Risk assessment of re-emerging Plasmodium falciparum on Ishigaki Island using a stochastic transmission model

ABSTRACT

On Ishigaki Island, <I>Plasmodium falciparum</I> and <I>Plasmodium vivax</I> 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 <I>P. falciparum</I> malaria patients stay on Ishigaki Island and also examined the re-emergence of the <I>P. falciparum</I> epidemic in 1951-1960 to determine the reliability of the model.<br>A stochastic transmission model of <I>P. falciparum</I> was constructed to detect a small number of infected persons. The seasonal fluctuation of the <I>Anopheles minimus</I> population obtained by observational data and meteorological data through statistical processing was introduced into the model.<br>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 <I>An. minimus</I> 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 <I>P. falciparum</I> infection.