Kaoru Fueda

Risk analysis of the re-emergence of Plasmodium vivax malaria in Japan using a stochastic transmission model.

ObjectivesThis study analyzed the risk of infection with Plasmodium vivax in local residents through a stochastic simulation in which an infected tourist, local resident, or immigrants from an endemic area would visit Himi-shi, Toyama prefecture, which is a formerly endemic area in Japan.MethodsIn Toyama, the habitats of Anopheles sinensis, which can transmit P. vivax, have been examined previously. We constructed a stochastic model of P. vivax transmission that can handle small numbers of infected persons and infected mosquitoes. The seasonal fluctuation in the numbers of captured An. sinensis was taken into account in the model.ResultsTen thousand trial simulations were carried out stochastically with a range of human blood indexes (HBI) of 1–10% for a range of months (June–September). The simulation results for a realistic assumption of a 1% HBI showed that the risk of infection for local residents was low (below 1%) except for the immigrants scenario.ConclusionsThe risk of infection among local residents (second cycle) was estimated to be very low for all situations. Therefore, there is little possibility for P. vivax infection to become established in this area of Japan.

How Did the Dengue Fever Outbreak Progress in Yoyogi Park, Tokyo, in 2014?—Evaluation Based on a Mathematical Model—

OBJECTIVES In the summer of 2014, an outbreak of autochthonous dengue fever occurred in Yoyogi Park and its vicinity, Tokyo, Japan. In this study, we investigated how the dengue fever outbreak progressed in Yoyogi Park using a mathematical model. METHODS This study was limited to the transmission of the dengue virus in Yoyogi Park and its vicinity. We estimated the distributions of the intrinsic incubation period and infection dates on the basis of epidemiological information on the dengue outbreak in 2014. We searched for an assumption that satisfactorily explains the outbreak in 2014 using rough estimates of secondary and tertiary infection cases. We constructed a mathematical model for the transmission of the dengue virus between humans and Aedes albopictus. RESULTS We carried out 1,000-trial stochastic simulations for all combinations of three kinds of assumption about Ae. albopictus and asymptomatic infection with each of three levels. Simulation results showed that the scale of the outbreak was markedly affected by the daily survival rate of Ae. albopictus. The outbreak involved a small number of secondary infection cases, reached a peak at tertiary infection, and transformed to termination at the fourth infection. Under some assumptions, the daily progress of onset cases was within a range between the 1st-3rd quartiles of 1,000 trials for 87% of dates and within a range between the minimum and maximum for all dates. CONCLUSIONS It is important to execute plans to detect asymptomatic cases and reduce the survival rate of Ae. albopictus to prevent the spread of tertiary infections unless an outbreak is suppressed at the secondary infection stage.