Japanese first JE case was recognized in Tokyo, Japan

Japanese encephalitis(JE), the leading
cause of vaccine preventable encephalitis in Asia and western pacific, and a
significant cause of disability 1,3,6. JE is a zoonotic disease caused by
single stranded positive sense RNA virus, of the family Flaviviridae 1,3,6
refer to appendix 1. More than  3
billion live in areas where
JE is transmitted 3. Nearly
68,000 clinical cases of JE are reported to WHO globally every year, with
approximately 13,600 to 20,400 deaths 4. The global incidence of JE is likely to be underreported due
to lack of good surveillance systems and varying availability of the diagnostic
laboratory testing across
the world 2. Extensive and devastating outbreaks are a feature of JE
disease in many countries 3. In 2005 epidemic in Uttar Pradesh, India, 5737
cases and 1344 deaths were reported in five months 3.

       
    The first JE case was
recognized in Tokyo, Japan in 1871 and thereafter major outbreaks have been
reported every 10 years 6. JE virus was first isolated from human brain in
1935 in Japan – Nakayama strain 6. 5 genotypes of JE
virus are recognized so far, and Indonesia – Malaysia(I-M) region is the only
region with all 5 genotypes8 refer to appendix 2. The genotypes 4 and 5,
which are thought to represent the oldest lineage are found only in
the I-M region, suggesting that the virus originated here and evolved into
different genotypes, which then spread across Asia8. The patterns in endemic
areas vary depending on factors like climate – they can
peak in late summer and autumn in temperate regions, can occur
year-round in tropical regions, with seasonal peaks during rainfall 5.

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Traveler’s risk is estimated to be

Vertebrate hosts such as pigs and
birds play an important role in the maintenance and amplification of JE virus
and mosquitoes are responsible for the transmission of virus 6 refer to
appendix 3. JE has been isolated from different mosquito species including
Culex, Anopheles, Mansonia but Culex Tritaeniorhynchus is the principal vector
which breeds in stagnant water and rice paddies 6. So JE is especially common
in rural and suburban areas of Asia where rice cultivation and pig farming co-exist
2. Humans are generally dead-end hosts as they seldom develop enough viremia
to infect feeding mosquitoes 2. Fewer than 1% human JE infections result in
JE 2.

In the past 50 years, geographical
areas affected by JE virus have expanded 6. The mechanism by which JE virus
emerges and establishes in a new area is unclear but the potential reasons
include bird migration, global warming, and changes in the usage of land 6,7.

Clinical manifestations 

            The incubation period in humans is estimated to be 5 to 15
days 5. Most JE infections are asymptomatic or mild and nonspecific –
fever, muscle aches, headache with vomiting, or gastrointestinal illness in
children 4,5. Most people recover after this stage, but.about 1 in 250 cases develop severe clinical
illness, with rapid onset of high fever, headache, neck stiffness,
disorientation, coma, seizures, spastic paralysis, and can lead to death 4,5.

Convulsions can range from mild to severe, ranging from subtle focal signs to
generalized seizures 5. Atypical presentations have also been reported, that
include acute onset of behavioral abnormalities which can be misdiagnosed as
psychiatric problems 5. Occasionally some serious cases can present syndromes
in other organs – pulmonary edema, upper gastrointestinal hemorrhage 5. Miscarriages have been seen in women who
are infected for the first time during pregnancy but are reported to be
uncommon in endemic areas 5..

            JE is considered in a patient with
evidence of neurological infection and who recently  traveled to or resided in an endemic
country9. Suspect’s serum or cerebrospinal fluid is tested for virus specific IgM
antibodies, and in the presence of IgM antibodies, a confirmatory neutralizing
antibody testing is done9.

Mortality & morbidity

            . The case fatality rate can be as
high as 30% among those with clinical illness and of those who survive, 30 to
50% have significant neurological sequelae – lifelong disability including physical,
cognitive, or behavioral problems leading to a long-term social and economic toll within
affected families and communities 2,3,4. Some of these people may gradually
improve but might take months to years 5. The vulnerable population for JE
are children and the rural poor, where most of the deaths occur 3. Mortality
and morbidity due to JE have not significantly reduced with interventions other
than immunization 3. Although good quality care improves outcomes, there is
no antiviral therapy specific for JE virus 3.

Changes in the disease epidemiology after a
vaccination program – 

JE affects all age groups, but in
endemic, unvaccinated areas it is primarily a disease of children as most
adults in endemic areas have natural immunity after childhood infection 2. In
areas where there is long standing, high quality vaccination programs, JE is
usually a rare disease of the non-immune adults, especially elderly 2. Dipti’s section here

The potential for eradication of the disease

            Human
exposure to the virus cannot be eliminated as JE virus is maintained in the
environment in enzootic cycles between birds and pigs 3. Humans are
incidental hosts and play no role in the maintenance or amplification of JE
virus, so by immunizing humans, there will be no change in the transmission
cycle or reduction in the JE virus levels in the environment 3.  Vector control has had limited effectiveness in most settings and is
expensive and resource-intensive, leaving immunization as the most important
and effective control measure 3. Therefore, high quality immunization
programs and maintenance of high immunization rates is essential for long-term
JE control 3. There is evidence of reduced JE incidence and maintenance of
low level of disease in countries that have implemented high quality JE
vaccination programs 2,3.