Japanese Encephalitis (Japanese B Encephalitis)

Japanese encephalitis is a serious mosquito-borne viral infection that primarily affects rural areas of Asia and the Western Pacific. While most people infected show no symptoms, those who develop brain inflammation face life-threatening complications, with up to one in four dying from the disease.

Table of contents

• What is Japanese encephalitis?
• How the disease spreads
• Where Japanese encephalitis is found
• Signs and symptoms
• Who is most affected
• Diagnosis
• Treatment and care
• Disease outcomes and complications
• Prevention

What is Japanese encephalitis?

Japanese encephalitis is an infection of the brain caused by the Japanese encephalitis virus. The virus belongs to a group called flaviviruses, which are viruses that have a similar structure and behavior. Japanese encephalitis virus is closely related to other serious viruses including dengue, yellow fever, West Nile virus, and Zika virus.^[1][3]

The disease was first documented in 1871 in Japan. Despite its name suggesting it is limited to Japan, Japanese encephalitis is now recognized as the main cause of viral brain inflammation in many countries across Asia.^[1]

The virus is caused by a single-stranded RNA virus, which means its genetic material is made of ribonucleic acid. This virus is surrounded by a protein coat that is about 50 nanometers in size.^[4][5]

How the disease spreads

Japanese encephalitis virus is spread to humans through the bite of infected mosquitoes. The main mosquito species that transmits the virus is Culex tritaeniorhynchus, although other Culex species can also spread the disease. These mosquitoes bite both during the day and at night.^[1][6]

The virus circulates in nature through a cycle involving mosquitoes and certain animals. Pigs and wild birds, particularly wading birds, serve as the main animal hosts where the virus multiplies. Mosquitoes become infected when they feed on these animals that have the virus in their blood. When these infected mosquitoes then bite humans, they can transmit the virus through their saliva.^[1][4]

Humans are considered “dead-end hosts” for the virus. This means that humans do not develop high enough levels of virus in their blood to infect mosquitoes that bite them. Therefore, the disease does not spread directly from person to person. You cannot get Japanese encephalitis by touching an infected person or animal, or by eating products from infected animals.^[4][5]

Where Japanese encephalitis is found

Japanese encephalitis is primarily found in East and Southeast Asia, as well as the Western Pacific region. Twenty-four countries in these areas have ongoing transmission of the virus, exposing more than 3 billion people to the risk of infection.^[1]

The disease is not found in the United Kingdom or other Western countries under normal circumstances. Places where Japanese encephalitis occurs include parts of India, China, Japan, South Korea, Indonesia, and Southeast Asian countries such as Thailand, Malaysia, and Vietnam.^[7]

The disease occurs most commonly in rural areas, particularly in agricultural settings such as farms and rice paddies. This is because these environments attract the pigs and wading birds that serve as hosts for the virus, and the flooding irrigation used in rice farming provides ideal breeding grounds for mosquitoes. However, in recent years, infections have been increasingly reported in suburban areas in countries like South Korea, China, Singapore, and Taiwan.^[5][6]

In many locations, the number of cases follows a seasonal pattern that corresponds with the mosquito breeding season. Cases often occur in outbreaks.^[1][4]

Signs and symptoms

Most people who are infected with Japanese encephalitis virus do not develop any symptoms at all. More than 99% of infected individuals either have no symptoms or experience only mild symptoms.^[2][6]

After a mosquito bite from an infected mosquito, there is an incubation period (the time between infection and the appearance of symptoms) that typically lasts 5 to 15 days, though it can range from 4 to 14 days or even extend to 2 to 26 days in some cases.^[1][2][3]

When symptoms do appear, they can range from mild to severe. Some people develop flu-like symptoms that include fever, headache, vomiting, and sometimes stomach pain. In children, stomach pain and vomiting may be the most noticeable early symptoms. These symptoms usually last between 1 and 6 days and may resolve on their own.^[1][7]

However, approximately 1 in 250 infections develops into severe clinical illness with encephalitis (inflammation of the brain). This severe form affects less than 1% of infected people but is very serious.^[1][2]

Severe disease is characterized by rapid onset of high fever (between 38-43°C or 100.4-109.4°F), severe headache, neck stiffness, disorientation, confusion, weakness, and potentially coma. The person may experience mental status changes, tremors, and movement disorders. Seizures are common, especially among children. Some people may develop paralysis or the inability to feel or move parts of their body.^[1][2][3]

Who is most affected

Japanese encephalitis primarily affects children. The majority of cases occur in children below 15 years of age. Children are most commonly affected because adults in areas where the disease is common usually develop natural immunity after childhood infection. However, individuals of any age can be affected, particularly if they have not been previously exposed to the virus.^[1][5]

