Japanese encephalitis is caused by the Japanese encephalitis virus (JEV), a single-stranded RNA virus in the Flavivirus genus. This virus is related to dengue, Zika, and yellow fever. While the virus itself causes the disease, several other factors help it spread. These include mosquitoes, animals, human behaviour, and environmental conditions.
Unlike infections that spread from person to person, Japanese encephalitis moves through a complex cycle involving animals and mosquitoes. Understanding the causes of Japanese encephalitis means looking at each step in this transmission chain—from the virus itself to the places and behaviours that let it thrive.
The Virus: Japanese Encephalitis Virus (JEV)
The Japanese encephalitis virus mainly targets the brain. After a mosquito bite, the virus multiplies in nearby tissues and then enters the bloodstream. In most people, the immune system clears it quickly. But in rare cases, the virus crosses into the brain and causes encephalitis—serious inflammation that can lead to death or lasting damage.
JEV survives in nature through a zoonotic cycle, meaning it spreads among animals and mosquitoes. Humans are not part of this natural cycle, making them accidental hosts.
Mosquitoes: The Main Vectors of Japanese Encephalitis
The Culex tritaeniorhynchus mosquito plays the biggest role in spreading JEV. These mosquitoes bite at night and breed in standing water like:
- Flooded rice fields
- Irrigation canals
- Marshes and stagnant pools
Several traits make Culex mosquitoes excellent virus carriers:
- They exist in high numbers in rural farm areas.
- They feed at night when people are outdoors.
- They can survive for weeks, infecting more than one host.
- They adapt easily to different water habitats.
After biting an infected animal, the mosquito stores the virus in its saliva. When it bites a human, it injects the virus into the bloodstream.
Amplifying Hosts: Pigs and Wading Birds
Pigs are key amplifiers of Japanese encephalitis. When a mosquito bites an infected pig, the virus multiplies to high levels in the pig’s blood. Other mosquitoes feeding on the same pig can then pick up the virus and spread it again.
This is especially risky in rural areas where pigs live near rice fields and homes. Pigs breed fast and are often raised in large groups, increasing the risk of viral spread.
Wading birds, such as egrets and herons, are natural virus reservoirs. They help the virus survive between seasons and carry it across long distances. Unlike pigs, birds can fly far and bring JEV into new areas.
Humans as Dead-End Hosts
Humans can get Japanese encephalitis through a mosquito bite, but they do not spread the virus further. The level of virus in human blood is too low to infect another mosquito. This makes humans “dead-end hosts.”
Although humans don’t continue the cycle, the virus can cause serious illness or death in people. Because human infection is accidental, outbreaks depend on the presence of infected mosquitoes and animals—not human-to-human spread.
Environmental and Climate Factors
The environment plays a major role in the spread of Japanese encephalitis. Key factors include:
a. Rainy Season and Standing Water
Mosquito numbers increase during and after heavy rains. Flooded fields and stagnant pools become ideal mosquito breeding grounds. Outbreaks often occur in the rainy season.
b. Rice Farming
Rice paddies hold water for long periods and attract both mosquitoes and pigs. This creates a hotspot for viral transmission.
c. Warm Temperatures and High Humidity
Hot weather speeds up how fast the virus multiplies inside mosquitoes. Humid conditions also help mosquitoes live longer and bite more often.
d. Deforestation and Urban Expansion
Clearing forests for farms or buildings forces animals, mosquitoes, and people to live closer together. This increases the chance of human exposure to infected mosquitoes or birds.
Human Activity and Risk Factors
Some daily habits and social issues also raise the risk of Japanese encephalitis, including:
- Working or spending time outdoors in the evening
- Living in homes without proper mosquito protection
- Poor waste disposal, which creates water-filled containers
- Lack of access to vaccines in rural or low-income areas
Rapid urban growth without good sanitation may also lead to new mosquito habitats, especially in crowded informal settlements.
Travel and the Spread Beyond Endemic Areas
Travellers visiting affected regions can also get infected. The risk is highest during the rainy season and in rural areas. Farming, camping, or hiking outdoors for long periods increases exposure.
Though global spread is rare, migrating birds or mosquitoes could carry JEV to new areas. The World Health Organization advises vaccination for travellers spending a month or more in high-risk areas—or less time if visiting rural zones with known outbreaks.
Conclusion
The causes of Japanese encephalitis are complex. They involve a virus, mosquitoes, animals, humans, and the environment. Unlike common infections that spread easily between people, this disease needs many steps to keep going.
Stopping the spread means breaking this chain. This can be done by:
- Controlling mosquitoes
- Keeping pigs away from homes
- Managing water storage
- Educating communities
Most importantly, wide access to vaccines and good public health planning can prevent serious outbreaks and save lives.
