Transmission Dynamics of Japanese Encephalitis, with Emphasis on Gaps in Understanding and Priority Areas for Research on Japanese Encephalitis and Other Acute Encephalitis Syndromes in India

Japanese encephalitis (JE), a vector-borne zoonotic viral disease, is endemic to large parts of Asia and the Pacific, but has potential to spread to other parts of the world. An estimated 3 billion people are at risk, and about 67,000 cases being reported annually, most of which are from India. The disease is transmitted by mosquitoes mostly belonging to Culex vishnui subgroup. It is considered vaccine preventable and, with the help of vector control, is largely kept under check notwithstanding periodic outbreaks. Vaccination programmes, increased living standards, and mechanization of agriculture are the key factors in the decline of incidence of this disease in Japan and South Korea, and in some other countries, including India (667 cases and 200 deaths in 1981 to 33 in 2013 without a death); Japanese encephalitis is on the decline since mid-1990s. However, transmission of JE is likely to increase in Bangladesh, Cambodia, Indonesia, Laos, Myanmar, North Korea and Pakistan because of population growth, intensified rice farming, failure of vector control in vast stretches of paddy ecosystems, unorganized pig rearing and the lack of effective vaccination programs and disease surveillance. A leading cause of acute encephalitis syndrome (AES), the JE coupled with an ever increasing trend in AES cases, recently poses many challenges as the infection is not only involving an array of animals as amplifiers, but is also transmitted by an overwhelmingly high number of mosquito species belonging to varied genera as Culex, Anopheles, Monsonia and Armigeres.