Prevalence of "Norwalk-like virus" infections in outbreaks of acute nonbacterial gastroenteritis observed during the 1999-2000 season in Osaka City, Japan.

We have investigated the incidence of Norwalk-like viruses (NLVs) associated with outbreaks of acute nonbacterial gastroenteritis in Osaka City, Japan, since April 1996 using reverse transcription (RT)-PCR and electron microscopy methods. From the results of the first 3 years, between April 1996 and March 1999, we previously reported that multiple genetic types of NLVs were detected in 71.9% of outbreaks using RT-PCR with Ando's primers except for one outbreak [Iritani et al., 2000]. However, during the 1999-2000 season, NLV outbreak strains, which could not be detected by RT-PCR with Ando's primers, were increased. From probe typing and sequence analysis, 76.9% of these undetectable outbreak strains were classified into the P1-B type and the others were untypable. These untypable strains were closely related with Alphatron type strains detected in the Netherlands. The P2-B probe type of the NLV outbreak strains was predominant (88.2%) in the 1999-2000 season. The phylogram based on the 81 nucleotide sequences from these P2-B outbreak strains formed 2 clusters closely related with Lordsdale virus. The dominant genetic type of the P2-B outbreak strains, during the 1996-1997 season in Osaka City, belonged in one of these 2 clusters. These findings of the emergence of NLVs escaping the RT-PCR method strongly indicated the importance of probe typing and sequence analysis to survey NLV infections. Our surveillance of NLV infection in the outbreaks, for these 4 years, showed that the predominant probe type and dominant genetic type of NLV outbreak strains changed each season.