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Arborviruses
INTRODUCTION
Japanese encephalitis virus (JEV) is a single-stranded RNA virus belonging to the family Flaviviridae, genus Flavivirus. The first major epidemic of Japanese encephalitis (JE) was described in 1924 in Japan, in which about 6000 cases were reported [1]. Since then, JE has been increasingly recognized from most countries of East and South-East Asia, causing an estimated 67 900 JE cases annually in 24 JE-endemic countries [2]. JE has a high case-fatality rate of 20-30% and is known to cause residual neurological or psychiatric sequelae in 30-50% of survivors [2].
In India, the preliminary evidence of the presence of JEV was found in 1952, and the primary human case was reported from Tamil Nadu in 1955 [1]. The disease was confined to the southerly parts of India with a low prevalence until 1973, when the first major epidemic occurred in the north-eastern state of West Bengal [3]. Currently, in India, JE is the principal cause of vaccine-preventable encephalitis. The annual incidence of JE varies from 1714 to 6594 and mortality varies from 367 to 1665 individuals [4]. JE is now endemic in several states in India, including Bihar, Uttar Pradesh (UP), Assam, Manipur, Andhra Pradesh, Karnataka, Madhya Pradesh, Tamil Nadu, Haryana, Kerala, West Bengal, Orissa, Union territories of Goa and Pondicherry [5], with epidemic activity in northern and central parts of India [6].
Since the occurrence of the first epidemic of JE in UP in 1978, annual outbreaks of the disease have been recorded from the northern and eastern districts of the state [1]. Factors conducive to JEV exist in these areas and include: the vector (Culex spp. mosquitoes); environmental conditions suitable for the vector; ample mosquito habitats such as irrigated paddy fields, swamps, marshes; and presence of amplifying hosts, including pigs and birds. Therefore, in 2006 the Government of India launched a one-time mass JE vaccination programme in highly endemic districts of UP under which a single dose of SA14-14-2 vaccine was given to children aged 1-15 years [7]. This was followed by incorporation of the vaccine in the Universal Immunization Programme in the same districts. Seven of 11 districts (Gorakhpur, Deoria, Kushinagar, Maharajganj, Lakhimpur-Khiri, Sant Kabir Nagar, Siddharthnagar) reported more than 95% coverage of the estimated target....