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

Abstract

Objectives: This 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.

Methods: In 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.

Results: Ten 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.

Conclusions: The 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.

Fig. 1

Numbers of Anopheles sinensis captured using a light-trap in Himi-shi, Toyama. The gray-solid, black-broken, and gray-broken lines indicate 1993, 1994, and 1995 observations, respectively, and the black-solid line indicates modification of the 1994 observations

Fig. 2

Model schemes for Plasmodium vivax transmission of infection in the first and second cycles. Dotted and dashed rectangles indicate the profiles of human and vector stages, respectively. Ovals and solid rectangles indicate the profiles of vector classes and human classes, respectively