Japanese encephalitis (JE) is a mosquito-borne encephalitis and is caused by flavivirus belonged to the family Flaviviridae. It is a zoonotic disease, infecting mainly animals and incidentally man. A severe epidemic of encephalitis occurred in Japan during 1924 and the disease was named Japanese B encephalitis (as it was caused by Arbovirus B group). Since then Japanese encephalitis virus became a major virus responsible for epidemics of encephalitis in Japan and other countries. In India, Japanese encephalitis epidemics have been recorded in Tamil Nadu, West Bengal, Assam, Uttar Pradesh, Andhra Pradesh, Karnataka, and Goa. In general, epidemics of JE coincide with the rainy season and the period of high mosquito prevalence. However, in endemic areas, sporadic occurrence of the disease has been recorded throughout the year.
Children bear the major threat from the disease. However, people of all age groups may be affected by JE. In most of the epidemics the incidence in males was higher than in females. There is no evidence of person-to-person transmission of JE virus through any mosquito vector.Several avian and mammalian hosts act as the maintenance hosts of the virus. Man is an incidental dead-end host. Among the animal hosts, pigs are the most efficient amplifier hosts of the virus. Although abortion in pigs due to JE has been recorded, infected pigs do not manifest any other symptoms of the illness. Cattle and buffaloes may be infected with JE virus. They act as mosquito attractants. Horses are only domestic animals so far known which show signs of encephalitis. Many avian species (especially those of the heron family) play an important role both in maintenance and dissemination of JE virus.
Clinical features: The incubation period in man varies from 4 to 14 days or even longer. The onset of symptoms is usually sudden and is characterized by general malaise, headache, fever and vomiting. Lethargy is a common feature and there are sensory and motor disturbances affecting speech, the eyes and limbs. There may be delirium, confusion and finally progressing to coma with convulsions particularly in children. Weakness and paralysis may affect any part of the body. Neck rigidity is common and reflexes are abnormal. The case fatality varies between 10 to 50 %.
Control and prevention: The control and prevention of JE should be attempted at three levels i.e., protection of man, protection of reservoirs and control of vectors.
1. Awareness and education of people about the risk of infection through pictorial posters, radio, television, newspaper, etc.
2. Use of mosquito nets and repellents to avoid exposure to mosquitoes.
3. Spraying of suitable insecticides to control vector population and reduction of mosquitogenic conditions and water management whenever possible.
4. Piggeries to be built away from human habitation and should be well protected from mosquitoes.
5. Immunization of population at risk with killed JE vaccine.