ON SUSCEPTIBILITY OF JAPANESE WILD BIRDS FOR JAPANESE B. ENCEPHALITIS VIRUS

Abstract

We have previously reported on the studies of the pattern of Japanese B encephalitis virus in mosquitoes (Culex pipiens var, pallens and Culex tryteniorhyncus), and various relationships between these species of mosquitoes and the virus, and concluded that Japanese B encephalitis virus which was sucked by mosquitoes reappered after a certain period of external incubation, and that the final concentration of the virus in mosquito bodies depends on the concentration of the virus in the first feeding to the mosquitoes. It was further concluded that the experimental transmission of the virus to mice through bite of these virus-sucking mosquitoes are only possible (10%) when the concentration of virus in the mosquito is high. We have considered, therefore, that there must be certain species of animals in nature, whose blood contained Japanese B encephalitis virus in high concentration as sources of the virus to these mosquitoes, if the latter had transmitted the virus in nature. In the same year (1941), W. McD. Hammon and W. C. Reeves in America, demonstrated the St. Louis virus and the western equine encephalomyelitis virus in the mosquitoes (Culex tarsalis), which were captured in Yakima Valley, Washington, and neutralizing antibodies in the blood of chickens in the same area, and proposed the hypothesis of infection cycle for these viruses (mosquito-chicken-mosquito) .These authors later concluded however, that the chickens were i not true reservoirs or latent carriers, though the possibility had not been ruled out of an occasional fowl having an unusual course of infection, because the domestic birds such as chicken, ducks and doves could not maintain these viruses in the blood for a long period, and they could not domenstrate the viruses in hibernated mosquitoes or transovalian infection of these viruses and mosquitoes.From the results of our foundamental experiments on the relationships between mosquitoes and J apanese B encephalitis virus and of the studies carried out by Hammon et al. in America, we thought that it was necessary to search for the reservoir of Japanese B encephalitis virus in Japan, from the standpoint of epidemiology and of prophylaxis against this disease. In the summer of 1947, Dr. Hammon, Dr. Casals and Dr. Meiklejohn came to Japan and, after a conference with these investigaters, and with the Virus and Rickettsia Committee of G. H. Q., staff of 406th Gen. Med. Lab. and N. I. H, it was decided to carry out the studies herein reported. We are indepted to Dr. W. McD. Hammon, Dr. J: Casals, Dr. G. Meiklejohn, Mr. Austine, Dr. H. Hamlin and Dr. Uchida for many valuable suggestions and assistance in our studies.