The Japanese encephalitis virus is the leading cause of viral encephalitis in Asia. It is caused by a flavivirus and is usually associated with chills, a general feeling of malaise, and fever.
Close to 90% of infections are asymptomatic. When symptoms of encephalitis (fever, chills, and severe headaches) appear suddenly, they are followed by various neuropsychiatric disturbances such as stupor, confusion, and vegetative disorders. The disease is fatal in about 20 to 50% of cases, and serious sequelae, including seizures, ataxia, and behaviour disorders, occur in 30% of survivors.
Japanese encephalitis is transmitted to humans through the bite of mosquitoes of the Culex species. Humans are accidental hosts for the virus that usually infects domestic pigs, certain mammals, and marsh birds. The mosquitoes thrive mostly in rural areas and tend to bite from dusk to dawn. Transmission of the virus to humans is occasional at best. The risk of contracting the disease is roughly 1 in 2,500 mosquito bites. The incubation period is about 1 week, and the infection confers long-term immunity on the host.
The usual precautions against mosquitoes should be followed: apply insect repellent, use bed netting and wear light-coloured long-sleeve shirts and long pants. The risk of contracting Japanese encephalitis is minimal. Since 1981, only 6 cases have been reported among Americans living in endemic regions in Asia.
The vaccine is recommended for travelers 2 months of age and older who plan to spend 1 month or more in endemic areas during the transmission season. In some situations, it may be recommended for travelers who plan to stay for less than 1 month. The vaccine is usually administered as two doses (Day 0 and Day 28). Between 10 and 20% of those who receive the vaccine experience headache, myalgia, fever and malaise.
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