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Questions
and Answers About Japanese Encephalitis |
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Q. How is
Japanese encephalitis transmitted?
A. By rice field breeding mosquitoes (primarily the Culex tritaeniorhynchus group)
that become infected with Japanese encephalitis virus (a flavivirus antigenically
related to St. Louis encephalitis virus). |
Q. How do people
get Japanese encephalitis?
A. By the bite of mosquitoes infected with the Japanese encephalitis
virus.
Q. What is
the basic transmission cycle?
A. Mosquitoes 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 during
the feeding process. The Japanese encephalitis virus is amplified in the
blood systems of domestic pigs and wild birds.
Q. Could you
get the Japanese encephalitis from another person?
A. No, 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 health care worker who has treated someone
with the disease.
Q. Could you
get Japanese encephalitis from animals other than domestic pigs, or from
insects other than mosquitoes?
A. No. Only domestic pigs and wild birds are carriers of the Japanese
encephalitis virus.
Q. What are
the symptoms of Japanese encephalitis?
A. Mild infections occur without 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) and spastic (but rarely flaccid) paralysis.
Q. What is
the incubation period for Japanese encephalitis?
A. Usually 5 to 15 days.
Q. What is
the mortality rate of Japanese encephalitis?
A. Case-fatality rates range from 0.3% to 60%.
Q. How many
cases of Japanese encephalitis occur in the world and the U.S.?
A. Japanese encephalitis is the leading cause of viral encephalitis
in Asia with 30-50,000 cases reported annually. Fewer than 1 case/year
is reported in U.S. civilians and military personnel traveling to and
living in Asia. Rare outbreaks in U.S. territories in Western Pacific
have occurred.
Q. How is Japanese
encephalitis treated?
A. There is no specific therapy. Intensive supportive therapy is
indicated.
Q. Is the disease
seasonal in its occurrence?
A. Seasonality of the illness varies by country (see
table).
Q. Who is at risk
for getting Japanese encephalitis?
A. Residents of rural areas in endemic locations, active duty military
deployed to endemic areas, and expatriates who visit rural areas. Japanese
encephalitis does not usually occur in urban areas (see table).
Q. Where is Japanese
encephalitis endemic?
A. See map.
Q. Where do
Japanese encephalitis outbreaks occur?
A. Japanese encephalitis outbreaks are usually circumscribed and
do not cover large areas. They usually do not last more than a couple
of months, dying out after the majority of the pig amplifying hosts have
become infected. Birds are the natural hosts for Japanese encephalitis.
Epidemics occur when the virus is brought into the peridomestic environment
by mosquito bridge vectors where there are pigs, which serve as amplification
hosts, infecting more mosquitoes which then may infect humans. 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
Viet Nam, Cambodia, Myanmar, India, Nepal, and Malaysia.
Q. Who should
be vaccinated against Japanese encephalitis?
A. According to the Advisory Committee on Immunization Practices
(ACIP) (Centers for Disease Control and Prevention. Inactivated Japanese
Encephalitis Virus Vaccine Recommendations of the Advisory Committee
on Immunization Practices [ACIP]. MMWR. Jan 8, 1993;42:11):
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U.S. Expatriates:
Japanese encephalitis vaccine is recommended for persons who plan
to reside in areas where Japanese encephalitis is endemic or epidemic
(residence during a transmission season). Risk for acquiring Japanese
encephalitis is highly variable within the endemic regions (see
table; see map).
The incidence of Japanese encephalitis in the location of intended
residence, the conditions of housing, nature of activities, and
the possibility of unexpected travel to high-risk areas are factors
that should be considered in the decision to seek vaccination.
Travelers: Japanese
encephalitis vaccine is NOT recommended for all travelers to Asia.
In general, vaccine should be offered to persons spending a month
or longer in endemic areas during the trans-mission season, especially
if travel will include rural areas. Under specific circumstances,
vaccine should be considered for persons spending <30 days in
endemic areas, e.g., travelers to areas experiencing epidemic transmission
and persons whose activities, such as extensive outdoor activities
in rural areas, place them at high risk for exposure. In all instances,
travelers should be advised to take personal precautions; e.g.,
to reduce exposure to mosquito bites. The decision to use Japanese
encephalitis vaccine should balance the risks for exposure to the
virus (see table; see
map) and for developing illness,
the availability and acceptability of repellents and other alternative
protective measures, and the side effects of vaccination. Risk assessments
should be interpreted cautiously (see table;
see map) since risk can vary
within areas and from year to year and available data are incomplete.
Estimates suggest that risk of Japanese encephalitis in highly endemic
areas during the transmission season can reach 1 per 5,000 per month
of exposure; risk for most short-term travelers may be 1 per million.
Although Japanese encephalitis vaccine is reactogenic, rates of
serious allergic reactions (generalized urticaria or angioedema)
are low (1 to 104 per 10,000). Advanced age may be a risk factor
for developing symptomatic illness after infection. Japanese encephalitis
acquired during pregnancy carries the potential for intrauterine
infection and fetal death. These special factors should be considered
when advising elderly persons and pregnant women who plan visits
to areas where Japanese encephalitis is endemic.
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Q. Where
can I get more information on Japanese encephalitis?
A. See the CDC Japanese Encephalitis Home Page (http://www.cdc.gov/ncidod/dvbid/jencephalitis)
and CDC Health Information for Travelers to Southeast Asia (http://www.cdc.gov/travel/seasia.htm).
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