Ecologic Studies of Japanese Encephalitis Virus in Japan

Abstract

Summary In 1956 and 1957, 98 to 100% of pigs on farms near Sagiyama north of Tokyo and in mosquito traps at Sagiyama, Tokyo, Zama and Shinhama became infected by JE virus during August. In 1957 extensive studies failed to detect swine infection between April and August. In two rural areas in 1956, swine became infected 2 to 3 weeks before those in Tokyo. Pigs were identified as a major natural source of JE virus for the vector mosquito, Culex tritaeniorhynchus, near Tokyo on the basis of: a) a high incidence of natural swine infection; b) the occurrence of viremia following natural infection; c) demonstration in the laboratory that viremia lasts up to 4 days11 and occurs in titers adequate to infect colonized C. tritaeniorhynchus and cause them to transmit virus to pigs and herons;5 d) the striking propensity of C. tritaeniorhynchus to bite pigs in nature;12 and e) the large, yearly-replenished populations of susceptible pigs near Tokyo.