[Recent data on the epidemiology of Japanese encephalitis]

Japanese encephalitis is an arbovirosis the incidence and geographic distribution of which are increasing in rural areas of tropical and temperate Asia. A total of about 45,000 to 50,000 clinical cases occur annually. The Flavivirus responsible for the disease shows birds as usual hosts, and Culex mosquitoes as vectors. After a first amplification cycle in birds, the virus can be transmitted to domestic pigs, then to man. This scheme, however, shows large variations in the different regions, according to the climate which determines the dynamics of mosquito and bird populations, and to the ways of life of human populations, particularly for the rice-growing technics and pig breeding. The epidemiologists schematically distinguish tropical endemic areas, subtropical endemic-epidemic zones, and temperate epidemic zones. However, our knowledge on the epidemiology of Japanese encephalitis carries many incompletely understood aspects, for instance the reasons of the actual geographic distribution of the disease, or the mechanisms for persistence of the virus between epidemics. Furthermore, these epidemiological situations are changing with time, particularly with the-development of the new agrosystems linked with demographic increase, while recent technics of genomic analysis allow the recognition of several viral genotypes. The prevention of Japanese encephalitis presently involves vector control, vaccination and, in some cases, particular modifications of environment. Each of these measures shows proper logistic, technical or financial difficulties, which often prevent their generalized use. However, one can observe that, in the countries where vaccination is systematically carried out, the incidence of the disease seems considerably decreasing, while elsewhere it shows a tendency to increase. The main difficulties lie in the high cost of the vaccine and in the weight of immunization scheme, which make uneasy its integration in the Expanded Vaccination Programme. For these reasons, the virologists are looking for other vaccine types, particularly recombinant vaccines relying on obtaining, by genetical manipulation, envelop proteins with protection effect. Finally, remains the question of immunization of travellers and expatriate people. It seems desirable to recommand this vaccine only to the really exposed persons; this means persons performing a rather long stay in a rural area of an endemo-epidemic region, or during the season of transmission in an epidemic region. However, the risk cannot be completely absent, and it is necessary to draw attention to other methods aiming at decreasing man-vector contact.