At least one person has died, and three others have been infected with Japanese encephalitis (JE) since the start of the monsoon some three weeks ago.

As monsoon rainfall has intensified, health officials anticipate more deaths and infections from the deadly virus in the coming days.

“One death and three cases of Japanese encephalitis have been confirmed in recent days,” said Dr Abhiyan Gautam, chief of the Immunisation Section at the Family Welfare Division under the Department of Health Services. “The coming days will be more challenging, as vector-borne diseases, including JE, tend to spread during monsoon season.”

JE is a viral brain infection endemic to Asia and parts of the Western Pacific. It is a mosquito-borne flavivirus belonging to the same family as dengue, Zika, yellow fever, and West Nile viruses.

According to the World Health Organisation, the virus kills about one-third of those who develop severe illness, while up to half of survivors are left with severe lifelong disabilities.

According to officials, the deceased was a 70-year-old man from the Itahari Sub-metropolitan City in Sunsari district. He had no travel history and was admitted to the BP Koirala Institute of Health Sciences. He had underlying conditions, including high blood sugar and high blood pressure, and was placed on a ventilator before being discharged at the request of his family members.

JE cases have so far been reported from four districts—Sunsari, Ilam, Surkhet and Chitwan.

In 2025, at least 41 people died, and 141 others were infected with the virus, which had spread to over 117 local units of more than 45 districts. The death rate from the infection was around 23 percent.

In 2024, twenty-three people, including one in Kathmandu Valley, died of JE and at least 80 people were infected.

JE can cause permanent injuries to the brain and the nervous system. As there is no specific cure, treatment focuses on managing symptoms. Experts say JE is the leading cause of death among vector-borne diseases in Nepal.

Immunisation experts, as well as members of the National Immunisation Advisory Committee, have recommended a single dose of the JE vaccine for everyone unvaccinated, saying they remain at high risk of infection, severe illness, and death. They say last year’s infection data show that most people who succumbed to the infection or had severe outcomes were among the unvaccinated population.

According to the data, 76 percent of those who died from JE were over 40 years old, while around 70 percent of those infected were older than 15 years.

Meanwhile, the government has allocated Rs 460 million to launch JE vaccination in 11 districts with a high JE burden. Officials say Chitwan, Kailali, Tanahun, Dang, Jhapa, Kapilvastu, Morang, Rupandehi, Sunsari, Rautahat and Sarlahi are the districts selected for JE vaccination, as these districts have recorded the highest morbidity and mortality rates.

However, the vaccination programme will not be started before April next year.

“Launching JE vaccination is not as easy as considered, as the procurement and supply of the vaccine takes time,” said Gautam.

In 2005, JE killed nearly 2,000 people in Nepal, mostly children in the Tarai districts. Nepal started administering the vaccine in 2006, eight years before the WHO officially issued prequalification certification, due to high rates of infection and deaths.

The vaccination programme was later expanded with support from donors and global health partners.

Experts say everyone must take at least one dose of the JE vaccine, which is effective in preventing morbidity and mortality.

Nepal has included the JE vaccine in its routine immunisation programme and administers it free of charge to children at state-run health facilities.

The JE virus is transmitted to humans through the bite of infected Culex mosquitoes. Pigs and ducks are considered natural reservoirs of the virus. Pig farmers and those residing near the paddy field are at particularly high risk. Along with vaccination, doctors urge authorities to launch awareness drives on JE.

An estimated 12.5 million people are thought to be at high risk of JE infection in Nepal.