Japanese encephalitis is the most common preventable cause of mosquito-borne encephalitis in Asia, Australia, and the western Pacific. The Culex species of mosquitoes transmit the virus with their bite. Transmission occurs most commonly in agricultural areas such as farms and rice paddies but may occur in urban areas under certain conditions. While the vast majority of infections are asymptomatic, those who do develop symptoms of encephalitis suffer significant morbidity and mortality. Symptomatic patients develop a high fever, headache, disorientation, coma, tremors and mental status changes due to cerebral inflammation. Movement disorders, neurologic deficits, and seizures are common, particularly in children. Approximately one in four symptomatic cases are fatal. Children are most commonly affected, and most people who reside in endemic areas will have immunity by adulthood. There is no specific therapy beyond supportive care, but there is an effective vaccine available to prevent infection. The vaccine is recommended for high-risk travelers to endemic areas. Many endemic areas have implemented childhood vaccination programs. The best protection is the prevention of mosquito bites.
Japanese encephalitis is a mosquito-borne illness caused by a single-stranded RNA virus, closely related to the West Nile flavivirus. Japanese encephalitis transmission is primarily due to the bite of Culex mosquito species, most commonly Culex tritaeniorhynchus. The virus is maintained and amplified in intermediate hosts, specifically pigs and wading birds. Humans are considered dead-end hosts in that they do not generally develop high enough levels of virus to transmit the infection to feeding mosquitoes. Because the amplifying hosts tend to be most abundant in agricultural areas such as farms and rice paddies where flooding irrigation attracts wading birds, most infections occur in rural areas. Recently, however, infections are being documented more commonly in suburban regions, particularly in South Korea, China, Singapore and Taiwan. This suggests that the vaccine recommendations for travelers should be expanded to include some suburban regions. While mosquitoes transmit the vast majority of infections, there is some concern that exposure to infected pigs, which are amplifying hosts, may result in virus transmission from close contact without vector involvement.
There are between 30,000 and 50,000 global cases of Japanese encephalitis each year. Severe disease is estimated to occur in about one in 250 infections. Transmission is seasonal in temperate climates and peaks between May and October, but the risk persists year-round in more tropical climates. The time of greatest risk for infection is during the rainy season and the pre-harvest period in rice cultivating areas due to increased mosquito vector populations. Most mosquito bites occur between dawn and dusk. Twenty-four countries in South-East Asia and the Western Pacific have endemic Japanese encephalitis virus transmission placing more than three billion people at risk for infection. Major outbreaks occur every 2 to 15 years. Between 1965 and 1975 more than one million cases were reported in China alone. The introduction of routine childhood vaccination programs in Japan, Korea, and Taiwan has nearly eliminated the risk in vaccinated patients despite ongoing infection in endemic animals and birds. Most cases in these areas are now reported in unvaccinated visitors.
The Japanese encephalitis virus attaches to host cell membranes, initially propagating at the site of the bite and nearby lymph nodes. Subsequent viremia develops but most cases are cleared before the virus enters the central nervous system, resulting in subclinical disease. If the virus is transmitted to the brain hematogenously with the invasion of the blood-brain barrier, neuroinvasive disease develops. Japanese encephalitis virus has both direct neurotoxic effects and the capacity to alter neuro stem cell development.
Most infected individuals will provide a history of mosquito exposure in an endemic area. The incubation period averages 6 to 8 days but ranges from 4 to 15 days. There is often a prodromal period of nonspecific symptoms of fever, headache, nausea, vomiting, diarrhea, and myalgias which may last for several days. Symptoms then progress to encephalitis, which is the most common neurologic manifestation. Symptoms may include altered mental status, agitation, confusion, and psychosis. A headache and meningismus are frequently reported in adults while children often develop seizures. More unusual presentations include mutism and flaccid paralysis. As the disease progresses, patients may develop dystonia, choreoathetoid movements that mimic extrapyramidal symptoms of Parkinson syndrome.
Patients with symptoms consistent with encephalitis are initially evaluated with neuroimaging and lumbar puncture. MRI or CT may show bilateral thalamic edema, lesions, or hemorrhage. A lumbar puncture may be significant for elevated opening pressure, elevated protein, and normal glucose. Blood work may reveal leukocytosis or hyponatremia. These findings are common in many forms of encephalitis or viral meningitis. If clinically suspected based on travel history, Japanese encephalitis virus immunoglobulin M (IgM) may be detected using an enzyme-linked immunoassay (ELISA) on serum or cerebrospinal fluid. Humans are dead-end hosts with low, transient viral loads making virus isolation difficult.
There is no effective antiviral therapy for Japanese encephalitis. Management is limited to supportive care with intravenous (IV) fluids and antipyretics. Anticonvulsants may be required for seizure control. Survivors often have poor neurologic outcomes requiring long-term care due to neurologic devastation and ongoing psychiatric symptoms. Up to 30% will suffer permanent intellectual, behavioral or neurologic issues ranging from paralysis, recurrent seizures or inability to speak or perform independent activities of daily living.
Since there is no effective treatment prevention is critical. This is best accomplished by avoiding mosquito bites entirely. Even very short periods of outdoor exposure can result in bites, so proper protective clothing that includes long sleeves, long pants, socks and closed-toe shoes should be worn. Pant legs can be tucked into socks to prevent bites to exposed ankles. Transmission is common during the warmer months and mosquitoes may bite through very thin clothing so treating clothing with repellents containing permethrin, DEET or other EPA-registered insect repellants will reduce this risk. Transmission is most frequent when mosquitoes feed, between dawn and dusk, so outdoor activities during this period should be avoided. Travelers should sleep in air-conditioned spaces or use mosquito nets or screens to prevent bites during sleep.
A safe, effective vaccine exists and may be administered in a short course regimen. The vaccine is quite underutilized. The current Center for Disease Control and Prevention (CDC) recommendations for the use of the Japanese encephalitis vaccine are as follows:
There are many differential diagnoses of Japanese encephalitis; therefore, a careful travel history important. It includes:
Only 1% of patients infected with the virus will progress to encephalitis. Unfortunately, mortality for patients who do develop encephalitis is 20% to 30%. While most cases will improve in 6 to 12 months, many patients who survive will have significant neurologic and psychiatric sequelae (30% to 50% of cases).
Japanese encephalitis is a potentially devastating but preventable disease. Travelers to endemic areas should review the CDC vaccination guidelines regarding immunization available at www.cdc.gov.