Arthropod-Borne Encephalitides in Japan and Southeast Asia

Abstract

The occurrence of Japanese encephalitis (JE) virus in the birds and mosquitoes of a heron-egret colony near Tokyo was investigated during the summers of 1952 and 1953. It was established that JE virus was disseminated among certain nestling birds (notably Black Crowned Night Herons) and mosquitoes (Culex tritaeniorhynchus) simultaneously during early August each year. Natural infection of other closely related fowl (egrets) or of other culecine mosquitoes was rare or absent. An explanation for the selective infection of herons was suggested by the behavior of the vector mosquito, C. tritaeniorhynchus, which was shown by experiment to feed preferentially upon herons. While frequency of mosquito infection tended to parallel that of birds in any season, extent of involvement of both nestlings and mosquitoes varied in different years. Variation of avian infection rates from year to year may be related to incidence of passively immune birds in the colony, Thus when the majority of heron nestlings were protected by transovarially acquired antibody, as in 1953, no active infection was demonstrated in this species. The study, besides implicating certain wild birds as natural hosts for JE virus, indicates that avian infection is influenced in part by the ecologic relationship between birds and mosquitoes, as well as by immune status of the nestlings. The potential significance of this bird-mosquito infection cycle in the natural history of Japanese encephalitis is discussed.