Japanese encephalitis was first recognized in Japan in 1871, and epidemics recurred every few years until the late 1960s (Ishii, 1983; Okuno, 1978). All parts of the country were affected except Hokkaido, the northern most island. The worst epidemics occurred in the years immediately following World War II, with 3,000 to 5,000 cases per year and 1,000 to 2,000 deaths (representing an attack rate of 4 to 6 per 100,000). Children were predominantly affected. In the immediate postwar period, major annual epidemics also were recorded in Korea (2,000 to 6,000 cases per year) (Okuno, 1978; Paik, 1983). The maritime provinces of mainland China were also affected. In these latter two countries, the disease was confined largely to children under the age of 15.
Between 1967 and 1970 the geographic distribution of JE shifted dramatically. Annual morbidity rates in Japan dropped from 2–4 to 0.1–0.2 per 100,000, and the peak age-specific attack rate shifted to adults over age 50 (Ishii, 1983). Simultaneously, rates in Korea dropped from 5–30 to 1–2 per 100,000 (Okuno, 1978). This regional decline has been attributed to a combination of increased distribution of vaccine, altered agricultural practices and insecticide use, and improved housing standards. Rates have remained low in Japan, but in 1982 the southwestern provinces of Korea were struck by the first major epidemic there in 12 years, involving almost 3,000 children (attack rates 5 to 10 per 100,000 in affected provinces) (Paik, 1983).
During the same 4-year period between 1967 and 1970, epidemic JE was recognized for the first time in northern Vietnam, where annual morbidity rates for acute encephalitis jumped from 2–4 to 9–22 per 100,000 (Okuno, 1978). Simultaneously, rates in Thailand increased from less than 0.1 to 3–4 per 100,000 total population (Jatanesen, personal communication, 1985; Okuno, 1978). In Thailand, epidemic JE was confined to the northern parts of the country, where rates reached 10 to 30 per 100,000; among children in northern Thailand, annual rates in excess of 100 per 100,000 have been recorded. Epidemics continue to recur in these countries. Three-fourths of all cases are among children less than 15 years old. In China, reported cases of JE, relatively stable at 2,000–9,000 cases per year, dramatically increased during the late 1960s, to 20,000–40,000 cases per year. Since 1975 the incidence of cases has declined to a relatively stable 10,000–15,000 cases per year (Quan, 1983). All provinces except the two most western, Xinjiang and Xizong, have the disease (Huang, 1982).
JEV has been known (by virus isolation) to exist in southern India and Sri Lanka for decades (Carey, 1969), but the proportion of the total number of acute encephalitis cases attributable to JE is uncertain; it may be less than 30 percent (Vitarana, 1982). In 1978, a major epidemic of JE was recorded in the northern India states of Bihar and Uttar Pradesh, with 7,600 recorded cases (Mathur et al., 1982; Rodrigues, 1982). Since then, smaller epidemics have recurred annually. Concurrently with the north Indian epidemics, JE appeared in the nearby