Methods: Patients were diagnosed with CHIK fever by a combination of virus isolation, viral RNA amplification, IgM antibody-, IgG antibody-, and/or neutralizing antibody detection. The whole-genome sequences of the CHIKV isolates were determined by next-generation sequencing.
Results: Prior to 2014, the source countries of the imported CHIK fever cases were limited to South and Southeast Asian countries. After 2014, when outbreaks occurred in the Pacific and Caribbean Islands and Latin American countries, there was an increase in the number of imported cases from these regions. A phylogenetic analysis of 14 isolates revealed that four isolates recovered from three patients who returned from Sri Lanka, Malaysia and Angola, belonged to the East/Central/South African genotype, while 10 isolates from 10 patients who returned from Indonesia, the Philippines, Tonga, the Commonwealth of Dominica, Colombia and Cuba, belonged to the Asian genotype.
Conclusion: Through the phylogenetic analysis of the isolates, we could predict the situations of the CHIK fever epidemics in Indonesia, Angola and Cuba. Although Japan has not yet experienced an autochthonous outbreak of CHIK fever, the possibility of the future introduction of CHIKV through an imported case and subsequent local transmission should be considered, especially during the mosquito-active season. The monitoring and reporting of imported cases will be useful to understand the situation of the global epidemic, to increase awareness of and facilitate the diagnosis of CHIK fever, and to identify a future CHIK fever outbreak in Japan.