Japanese encephalitis
Japanese encephalitis (JE) is the main cause of viral encephalitis in many countries of Asia. The infection is mosquito-borne and caused by the JE virus, a flavivirus related to dengue, yellow fever and West Nile viruses. The virus exists in a transmission cycle between mosquitoes and pigs and/or water birds (enzootic cycle). Humans become infected only incidentally when bitten by an infected mosquito and the disease is predominantly found in rural and periurban settings.
The disease is endemic with seasonal distribution in parts of China, the Russian Federation’s south-east, and South and South-East Asia. All year transmission is observed in tropical climate zones. Currently, JE is considered hyperendemic in northern India and southern Nepal as well as in parts of central and southern India, and authorities have responded with immunization campaigns. The spread of JE in new areas has been correlated with agricultural development and intensive rice cultivation supported by irrigation programmes.
JE vaccines have been available since decades and haven prove their potential to control the disease. Other control measures such as mosquito control or amplifying pig control have shown to be less reliable. WHO recommends JE immunization in all regions where the disease is a recognized public health problem (WER2006; 81:325). Until recently, the use of vaccine has been limited due to vaccine price, cumbersome immunization schedules, and lack of recognition of the burden of disease.
Surveillance of the disease is mostly syndromic for acute encephalitis syndrome, while case confirmation and distinction from other causes of encephalitis requires a laboratory diagnosis on serum or, preferentially, cerebro spinal fluid (CSF). Laboratory testing is often conducted in dedicated sentinel sites, and efforts are undertaken to expand laboratory-based surveillance. Case-based surveillance is established in countries that effectively control JE through vaccination.
Overall, the disease is considered under-reported, and annual mortality is estimated to range from 10-15,000 deaths. A recent literature review estimates a total number of 68,000 clinical cases of JE (Bulletin of WHO, October 2011). Of these cases, 30 % or more result in permanent neuropsychiatric sequelae.
The portfolio of licensed JE vaccine is evolving rapidly. Traditionally, the most widely used vaccine was a purified inactivated product made from either Nakayama or Beijing strains propagated in mouse-brain tissue. It is still produced and used in several South-East Asian countries. Over the past years, the live attenuated SA14-14-2 vaccine manufactured in China has become the most widely used vaccine in endemic countries, which requires only one dose for primary immunization.
Recently, two cell-culture based vaccine inactivated vaccine have been licensed, one as traveler's vaccine and the other for paediatric use in Japan. In addition, a live, recombinant product based on the yellow fever vaccine strain has been licensed in two JE-endemic countries.
Last updated: November 2011
