,063 bp), and both contained single open reading frames encoding 332 amino acids. The alignment of two sequences revealed 46 nucleotide substitutions, 11 of which were predicted to give amino acid changes. The deduced amino acid sequence of VP7 from OK118 was similar to published sequences of a Japanese isolate and three foreign isolates (more than 98.4% identity), whereas the VP7 sequence of OK450 revealed around 96% identity with these isolates and had nine unique amino acid substitutions. The VP7 genes of nine Okayama isolates were then analyzed by dot blot hybridization with the VP7 probes of OK118 and OK450. Under highly stringent conditions, the OK118 probe produced strong hybridization signals with the genes of five pattern I strains and one pattern II strain, while the OK450 probe strongly reacted only with those of three pattern II strains. Our results concluded that relative sequence diversity in the VP7 gene was observed between two different electropherotypes prevalent in a limited area.