Case-based surveillance enhanced with measles virus detection/genotyping is essential to maintain measles elimination in Aichi Prefecture, Japan.
BACKGROUND
Japan was verified as having achieved measles elimination by the Measles Regional Verification Commission in the Western Pacific Region in March 2015. Verification of measles elimination implies the absence of continuous endemic transmission. After the last epidemic in 2007 with an estimated 18,000 cases, Japan introduced nationwide case-based measles surveillance in January 2008. Laboratory diagnosis for all suspected measles cases is essentially required by law, and virus detection tests are mostly performed by municipal public health institutes. Despite relatively high vaccination coverage and vigorous response to every case by the local health center staff, outbreak of measles is repeatedly observed in Aichi Prefecture, Japan.


METHODS
Measles virus N and H gene detection by nested double RT-PCR was performed with all specimens collected from suspected cases and transferred to our institute. Genotyping and further molecular epidemiological analyses were performed with the direct nucleotide sequence data of appropriate PCR products.


RESULTS
Between 2010 and 2014, specimens from 389 patients suspected for measles were tested in our institute. Genotypes D9, D8, H1 and B3 were detected. Further molecular epidemiological analyses were helpful to establish links between patients, and sometimes useful to discriminate one outbreak from another. All virus-positive cases, including 49 cases involved in three outbreaks without any obvious epidemiological link with importation, were considered as import-related based on the nucleotide sequence information. Chain of transmission in the latest outbreak in 2014 terminated after the third generations, much earlier than the 2010-11 outbreak (6th generations).


CONCLUSIONS
Since 2010, almost all measles cases reported in Aichi Prefecture are either import or import-related, based primarily on genotypes and nucleotide sequences of measles virus detected. In addition, genotyping and molecular epidemiological analyses are indispensable to prove the interruption of endemic transmission when the importations of measles are repeatedly observed.
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan. Electronic address: hiroko_minagawa@pref.aichi.lg.jp.
Hiroko
H
Minagawa
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Yoshihiro
Y
Yasui
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Hirokazu
H
Adachi
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Miyabi
M
Ito
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Emi
E
Hirose
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Noriko
N
Nakamura
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Mami
M
Hata
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Shinichi
S
Kobayashi
Aichi Prefectural Institute of Public Health, Tsuji-cho 7-6, Kita-ku, Nagoya 462-8576, Japan.
Teruo
T
Yamashita
D060740:Disease Eradication / Q000379:methods*; D004196:Disease Outbreaks; D062665:Epidemiological Monitoring; D005260:Female; D005838:Genotype; D006801:Humans; D007564:Japan / Q000453:epidemiology; D008297:Male; D008457:Measles / Q000175:diagnosis / Q000453:epidemiology* / Q000517:prevention & control* / Q000821:virology; D008459:Measles virus / Q000235:genetics* / Q000302:isolation & purification*; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D012367:RNA, Viral / Q000235:genetics; D017422:Sequence Analysis, DNA
D012367:RNA, Viral
Elimination; Genotype; Importation; Measles; Surveillance