Vaccine-Preventable Diseases

Japanese Encephalitis

Japanese encephalitis (JE) virus is the leading cause of viral encephalitis in Asia, where more than 50,000 cases occur each year. Clinically apparent infection with JE virus is seen only rarely in travellers. Countries where the disease occurs are listed in Table 8. The incidence of JE varies widely from year to year and between regions within countries. JE virus is an arthropod-borne flavivirus, a group that also includes yellow fever virus, West Nile virus and St. Louis encephalitis virus. The principal vectors are Culex mosquitoes that breed mainly in rice fields. Swine and certain species of wild birds are intermediate hosts in the transmission cycle. Conditions that support transmission of JE virus are primarily rural agricultural ones, but occasionally cases are reported from urban areas. Culex mosquitoes tend to bite in the evening and night, but day-biting species predominate in some regions.

Table 8. Areas where Japanese Encephalitis Has Been Recognized and Season of Epidemic Risk

Zone	Country
Temperate regions (Risk greatest July to October)	Bangladesh China Northern India Japan Kampuchea Korea Laos Myanmar Nepal Far Eastern Russia Northern Thailand Northern Vietnam
Tropical regions (Risk greatest during the rainy season. Note that the rainy season varies some what from region to region but is typically May to November)	Southern India Pakistan Indonesia Malaysia Philippines Sri Lanka Taiwan Southern Thailand Southern Vietnam

Epidemiology

The disease occurs in epidemic form in temperate and northern tropical regions and is endemic in southern tropical regions of Asia. Cases occur chiefly during the summer and autumn in temperate zones and during the rainy season in tropical zones. In areas where irrigation is the main factor affecting the abundance of vector mosquitoes, transmission may occur year round. For these reasons, the periods of greatest risk for JE virus transmission to travellers are highly variable and depend on such factors as season, location, duration of stay and the type of activities undertaken. Crude estimates for North Americans travelling to Asia place the overall risk of JE illness at less than 1 per million. However, for travellers to rural areas during the transmission season, the risk per month of exposure can be as high as 1 per 5,000. Rare case reports suggest that even short-term, resort-based travellers can occasionally contract JE.

The risk of JE transmission to the traveller may be significantly reduced by the appropriate use of bednets, repellents and protective clothing.

Most JE infections do not result in obvious illness. Between 50 and 300 infections occur for each clinical case identified. However, when encephalitis does occur, it is usually severe, with 10% to 25% mortality rates and residual neuropsychiatric problems in 50% of survivors.

The disease usually affects children, but in countries where it has been recently introduced all age groups may be affected. In addition to children < 10 years of age, advanced age may be a risk factor for developing symptomatic illness.

Japanese encephalitis acquired during pregnancy carries the risk of intrauterine infection and miscarriage.

Source: Canadian Immunization Guide, 7^th edition, 2006

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Links to more information

Guidelines and Recommendations

• For information about Japanese Encephalitis vaccine, see the Canadian Immunization Guide, 7^th edition 2006

Travel Medicine Program

• Immunizations Recommended for Travel Outside of Canada