An epidemiological review of Japanese encephalitis.

Twenty-five years ago, Japanese encephalitis (JE) was known as an endemic, mosquito-borne disease in East Asia. Today, the causative virus is known to be distributed from maritime Siberia to the north, eastern India to the west, the Archipelago of Japan presumably the Philippines to the east, and from Indonesia to the south. Since the late-1960s, the geopathological status of JE epidemics has undergone considerable changes. The sizes of JE epidemics in Japan and China (Province of Taiwan) have steadily declined. In fact the JE virus itself appears to be disappearing from Japan. Long-term prediction of JE epidemics is more difficult for Korean Peninsula, nevertheless, reported JE morbidity rate in the Republic of Korea has remained at a relatively low level since 1969. In contrast to the trend in East Asia, new epidemic foci of JE have been emerging in the northern part of Tropical Eastern South Asia starting in 1969. Of particular importance are the continued high incidence (annual morbidity rate: 8.67-22.04/100 000) of reported JE in the northern part of Viet Nam between 1969 and 1974, and the high incidence (14.7/100 000) recorded in Chiang Mai Valley, Thailand, between 1969 and 1970. Apparently, the epidemic in Chiang Mai Valley spread to the neighbouring Shan State of Burma in 1974. Another JE epidemic broke out in West Bengal State, India in 1973. The occurrence of JE in the endemic zone south of these areas has remained sporadic. An increasing number of pathogens have been shown to cause signs and symptoms clinically indistinguishable from JE. In this review, the quality and international comparability of available JE statistics are also examined. Only a few countries and areas with reasonably developed statistical and laboratory services are able to provide national JE statistics in a form ready for epidemiological analysis. A practical surveillance system for JE needs to be organized in those countries where JE is a newly emerging health problem.