An outbreak of measles virus infection due to a genotype D9 at a junior high school in Yamagata, Japan in 2004.

We investigated a measles virus (MV) outbreak that occurred at a junior high school in Yamagata, Japan between January and February, 2004. We received throat swab specimens from three patients at this school and carried out virus isolation with Vero/hSLAM cells and virus genome detection by reverse-transcription polymerase chain reaction. As a result, we isolated the virus from one patient and succeeded in amplifying the MV genome from the others. Further sequence analysis of the N gene revealed that these viruses were completely identical, and that their genotype could be characterized as type D9, which has not been reported in Japan previously. We also identified D9 viruses in two students at other junior high schools in Yamagata. These results suggested that D9 strains were imported from a region outside Japan. The genotypes of MVs found in Yamagata have changed in recent years, with D5 predominating in 2001 and H1 predominating in 2002 and 2003 as reported as national surveillance data. Therefore, we should monitor carefully to be sure that D9 strains do not become the next predominant virus. The more the number of measles cases decrease, the more important become the roles of public health laboratories, which genotype MVs and monitor their circulation and transmission pathways.