Dissecting the origin of the 2014 dengue outbreak in Japan

Introduction: Endemic in at least 100 countries, dengue is currently regarded as world's most important mosquito borne viral disease. While most of the disease burden is limited to areas with tropical and sub-tropical climates, evidence suggests that temperate areas may be increasingly at risk as the geographic distribution of relevant vectors expands. Japan, a country with a temperate climate, reported the first major dengue outbreak in 2014. We examined the factors that may have facilitated the dengue outbreak in Tokyo during 2014. Methods: Multiple sequence alignment of the dengue virus 1 (DENV1) sequence from the 2014 dengue outbreak in Tokyo was carried out using a fast Fourier transformation method in MAFFT v6.940b. We collected the Japan National Tourism Organization’s data on inbound travelers between January and September 2014 from dengue endemic countries in Asia to Japan. Daily observations of temperature (minimum, maximum, and mean) and precipitation were obtained from the MIDAS dataset for Tokyo. We calculated the relative vectorial capacity (rVc) for Aedes vectors to quantify the dengue epidemic potential based on temperature dependent parameters, by applying a modified Ross-McDonald model. Findings: Tourist arrivals into Japan in 2014 coincided by 70% with its warm summer months suitable for dengue transmission, The phylogenetic similarity of DENV-1 isolated from the 2014 outbreak in Japan with viruses from China, Indonesia, Singapore, and Vietnam renders any of these four countries a likely source of importation. Several conducive climate factors converged preceding and during the time of the dengue outbreak in Tokyo, August until October 2014. Climate conditions, in particular mean temperature and precipitation, were favorable for the amplification of Aedes vectors. Furthermore, the ability for the vector to transmit dengue, as measured by the relative vectorial capacity, was highest at the time of the 2014 outbreak. Conclusions: Taking into account th