Japanese B encephalitis

Japanese B encephalitis

What is Japanese B encephalitis?

Japanese B encephalitis is the leading cause of viral encephalitis in Asia, with 30-50,000 cases reported annually. It has a fatality ratio of 30% and there are serious neuralgic (affected by pain in the sensory nerve) consequences in another 30% of cases.

What are the symptoms of Japanese B encephalitis?

The incubation period is usually five to 15 days. The majority of Japanese B encephalitis infections are subclinical (have no symptoms) and many infected people become symptomless carriers. Mild infections occur with few apparent symptoms other than fever with headache.

More severe infection is marked by:

  • Quick onset.
  • Headache
  • High fever.
  • Neck stiffness.
  • Stupor.
  • Disorientation.
  • Coma.
  • Tremors.
  • Occasional convulsions (especially in infants).
  • Spastic (occasionally flaccid) paralysis.

Acute encephalitis can progress to paralysis, seizures, coma and death.

How is Japanese B encephalitis transmitted?

Rice field breeding mosquitoes (primarily the Culex tritaeniorhynchus group) become infected by feeding on domestic pigs and wild birds infected with the Japanese encephalitis virus.

Infected mosquitoes then transmit the Japanese encephalitis virus to humans and animals by biting them. Only domestic pigs and wild birds are carriers of the Japanese encephalitis virus.

Where are the high risk destinations for Japanese B encephalitis infection?

It is found almost exclusively in Asia but there is a growing incidence of Japanese B encephalitis in Northern Australia. Countries which have had major epidemics in the past but which have controlled the disease primarily by vaccination include China, Korea, Japan, Taiwan and Thailand.

Other countries that still have periodic epidemics include Vietnam, Cambodia, Myanmar, India, Nepal, and Malaysia. Seasonality of the illness varies by country, for example May to September in Northern China, May to October in Thailand and Goa; July to December in Northern India; June to September in Japan; April to October in Southern China; and year round transmission with an April peak in Singapore.

How can infection be prevented?

The only means of infection is the bite of mosquitoes infected with the Japanese encephalitis virus. Japanese encephalitis virus is NOT transmitted from person-to-person. For example, you cannot get the virus from touching or kissing a person who has the disease, or from a healthcare worker who has treated someone with the disease.

How is Japanese B encephalitis treated?

There is no specific therapy. Intensive supportive therapy is indicated.

Should I be vaccinated against Japanese B encephalitis?

Japanese encephalitis vaccine is NOT recommended for all travellers to Asia. In general, vaccine should be offered to persons spending a month or longer in endemic areas during the transmission season, especially if travel will include rural areas.

What can I do?

Japanese encephalitis does not usually occur in urban areas and rarely infects travellers. Residents of rural areas in endemic locations are most at risk. Risk for most short-term travellers may be one per million. Special factors should be considered by elderly persons and pregnant women who plan visits to areas where Japanese encephalitis is endemic. Japanese encephalitis acquired during pregnancy carries the potential for intrauterine infection and foetal death.

Where can I get further information?

Your GP will be able to advise you on the vaccinations necessary for travel to various regions. Travel vaccination information is also available at www.cdc.gov/travel/

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