Different genotypes of C1, D3, D5, and H1 were isolated in outbreaks of 1984, 1987-1988, 1991-1993, and 2001, respectively, when the previous circulating genotype was replaced successively by a new genotype, through molecular studies of measles since 1984 in Japan. In March 2007, several patients with measles were observed in outpatient clinics, who were all young adolescents in high school and university students. The outbreak expanded subsequently throughout Japanese districts in May and is still ongoing in 2008. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) was used to detect the measles genome from 18 clinical samples obtained from patients suspected of modified measles infection with a very mild febrile illness. The measles genome was detected in nine patients by reverse transcription polymerase chain reaction (RT-PCR) and in 12 patients by RT-LAMP. Six measles strains were isolated in the 2007-2008 outbreak and identified as the D5 genotype (MVi/Bangkok.THA/93 type) different from the D5 sub-cluster (MVi/Palau.BLA/93 type) isolated in 1990-2005. Similar Bangkok type D5 strains were isolated in Phnom Penh in 2002 and in Taiwan in 2003, suggesting that the D5 strains might have been introduced via South East Asia, rather than resulting from the accumulation of mutations in the D5 strains of 1990-2005. One D9 strain was isolated from a sporadic case in Aichi in 2006. There was no difference in the antigenicity of the D9 and D5 strains in comparison with the vaccine strain. Infrastructure of systematic laboratory-based surveillance system should be established in order to confirm measles virus infection in Japan.
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