Estimation of reproduction number and probable vector density of the first autochthonous dengue outbreak in Japan in the last 70 years.

Abstract

The first autochthonous case of dengue fever in Japan since 1945 was reported on August 27, 2014. Infection was transmitted by Aedes albopictus mosquitoes in Tokyo's Yoyogi Park. A total of 65 cases with no history of overseas travel and who may have been infected around the park were reported as of September 5, 2014. To quantify infection risk of the local epidemic, the reproduction number and vector density per person at the onset of the epidemic were estimated. The estimated probability distribution and the number of female mosquitoes per person (MPP) were determined from the data of the initial epidemic. The estimated distribution R(0i) for the initial epidemic was fitted to a Gamma distribution using location parameter 4.25, scale parameter 0.19, and shape parameter 7.76 with median 7.78 and IQR (7.21-8.40). The MPP was fitted to a normal distribution with mean 5.71 and standard deviation 0.53. Both estimated reproduction number and vector density per person at the onset of the epidemic were higher than previously reported values. These results indicate the potential for dengue outbreaks in places with elevated vector density per person, even in dengue non-endemic countries. To investigate the cause of this outbreak, further studies will be needed, including assessments of social, behavioral, and environmental factors that may have contributed to this epidemic by altering host and vector conditions in the park.