On Ishigaki Island, Plasmodium falciparum and Plasmodium vivax epidemics occurred in 1945-1946 and were successfully suppressed. The epidemic re-emerged in 1949 because many settlers immigrated to the former endemic areas, but it terminated in 1961. The present study aimed at predicting an outbreak of a new epidemic based on the situation in which P. falciparum malaria patients stay on Ishigaki Island and also examined the re-emergence of the P. falciparum epidemic in 1951-1960 to determine the reliability of the model.
A stochastic transmission model of P. falciparum was constructed to detect a small number of infected persons. The seasonal fluctuation of the Anopheles minimus population obtained by observational data and meteorological data through statistical processing was introduced into the model.
Simulations were carried out to predict the risk of a new epidemic with scenarios in which the attribute of index patient, visiting season, and reduced inoculation rates of An. minimus were assumed. When an infected person visited the island in summer, a small number of patients with primary infections derived from the index patient appeared for all 1,000-iterations. On the other hand, when an infected person visited the island in winter, few or no patients with primary infections appeared for any of the 1,000-iterations because of the low mosquito density. In realistic conditions, the simulation results showed that there was little possibility of the occurrence of P. falciparum infection.