Japanese Encephalitis (JE)
1. Causative Agent
Flavivirus (+RNA) belonging to the family flaviviridae transmitted by infected mosquitoes. Due to this specific route of transmission by infected mosquitoes, it is also called an arbovirus (arthropod-borne). The JR virus is antigenically linked to West Nile virus, St. Louis encephalitis virus and Murray Valley encephalitis virus.
2. Incubation Period
5-15 days
3. Risk to the Traveller
3.1 Incidence
1 per million for short-term travel (< 4 weeks) depending on factors such as season, location, and duration of travel.
Risk of illness to travellers in rural areas during transmission season – rate per month of exposure is 1 per 5,000.
Between 1978 and 1992 24 cases of JE occurred among expatriated and travelers from the US.
3.2 Mortality
Symptomatic disease 10-25% (especially when encephalitis occurs).
4. Transmission
- The disease agent is transmitted to man by Culex mosquitoes. (Culex tritaeniorhynchus is the principal vector in Asia). They are night-time biters, and breed in dirty polluted water, mainly in the rural rice-growing and pig farming regions.
- Wild and domesticated animals, such as pigs and birds, are the intermediate hosts for JE virus. The virus is amplified in the blood systems of domestic pigs and wild birds.
- In the basic transmission cycle, mosquitoes become infected by feeding on domestic pigs and wild birds infected with JE virus. Infected mosquitoes then transmit the virus to humans and animals during the feeding process. Only a small portion (1-3%) of these mosquitoes are infected. Humans are a dead-end host.
5. Geographic Considerations
Occurs mainly in 3 areas:
- China and Korea
- Indian Sub-Continent – India, Bangladesh, South Nepal, Sri Lanka
- South-East Asia – Burma, Thailand, Cambodia, Laos, Vietnam, Malaysia, Indonesia, Phillippines
Although it has been decreasing in China, Korea and Japan, it is holding steady or even increasing in areas of South and Southeast Asia.
Transmission occurs year-round, but epidemics/outbreaks usually begin during the rainy season when the mosquito population is at its maximum.
- Temperate Regions
May to September
- Sub-Tropical and Tropical Regions
Transmission is correlated with the abundance of mosquitoes and amplifying hosts, which fluctuate with the season, the amount of rainfall, and the migratory patterns of birds.
Agricultural irrigation is also an important factor, as flooded rice fields are excellent breeding grounds that may cause transmission to occur throughout the year.
Occurrence in Asia – 30,000-50,000 cases annually.
Most infections are asymptomatic (subclinical). Between 50 and 300 infections occur for each clinical case identified.
Mild case: Fever and headache
Severe infection: Acute onset, headache, high fever, neck stiffness, stupor, disorientation, coma, tremor, occasional convulsions (especially infants), and spastic (rarely flaccid) paralysis, neurologic deficits.
Serious neurologic sequelae – 30-50% (intellectual impairment, emotional instability, aphasia, hemiplegia)
7. Prevention
7.1 Control of the mosquito population
Eliminate breeding places to control the vector.
Vaccinate animals
Protection of animal reservoirs by screening of pig stalls.
7.2 Protective clothes and repellents
Solid footwear, socks, close-fitting trousers, smooth fabrics, etc. may help to make access to the skin more difficult for ticks.
Light-colored clothing
Insect repellents such as DEET and permethrin (body and/or clothes).
7.3 Active immunization
Given as a subcutaneous injection on days 0, 7 and 30. When time does not permit, they may be administered 5 to 7 days apart, but the antibody response to this accelerated schedule is lower and may not be as durable. Two doses of vaccine 7 to 14 days apart can provide reasonable protection (80% efficacy) for short periods of time (< 1 year).
Children 1-2
years 0.5 mL
Children 3 years and older, and adults 1.0 mL
Duration of protection is 3 years – the a booster dose every 3 years for adults and every 2-3 years for children < 3 years.
Immunogenicity – 90%
Efficacy – 91%
8. Indications for Vaccination:
Travellers spending more than 30 days in rural parts of endemic areas during transmission season.
