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Japanese encephalitis

General
JE is an important mosquito-borne viral disease and one of the leading causes of viral encephalitis and neurological infections in Asia. Although severely under-reported, 50,000 cases are annually recorded throughout Asia, with 15,000 deaths (5-35% case fatality rate) and a 75% JE-related disability rate (767,000 DALYs, WHO, 2002).

The disease was first recognized in India in 1955, when cases of encephalitis from North Arcot district of Tamil Nadu and neighboring districts of Andhra Pradesh, admitted to Christian Medical College Hospital, Vellore, were serologically diagnosed as Japanese Encephalitis. JE virus was isolated from wild caught mosquitoes in the same year, followed by isolations from patients
from the same area in 1958. JE continues to be endemic in these states.

Since 1972, JE has spread to newer areas and epidemics / outbreaks have been reported from West Bengal, Uttar Pradesh, Assam, Manipur, Bihar, Andhra Pradesh, Pondicherry, Karnataka, Goa and recently from Kerala and Maharashtra. The disease in southern India affects children below 15 years, while in north India all age groups are affected. In most of the epidemics, the incidence has been higher in males than in females (M : F=1.2 -1.5:1).
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Key facts

  • Japanese encephalitis virus (JEV) is a flavivirus related to dengue, yellow fever and West Nile viruses, and is spread by mosquitoes.
  • JEV is the main cause of viral encephalitis in many countries of Asia with an estimated 68 000 clinical cases every year.
  • Although symptomatic Japanese encephalitis (JE) is rare, the case-fatality rate among those with encephalitis can be as high as 30%. Permanent neurologic or psychiatric sequelae can occur in 30%–50% of those with encephalitis.
  • 24 countries in the WHO South-East Asia and Western Pacific regions have endemic JEV transmission, exposing more than 3 billion people to risks of infection.
  • There is no cure for the disease. Treatment is focused on relieving severe clinical signs and supporting the patient to overcome the infection.
  • Safe and effective vaccines are available to prevent JE. WHO recommends that JE vaccination be integrated into national immunization schedules in all areas where JE disease is recognized as a public health issue.

Japanese encephalitis virus JEV is the most important cause of viral encephalitis in Asia. It is a mosquito-borne flavivirus, and belongs to the same genus as dengue, yellow fever and West Nile viruses.

The first case of Japanese encephalitis viral disease (JE) was documented in 1871 in Japan.

The annual incidence of clinical disease varies both across and within endemic countries, ranging from <1 to >10 per 100 000 population or higher during outbreaks. A literature review estimates nearly 68 000 clinical cases of JE globally each year, with approximately 13 600 to 20 400 deaths. JE primarily affects children. Most adults in endemic countries have natural immunity after childhood infection, but individuals of any age may be affected.
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