For travelers to areas where Japanese encephalitis is found, the risk varies based on several factors including the destination, length of travel, season, and planned activities. People who work or spend time outdoors in high-risk rural areas, particularly those near rice fields, wetlands, or pig farms, have a higher chance of being exposed to infected mosquitoes. Those staying for more than a month in rural areas or visiting during mosquito season face increased risk.^[6][7]

Diagnosis

If you have traveled to an area where Japanese encephalitis is present and develop symptoms such as fever, headache, vomiting, or more serious symptoms like confusion, seizures, or weakness, you should see a healthcare provider immediately. It is important to tell your doctor when and where you traveled.^[2]

Your healthcare provider can order specific tests to look for Japanese encephalitis virus infection. Diagnosis is typically based on testing blood or cerebrospinal fluid (the fluid that surrounds the brain and spinal cord). These tests can detect the virus itself or measure levels of antibodies that your body produces in response to the infection.^[2][3]

The virus can be isolated from brain tissue or other samples and identified using laboratory techniques. Tests may include inoculating samples into special cell cultures or mice, followed by identification tests. Alternatively, viral material in tissues can be identified using special staining techniques.^[4]

Treatment and care

There is no specific medicine or antiviral treatment available to cure Japanese encephalitis. There are no medicines that can directly target and eliminate the Japanese encephalitis virus from the body.^[1][2]

Treatment focuses on relieving symptoms and supporting the patient while their body fights the infection. This is called supportive care. Rest, fluids, and over-the-counter pain medications may help relieve some symptoms such as fever and headaches. Medicines like acetaminophen (paracetamol) may be given to reduce fever and relieve pain. Nonsteroidal anti-inflammatory drugs are generally avoided due to the risk of bleeding complications.^[2][4]

People with severe disease require hospitalization for close monitoring and medical care. Hospital treatment may include intravenous fluids to maintain hydration, especially if the patient has high fever, vomiting, or diarrhea. Patients with severe symptoms often need help managing headaches with pain control medications and antiemetic therapy for nausea and vomiting.^[2][4]

Patients who develop encephalitis require particularly close monitoring for complications including elevated pressure inside the skull, seizures, and difficulty protecting their airway. Hospitalization for supportive care and close observation is generally required for anyone with severe symptoms.^[2][9]

Disease outcomes and complications

Among those who develop encephalitis (brain inflammation), the disease is very serious. The death rate among people with encephalitis is high, with about 1 in 4 people (20-30%) dying from the disease. This means the case-fatality rate can be as high as 30%.^[1][2]

For those who survive severe Japanese encephalitis, permanent complications are common. Between 30% and 50% of those who develop encephalitis and survive continue to have lasting problems. These long-term complications can include movement disorders, problems with thinking and memory (cognitive problems), and behavioral changes.^[1][2]

Specific long-term complications that may occur include deafness, emotional instability, weakness or paralysis on one side of the body, seizures, and loss of speech. Some people experience ongoing neurological deficits, particularly children. The disease can cause lifelong disabilities in survivors.^[3][7]

Infection during pregnancy can potentially harm the unborn baby, as the virus can cross the placenta. Pregnant women are generally advised not to receive the vaccine and to take extra precautions to avoid mosquito bites.^[3][5]

Prevention

The best way to prevent Japanese encephalitis is to protect yourself from mosquito bites and get vaccinated before traveling to areas where the disease is found, if vaccination is recommended for you.^[6][18]

Safe and effective vaccines are available to prevent Japanese encephalitis. The World Health Organization recommends that vaccination be integrated into national immunization schedules in all areas where Japanese encephalitis is recognized as a public health issue. Many countries where the disease is common have implemented childhood vaccination programs.^[1][5]

For travelers, vaccination is recommended if you are traveling to parts of the world where the virus is found, especially if you will be staying for more than a month, staying in rural areas, or staying near or visiting rice fields, wetlands, or places where pigs are kept. One vaccine called IXIARO is available in the United States. The vaccine is given as a 2-dose series, with a booster dose recommended after a year for people who remain at risk.^[7][9]

Regardless of vaccination status, everyone should take steps to avoid mosquito bites. Protection measures include:

• Using insect repellent on exposed skin, ideally containing at least 50% DEET
• Wearing long-sleeved shirts, long trousers or long skirts, socks, and shoes to protect your skin from mosquito bites
• Treating clothing and gear with 0.5% permethrin
• Sleeping under a mosquito net treated with insecticide when air conditioning or window screens are not available
• Choosing hotels or lodging with air conditioning or screens on windows and doors

These protective measures are important because mosquitoes that carry Japanese encephalitis virus bite both during the day and at night.^[6][7][18]

If you are planning to travel to an area where Japanese encephalitis is found, you should discuss the need for vaccination with your healthcare provider or visit a travel health clinic before your trip.^[6][7]