Travellers spending less than 30 days, if travel includes areas experiencing epidemic transmission and extensive outdoor activities, in rural areas.
9. Warnings/Precautions/Contraindications against Vaccination
9.1 Warnings
Adverse reactions to JE vaccine may occur within minutes following vaccination, but also as late as 17 days following, most occurring within 10 days – with the majority occurring within 48 hours.
All vaccinees should be observed for 30 minutes following vaccination, and warned of possible delayed adverse events. Therefore, all vaccinees should remain in areas of ready access to medical care for 10 days following vaccination.
Persons should not embark on international travel within 10 days of JE vaccine because of possibility of delayed reactions.
Persons with a past history of urticaria after bee, wasp or ant stings/bites (hymenoptera), drugs, idiopathic cause, appear to have a greater risk of developing reactions to JE vaccine.
Should live virus vaccines need to be administered (e.g. MMR), and time permits, it would be better to administer at least 2 doses of JE vaccine before administration of the live viral agents.
Also, theoretically, serologic responses may be diminished if JE vaccine is administered while chloroquine is being taken for prophylaxis against malaria.
9.2 Precautions
Moderate to severe illness with or without fever
Allergies to components of the vaccine.
9.3 Contraindications
Anaphylaxis to components of the vaccine.
Anaphylaxis to a previous dose of this vaccine
Children less than 1years of age
No information available on the safety of these vaccines during pregnancy and breastfeeding and should be used with caution, and only if the benefit outweighs any potential risk to the infant.
10. Adverse Effects
Local tenderness, redness, swelling – 20% of vaccinees
Systemic: fever, headache, malaise, rash, chills, dizziness, myalgia, nausea, vomiting and abdominal pain in 5-10% of vaccinees
New pattern of adverse events reported since 1989 in developed countries: urticaria, angioedema of extremities, face (especially lips), oropharynx, within 12 hours or as long as 14 days after vaccination.
About 1:260 vaccinees experience a general rash, itching or swelling, especially of the extremities, face, lips and throat.
Late reactions occur at a rate of 6.3/100,000 in the US.
11. Storage and Administration
2-8 °C until reconstituted with diluent. After reconstiution, the vaccine should be stored at a temperature between 2-8 °C and used within 8 hours.
12. Diagnosis:
Demonstrating specific IgM in acute-phase serum of cerebrospinal fluid (CSF), or antibody rises between early and late specimens of serum by neutralization, CF, HI, FA, ELISA or other serologic test. Cross reactions may occur within a virus group.
13. Treatment:
No specific treatment, only supportive measures.
14. References
1. Canadian Immunization Guide, 2002
2. Caumes E. Health and Travel. Aventis Pasteur. 2000
3. Chin J. Control of Communicable Diseases Manual, 2000
4. Communicable Disease Control Report. August 15, 1998
5. Dawood R. Travellers’ Health. How to stay healthy abroad. Oxford University Press. 2002
6. DuPont HL, Steffen R. Textbook of Travel Medicine and Health. Second Edition. BC Decker Inc, Hamilton, London. 2001
7. MMWR, Inactivated Japanese Encephalitis Virus Vaccine, January 8, 1993
8. Plotkin and Orenstein. Vaccines. 1999
9. Shlim D and Solomon T. Japanese Encephalitis Vaccine for Travelers: Exploring the Limits of Risk. Clinical Infectious Diseases 2002; 35:183-188
10. Thompson RF. Travel & Routine Immunizations. A Practical Guide for the Medical Office. Shoreland, 2002
11. Wise, M. The Travel Doctor. Your guide to staying healthy while you travel. Firefly Books. 2002
12. World Health Organization/International Travel and Health. 2002
Contributor:
Dr Tania Diener
Executive Director/Medical Health Officer
Population and Public Health Services
Regina Qu'Appelle Health Region
2110 Hamilton Street
Regina
Saskatchewan
S4P 2E3
Canada
Tel: (306) 766-7774
Fax: (306) 766-7607
E-Mail: Tania.Diener@rqhealth.ca