Japanese encephalitis

Japanese encephalitis is a serious infection of the brain. It is caused by the Japanese encephalitis virus, also known as JEV. This virus belongs to the Flaviviridae family. It is part of a group of nine related viruses called the Japanese encephalitis serocomplex. While many of these viruses are dangerous to horses, four of them can infect humans. One well-known relative of JEV is the West Nile virus.

Most people who catch JEV show no symptoms at all. However, for some, the virus causes inflammation of the brain. This condition is called encephalitis. It can lead to severe problems like fever, headache, vomiting, confusion, and seizures. These symptoms usually appear 5 to 15 days after the initial infection.

The virus moves through a specific cycle involving animals and insects. Mosquitoes of the Culex type are the primary vectors, or carriers, of the disease. These mosquitoes pick up the virus from reservoirs like wild birds and pigs. Pigs play a very special role as amplifying hosts. This means the virus can grow and spread quickly inside them. Humans and horses are considered dead-end hosts. This means the virus does not continue to spread to new people from an infected human.

The disease mostly occurs in rural areas rather than in big cities.

Inside the body, the virus works by attacking cells. JEV is an enveloped virus with a single-stranded RNA genome. This genome is protected by a protein shell called a capsid. The outer envelope helps the virus enter a host cell. Once inside, the virus infects the lumen of the endoplasmic reticulum, which is a part of the cell. The virus then produces many viral proteins. In the brain, the virus can trigger a very intense immune response. Microglia are the resident immune cells of the central nervous system. When they are activated to fight the virus, they release substances called cytokines. While these cytokines try to kill the virus, they can also cause toxic effects that damage the brain itself. This can create a dangerous loop of damage.

Scientists have studied the history and types of JEV for a long time. The disease was first described in Japan in 1871. Researchers believe the virus may have started evolving in the mid-16th century. There are five known genotypes of the virus, labeled I through V. One important version is the Muar strain from Malaya, which is the prototype for genotype V. As of 2024, scientists have sequenced the complete genomes of 372 different strains. This detailed mapping helps experts understand how the virus changes and moves across different regions.

The impact of this disease is quite large across the globe. About 3 billion people live in areas where JEV is present. This includes much of East Asia, Southeast Asia, and the Western Pacific. Every year, there are approximately 68,000 symptomatic cases. Around 17,000 people die from the infection annually. The mortality rate is about 25%. This rate is often higher for children under five, the elderly, or people with weak immune systems.

Even for those who survive, the effects can be long-lasting. Up to 50% of survivors may face permanent neurological problems. These can include deafness, emotional changes, or weakness on one side of the body.

Preventing the disease is a major goal for health officials. The most effective tool is the Japanese encephalitis vaccine. There are currently three different vaccines available, such as IXIARO and IMOJEV. All of these current vaccines are based on the genotype III virus.

In the past, some vaccines were grown in mouse brains, which carried a small risk of neurological complications. Newer vaccines are produced using cell culture, which has fewer side effects. Because protection might fade after a few years, doctors often recommend booster shots every 11 months for people at risk. Other prevention methods include simply avoiding mosquito bites.

There is currently no specific medicine to cure the infection once it starts. Instead, doctors provide supportive care in a hospital setting. This care helps with breathing, eating, and controlling seizures. Doctors may also use medicine like mannitol to manage pressure inside the brain.

While there is no cure, understanding the disease is improving. Scientists are studying how specific receptors on immune cells affect how severe the infection becomes. This research could lead to new ways to treat the disease in the future. For now, vaccination and mosquito control remain the best ways to stay safe.