Page 103 - Journal of Special Operations Medicine - Summer 2016

An Ongoing Series

Japanese Encephalitis

Mark W. Burnett, MD

Introduction
Japanese encephalitis is the main cause of encephalitis parkinsonian syndrome with masklike facies, tremor,
in many countries of Asia and the Western Pacific, and cogwheel rigidity, and choreoathetoid movements.”
is the most common vaccine-preventable etiology of this
type of disease in the region. It is caused by a single- The case fatality ratio is between 20% and 30% of those
stranded RNA virus of the genus Flavivirus, which is infected who show symptoms. In these patients, one-
related to dengue virus and even more closely related to third to one-half of the survivors will have significant
the West Nile virus. It is transmitted to humans through neurologic, cognitive, or psychiatric disabilities. Patients
the bite of an infected mosquito, most commonly of may also have milder symptomatic infections manifest-
the Culex species, which tends to bite during the cooler ing as aseptic meningitis or a nonspecific febrile illness,
times of the day when the sun is setting or rising. but JE may often be an unrecognized source of infection
in this population.
Humans are considered “dead-end hosts” because the
virus is usually maintained in an enzootic cycle between
mosquitoes and amplifying vertebrate hosts, usually Diagnosis
pigs and “wading birds,” such as herons. When humans Diagnosis in the field is challenging, but JE should be
are infected, they most often do not develop viremia to suspected in patients with meningitis, encephalitis, or
the extent that feeding mosquitoes could then become acute flaccid paralysis who have recently been in areas
infected. The disease is endemic to areas of Asia and where the disease is endemic. Other life-threatening
the Western Pacific where rice growing or flooding oc- diseases with potentially similar presentations such as
curs. Japanese encephalitis (JE) infections are seasonal bacterial meningitis and malaria must also be consid-
in temperate areas, occurring during the warmer months ered in the differential diagnosis. Laboratory diagnosis
of the year, but may be year-round in areas where fre- is most reliably made by sending serum or cerebrospinal
quent heavy rains occur or where fields are intentionally fluid for JE virus-specific immunoglobulin-M-capture
flooded for crop growth. enzyme-linked immunosorbent assay, which can be de-
tected in spinal fluid and serum in 4 days and 7 days
after the onset of symptoms, respectively. Follow-up
Clinical Presentation
convalescent titers can serve to confirm the diagnosis.
JE is most often a disease of children in countries where
it is endemic, because older members of the popula-
tion often have acquired immunity through previous Treatment
vaccination campaigns or via infection. Less than 1% The treatment of symptomatic JEs infections is support-
of humans who are infected show evidence of clinical ive and should be performed in a hospital for the proper
disease. The incubation period after the bite of an in- management of complications. Again, other causes of
fected mosquito is between 5 and 15 days. The onset illness must be considered.
of symptomatic disease starts with sudden fevers, head-
ache, nausea, and vomiting, followed by mental status
changes, weakness, focal neurologic deficits, and, often, Vaccination
seizures. According to the Centers for Disease Control The sole vaccine licensed in the United States is the
and Prevention, the classic description of JE includes “a inactivated Vero cell culture-derived vaccine IXIARO

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