What is japanese encephalitis?

Japanese encephalitis is a viral infection that causes inflammation of the brain. The NHS describes Japanese encephalitis as a rare and serious infection that develops in some parts of the world if you are bitten by a mosquito. This is because mosquito bites can spread the virus, causing Japanese encephalitis. The virus belongs to a group of viruses called flaviviruses. Flaviviruses are a subset of viruses which are usually spread by an infected vector, such as mosquitoes or ticks. Other flaviviruses cause similar diseases, including West Nile virus, St. Louis encephalitis virus, tick-borne encephalitis virus, and Powassan virus.¹

Japanese encephalitis is not found in the UK, but it is prevalent in rural parts of Asia, Australia and the western Pacific. It is unlikely for travellers to catch the disease, but you should still get vaccinated before travelling to these areas. You can find advice on your travel risk here.

Most people who get Japanese encephalitis don’t experience any symptoms. Some people get the following symptoms: headache, high temperature, nausea, and stomach pain. In some cases, the virus spreads to the brain and causes more serious symptoms, such as more severe headaches, paralysis and seizures. It can also affect your cognitive function, causing confusion and changes in behaviour.

Japanese encephalitis can be life-threatening, so it’s important that you seek medical advice and attention if you have travelled to an area where the virus is found and if you are experiencing the associated symptoms.

Pathophysiology of japanese encephalitis

The Japanese encephalitis virus works by attaching to the cell membrane of host cells, initially spreading from where you were bitten and nearby lymph nodes. After infection, the viral levels in the bloodstream rise, but most cases in humans are cleared out before the virus enters the central nervous system, resulting in little/no symptoms being observed. In a smaller proportion of cases, the virus crosses the blood-brain barrier and enters the brain. When this happens, the nervous system can be seriously affected. Japanese encephalitis virus can cause inflammation of the brain, which can damage neurons inside of the brain and cause headaches, seizures, fever, confusion, muscle weakness and sometimes comas².

Mosquitoes spreading japanese encephalitis

Japanese encephalitis is spread to humans when they are bitten by infected mosquitoes, mainly of the Culex species (especially Culex tritaeniorhynchus), although many other species of mosquito can also spread the disease. As a result, mosquitoes are considered the primary vector for Japanese encephalitis, meaning that mosquitoes are the main way that the disease is spread. 

A mosquito which is infected with the Japanese encephalitis virus carries the virus in its saliva, so when the mosquito bites you, the virus is sent into your bloodstream.

Humans do not develop a high enough level of the Japanese encephalitis virus in their bloodstream, so even if you are infected, you will not infect anyone else, and uninfected mosquitoes can’t catch the virus from you either. For this reason, humans are called ‘dead-end hosts’ as they do not spread the disease.³

Understanding that mosquitoes are the primary vector for Japanese encephalitis and play a significant role in the transmission of the disease has been very important for our understanding of how to control the transmission of the disease and how to prevent people from catching it.

Transmission cycle of japanese encephalitis

The virus exists in a transmission cycle between mosquitoes, pigs and/or birds which live near water. This means that the Japanese encephalitis virus is kept in the environment by spreading between mosquitoes, wading birds and pigs.

When a mosquito carrying the Japanese encephalitis virus bites a pig or bird, the virus enters the bloodstream of the animal. Once the animals are infected with the virus, the virus rapidly replicates so that there’s a high level of viral particles in the bloodstream. For this reason, birds and pigs are called ‘amplifying hosts’ as they allow many copies of the virus to accumulate. This makes it a lot more likely that any uninfected mosquito that bites the bird or pig will become infected. If an infected mosquito then bites you, the virus will be transmitted into your bloodstream.

Pigs and birds play an important role in the transmission of Japanese encephalitis as pigs can carry high levels of the virus without showing any severe symptoms, making the disease hard to track. Additionally, birds are capable of quickly spreading the disease to different areas.³

The virus can be transmitted between people through blood transfusions. As a result, people who have been recently diagnosed with Japanese encephalitis virus infection should not donate blood and bone marrow for 120 days following infection. 

Risk factors for transmission of japanese encephalitis

The World Health Organisation outlines the following factors which influence variations in the transmission of Japanese encephalitis:

• Geographic location: Areas where you are particularly at risk of developing Japanese encephalitis include Asia and parts of the western Pacific. Additionally, Japanese encephalitis usually occurs in agricultural and rural areas.
• Seasonal patterns: Seasonal changes have been linked to differences in the transmission of Japanese encephalitis. For example, transmission is usually greater during rainy seasons, likely due to mosquitoes laying eggs in water. Transmission also increases during warmer months when mosquitoes are more active.
• Agricultural practices: Transmission occurs most commonly in agricultural areas such as farms and rice paddies but may occur in urban areas under certain conditions.

Prevention and control of transmission

The CDC has provided the following advice for preventing Japanese encephalitis infections:

• Mosquito control measures: You should keep your property dry to reduce the risk of mosquitos accumulating - mosquitoes lay their eggs in standing water, so you should keep your belongings dry and spray them with mosquito repellant. You should take care to protect against mosquito bites, for example, by wearing proper protective clothing, including long sleeves and minimising any exposed skin. Furthermore, investing in mosquito nets and mosquito repellant sprays are also sensible ideas.
• Vaccination: there are currently vaccinations available for Japanese encephalitis, which are both safe and effective at protecting against Japanese encephalitis. If you are planning to travel to any areas where the virus exists, you should get vaccinated first and try to avoid mosquito bites. Although the risk of travellers developing Japanese encephalitis is low, you should still be careful. Your healthcare provider will ask about your medical history before giving you the vaccine because some medications interact with the vaccination, meaning the vaccination may not work as intended. The vaccination is injected directly into a muscle. To read more about the vaccine, see here.
• Public health awareness: Public health awareness is important in reducing the prevalence of Japanese encephalitis by educating people, promoting preventative measures against Japanese encephalitis, and encouraging vaccination in areas which are at a high risk of Japanese encephalitis. By educating as many people as possible about the causes and risks of Japanese encephalitis, more people will know how to avoid Japanese encephalitis, as well as the importance of doing so, which will reduce the incidence rate of Japanese encephalitis. 

Summary

Japanese encephalitis is a rare, and serious infection that is transmitted to humans in some parts of the world if you are bitten by an infected mosquito. The virus flourishes in a cycle between mosquitoes, pigs and wading birds. Japanese encephalitis is not found in the UK, but it is prevalent in rural parts of Asia, Australia and the western Pacific. Most people who get Japanese encephalitis don’t experience any symptoms. The most serious cases of Japanese encephalitis occur if the virus crosses the blood-brain barrier and affects the nervous system. In these cases, symptoms include severe headaches, paralysis, confusion, fever, seizures, and sometimes falling into a coma. Humans are ‘dead-end hosts’ as they do not spread the disease further. Birds and pigs are called ‘amplifying hosts’ as they allow many copies of the virus to accumulate. The virus can be transmitted between people through blood transfusions; you should not donate blood for at least 120 days after infection. Transmission is greater during warmer, wetter seasons. The risk of transmission can be reduced by reducing the risk of mosquito bites, vaccination, and by educating people in high-risk areas