Potential Transmission of Dengue Virus in Japan

The global burden of dengue is increasing at an alarming rate and increased international travel will lead to constant importation of dengue virus into non-endemic areas. The potential for dengue epidemics in such countries during seasons with permissive temperatures has already been underlined by epidemics in Japan and Madeira. While improved surveillance can help identify clinical cases of dengue, differentiating between imported and autochthonous cases remains problematical. Implementation of a threshold criterion can help in identifying aberrant incidences of dengue. This threshold approach was applied to dengue cases reported in the Japanese surveillance system from 2011–2019. Several aberrant incidences occurring during consecutive weeks were detected, one of which was concomitant to the Yoyogi Park Tokyo epidemic but in another area, Kanagawa, and another above threshold week was coincidental with a symptomatic case of a German traveller. This indicates autochthonous transmission. Despite the occurrence of several alert periods, however, on no occasion did the spread of dengue progress into a full epidemic as was seen in Yoyogi. It thus seems likely that Yoyogi Park was a particular event and that stochastic die-out of viruses is occurring frequently without progression, perhaps reflecting the negative impact of societal infra-structure on dengue transmission despite permissive temperatures. Implementation of a dengue epidemic threshold as used for seasonal influenza may provide a basis for future seroprevalence studies to assess the true prevalence of dengue in light of the high frequency of subclinical, asymptomatic infections.