Ecologic Studies of Japanese Encephalitis Virus in Japan

Abstract

Summary At the Shinhama heronry and nearby Minamigyotoku village near Tokyo, Japan, in 1953, C. tritaeniorhynchus, the major vector of Japanese encephalitis virus in Japan, entered traps baited with Black-crowned Night Herons in numbers 3 to 15 times greater than traps containing egrets or chickens; Blue Magpies, Tree Sparrows, Dusky Thrushes, and Grey Starlings failed to attract significant numbers. In 1956, at two rural and one urban location near Tokyo, pigs attracted and were bitten by many more C. tritaeniorhynchus than BCNH or man. C. tritaeniorhynchus were found in traps as high as 50 feet above ground at the Sagiyama heronry. The behavior of the vector mosquito explained in part the different incidences of virus infection among bird species, the high incidence of swine infection in nature, and infection of nestling birds high in trees at Sagiyama. Whatever animal mechanism exists for attracting C. tritaeniorhynchus, it persisted in traps for 1 to 3 days after removal of the animal. Other mosquitoes behaved differently from C. tritaeniorhynchus. C. pipiens frequently entered traps with all seven species of birds and was attracted to BCNH in traps more than to pigs. A. sinensis was found in significant numbers only in pig-baited traps in rural areas. Other species were recovered only in small numbers. Microhabitats affected entry of mosquitoes into traps and necessitated study of multiple trap sites with duplicate traps. On one occasion C. tritaeniorhynchus collections were influenced by factors related to the individual trap per se and independent of trap design or location. These studies emphasized that predictions of virus ecology from knowledge of mosquito behavior alone may be erroneous.