RECENT DATA ON THE EPIDEMIOLOGY OF JAPANE SE ENCEPHALITIS

Japanese encephalitis is an arbovirosis the incidence and geographic d istribution of which are increasing in rural areas of tropical and tem perate Asia. A total of about 45 000 to 50 000 clinical cases occur an nually. The Flavivirus responsible for the disease shows birds as usua l 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 re gions, according to the climate which determines the dynamics of mosqu ito and bird populations, and to the ways of life of human populations , particularly for the rice-growing technics and pig breeding. The epi demiologists schematically distinguish tropical endemic areas, subtrop ical endemo-epidemic zones, and temperate epidemic zones. However, our knowledge on the epidemiology of Japanese encephalitis carries many i ncompletely understood aspects, for instance the reasons of the actual geographic distribution of the disease, or the mechanisms for persist ence of the virus between epidemics. Furthermore, these epidemiologica l 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 g enotypes. The prevention of Japanese encephalitis presently involves v ector control, vaccination and, in some cases, particular modification s of environment. Each of these measures shows proper logistic, techni cal or financial difficulties, which often prevent their generalized u se. However, one can observe that, in the countries where vaccination is systematically carried out, the incidence of the disease seems cons iderably decreasing, while elsewhere it shows a tendency to increase. The main difficulties lie in the high cost of the vaccine and in the w eight of immunization scheme, which make uneasy its integration in the Expanded Vaccination Programme. For these reasons, the virologists an looking for other vaccine types, particularly recombinant vaccines re lying on obtaining, by genetical manipulation, envelop proteins with p rotection effect. Finally, remains the question of immunization of tra vellers and expatriate people. It seems desirable to recommand this va ccine only to the really exposed persons; this means persons performin g 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 attentio n to other methods aiming at decreasing man-vector contact.