nebraska health articles: Japanese Encephalitis (JE)

JEV is transmitted to humans through the bite of an infected mosquito, primarily Culex species. Wading birds are the main animal reservoir for the virus, but the presence of pigs greatly amplifies the transmission of JEV.
Humans are a dead-end host in the JEV transmission cycle.

JEV is the most common cause of encephalitis in Asia, occurring throughout most of Asia and parts of the western Pacific (Map 2-5). JEV has not been locally transmitted in Africa, Europe, or the Americas.
JEV transmission principally occurs in rural agricultural areas, often associated with rice production and flooding irrigation. In some areas of Asia, these ecologic conditions may occur near or occasionally within urban centers.
In temperate areas of Asia, transmission is seasonal, and human disease usually peaks in summer and fall. In the subtropics and tropics, seasonal transmission varies with monsoon rains and irrigation practices and may be extended or even occur year-round.
In endemic countries, JE is primarily a disease of children. However, travel-associated JE can occur among persons of any age.

The risk for JE for most travelers to Asia is extremely low but varies according to season, destination, duration, and activities. Fewer than 40 cases of confirmed JE have been reported in travelers in the last 40 years.
The overall incidence of JE reported among people from nonendemic countries traveling to Asia is <1>
Travelers on even brief trips are probably at increased risk if they have extensive outdoor or nighttime exposure in rural areas, including persons staying in resort areas or with family.
Short-term travelers whose visits are restricted to major urban areas are at very minimal risk for JE.
In endemic areas where there are few human cases among residents because of vaccination or natural immunity, JEV is often maintained in an enzootic cycle between animals and mosquitoes. Therefore, susceptible visitors still may be at risk for infection.

Most human infections with JEV are asymptomatic; <1%>
Acute encephalitis is the most commonly recognized clinical manifestation of JEV infection. Milder forms of disease such as aseptic meningitis or undifferentiated febrile illness can also occur.
The incubation period is 5–15 days. Illness usually begins with sudden onset of fever, headache, and vomiting. Mental status changes, focal neurologic deficits, generalized weakness, and movement disorders may develop over the next few days.
- A parkinsonian syndrome resulting from extrapyramidal involvement is a very distinctive clinical presentation of JE.
- Acute flaccid paralysis, with clinical and pathological features similar to poliomyelitis, has also been associated with JEV infection.
- Seizures are very common, especially among children.
Clinical laboratory findings include moderate leukocytosis, mild anemia, hyponatremia, and cerebrospinal fluid (CSF) pleocytosis with a lymphocytic predominance.
Case–fatality ratio is approximately 20%–30%. Among survivors, 30%–50% may still have significant neurologic or psychiatric sequelae, even years after their acute illness.

JE should be suspected in a patient with evidence of a neurologic infection (e.g., encephalitis, meningitis, or acute flaccid paralysis) who has recently traveled or resided in an endemic country in Asia or the western Pacific.
Laboratory diagnosis of JEV infection should be performed by using JE-specific IgM-capture enzyme-linked immunosorbent assay (ELISA) on CSF or serum. JE-specific IgM antibodies will be present in the CSF or blood of almost all patients by 7 days following onset of symptoms. A fourfold or greater rise in JEV-specific neutralizing antibodies between acute- and convalescent-phase serum specimens may be used to confirm the diagnosis.
Vaccination history, date of onset of symptoms, and information regarding other flaviviruses known to circulate in the geographic area that may cross-react in serologic assays need to be considered when interpreting results.
Humans have low levels of transient viremia and usually have neutralizing antibodies by the time distinctive clinical symptoms are recognized. Virus isolation and nucleic-acid amplification tests (NAATs) are insensitive for the detection of JEV or JE viral RNA in blood or CSF and should not be used for ruling out a diagnosis of JE.
Health-care providers should contact their state or local health department or CDC’s Division of Vector Borne Infectious Diseases at 970-221-6400 for assistance with diagnostic testing.

There is no specific antiviral treatment for JE; therapy consists of supportive care and management of complications.

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