Japanese encephalitis is a serious viral brain infection spread by infected mosquitoes. It mainly occurs in Asia and the Western Pacific. Though not well known outside these areas, it is the leading cause of viral encephalitis in Asia, causing thousands of cases each year. The virus responsible, called Japanese encephalitis virus (JEV), belongs to the Flavivirus family, which also includes yellow fever, Zika, and dengue viruses. Japanese encephalitis is a zoonotic disease. This means it naturally cycles between animals and mosquitoes, with humans becoming infected by chance.
Most people infected with JEV show no symptoms. However, about 1 in 250 develop severe illness. This usually means inflammation of the brain, called encephalitis, which can cause coma, long-term disability, or death. Children under 15 years old are affected most, especially where vaccination is low. Survivors may suffer lasting problems like learning difficulties, seizures, or paralysis.
The virus spreads silently between mosquitoes and animal hosts, mainly pigs and wading birds. These animals do not show illness but carry the virus. Humans get infected when bitten by an infected mosquito but do not spread the virus further. Humans are dead-end hosts. They cannot keep the infection cycle going but can suffer severe effects.
In recent years, changes such as more rice farming, pig rearing, and shifting climates have helped the disease spread to wider areas. It now affects parts of Australia, Pakistan, and urban edges of Asia. The World Health Organization (WHO) estimates over 3 billion people live in areas at risk of Japanese encephalitis.
Transmission and Reservoirs
The main mosquito spreading JEV is Culex tritaeniorhynchus. These mosquitoes breed in still water, common in rice fields and flooded farms. When they bite infected pigs or birds, they pick up the virus. Pigs are important amplifying hosts because they develop high virus levels without becoming sick. This allows mosquitoes to become infected again after biting pigs.
Humans do not spread the virus. So, controlling Japanese encephalitis means focusing on mosquitoes, vaccination, and managing contact with animals, not stopping human-to-human spread.
Disease Presentation
Over 99% of JEV infections cause no symptoms or only mild signs like fever or headache. Severe illness causes acute encephalitis syndrome (AES). Patients may suddenly develop high fever, confusion, seizures, movement problems, and coma. The death rate in symptomatic cases is 20% to 30%. Among survivors, up to 50% may have lasting brain damage.
The incubation period is 5 to 15 days. This is the time from infection to symptoms. Without treatment, death can occur within days of symptom start.
At-Risk Groups
Certain people face higher risks of Japanese encephalitis:
- Children under 15 in endemic regions without vaccination
- Travellers to rural areas during mosquito season
- Farmers working in rice paddies or pig farms
- Communities lacking immunisation or mosquito control
Many countries have lowered disease rates with vaccination campaigns. However, millions remain unprotected due to cost, logistics, or politics.
Global Impact and Prevention
Around 68,000 clinical cases of Japanese encephalitis occur worldwide each year. Most cases happen in India, China, Nepal, Vietnam, and Southeast Asia. Yet, the disease could spread to new areas because of:
- Climate change, which lengthens mosquito breeding seasons
- Urbanisation, bringing people closer to animal hosts
- International travel, moving the virus to new places
The good news is that Japanese encephalitis is preventable. A safe, effective vaccine exists and is used in many endemic countries. Other prevention steps include:
- Using insecticide-treated bed nets
- Improving pig farming practices
- Reducing mosquito breeding sites
Importance of Awareness
Despite its impact, Japanese encephalitis is often overlooked, especially outside endemic areas. Travellers and health workers may underestimate the risk. Stronger surveillance, public education, and international cooperation are vital to reduce disease spread.
Raising awareness about how the virus spreads, its symptoms, and dangers helps protect high-risk groups. Early diagnosis and treatment improve outcomes. Although no specific antiviral exists, supportive care can save lives.
