The most significant cause of viral encephalitis in Asia is Japanese encephalitis virus JEV. It is a mosquito-borne flavivirus and belongs to the same genus as dengue, yellow fever and viruses of West Nile.
The first case of Japanese encephalitis viral disease (JE) was documented in 1871 in Japan.
According to WHO, the annual incidence of the 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.
Signs and symptoms
Most JEV infections are mild (fever and headache) or without apparent symptoms, but approximately 1 in 250 infections results in severe clinical illness. The incubation period is between 4-14 days.
In children, gastrointestinal pain and vomiting may be the dominant initial symptoms.
Severe disease is characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and ultimately death. The case-fatality rate can be as high as 30% among those with disease symptoms.
Of those who survive, 20%–30% suffer permanent intellectual, behavioural or neurological sequelae such as paralysis, recurrent seizures or the inability to speak.
Transmission
JEV is transmitted to humans by the Culex species (mainly Culex tritaeniorhynchus) infected mosquito bites. Once infected, humans do not develop enough viraemia to infect mosquito feeding. The virus occurs between mosquitoes, pigs and/or water birds (enzootic cycle) in a transmission cycle. The disease is discovered predominantly in rural and periurban environments, where beings reside nearer to these hosts.
Assam is the ecologically favourable region for the spread of J.E. due to heavy rainfall, large paddy fields with big water body, piggery farming or domestic pig rearing almost throughout the state which supports the virus propagation.
Diagnosis
Individuals who live in or have travelled to a JE-endemic area and experience encephalitis are considered a suspected JE case. A laboratory test is required in order to confirm JEV infection and to rule out other causes of encephalitis. WHO recommends testing for JEV-specific IgM antibody in a single sample of cerebrospinal fluid (CSF) or serum, using an IgM-capture ELISA. Testing of CSF sample is preferred to reduce false-positivity rates from previous infection or vaccination
Surveillance of the disease is mostly syndromic for acute encephalitis syndrome. Confirmatory 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.
Treatment
There is no antiviral treatment for patients with JE. Treatment is supportive to relieve symptoms and stabilize the patient.
Prevention and control
Safe and effective JE vaccines are available to prevent disease. WHO recommends having strong JE prevention and control activities, including JE immunization in all regions where the disease is a recognized public health priority, along with strengthening surveillance and reporting mechanisms. Even if the number of JE-confirmed cases is low, vaccination should be considered where there is a suitable environment for JE virus transmission. There is little evidence to support a reduction in JE disease burden from interventions other than the vaccination of humans. Thus, vaccination of humans should be prioritized over vaccination of pigs and mosquito control measures.
