Japanese encephalitis and other forms of viral encephalitis transmitted by mosquito

Written by Dr Charlie Easmon, specialist adviser in travel medicine

What is Japanese encephalitis?

Japanese encephalitis is a viral disease, transmitted by mosquito bites. It causes serious inflammation of the brain, which may lead to permanent brain damage, and has a high mortality rate. However, the great majority of cases only produce a mild, influenza-like illness with no effect on the brain.

The disease is prevalent in large areas of Asia. However, it is uncommon among tourists and short-term visitors in the areas affected.

There is a vaccine, which forms part of the child vaccination programme in countries like Thailand, Korea, and Japan.

What causes Japanese encephalitis?

Japanese encephalitis virus belongs to the Flaviviridae family, which also includes dengue virus and yellow fever virus. When the virus enters the bitten person's bloodstream via the mosquito's saliva, it replicates itself in a variety of cells. Eventually the virus attacks the cells of the central nervous system, causing the typical symptoms of the disease to appear.

In the majority of cases the disease does not develop sufficiently to produce any obvious signs. It is therefore thought that only 1 out of 100 adults who come into contact with Japanese encephalitis actually develop any visible signs of it.

Which other viruses cause encephalitis?

There are other flaviviruses, which cause viral encephalitis. They have a similar disease picture, and their names often relate to where they occur. They include:

St Louis and Murray Valley flaviviruses

Russian Spring Summer flaviviruses, transmitted by ticks. A vaccine is available for it.

Three other encephalitis viruses belong to the Alphavirus genus and are also transmitted by mosquito bite. They are Western, Eastern and Venezuelan equine encephalitis, or WEE, EEE and VEE respectively. These all occur in North and South America. There are also vaccines that protect against EEE and WEE.

Like Japanese encephalitis these viruses often only produce mild general symptoms, similar to mild influenza.

How does Japanese encephalitis spread?

The virus is passed on by the bite of an infected mosquito that has previously sucked blood from an infected animal or person. Birds are the common hosts for several of the various encephalitis viruses. The Japanese encephalitis virus in particular is also able to replicate itself in pigs and birds. This means that the pig and bird populations constitute a reservoir of the disease, which may be difficult to eradicate. Infected mosquitoes are particularly common in the countryside and slums of large cities.

Where does Japanese encephalitis occur and how many people are affected?

The virus is prevalent in many countries in Asia. The disease is very widespread in China, Sri Lanka, the Philippines, Nepal, Indonesia and South East Asia, with epidemics breaking out every few years. In the UK, rare cases of Japanese encephalitis are reported in travellers from abroad.

The World Health Organisation estimates that there are at least 50,000 serious cases of the disease in Asia each year. Approximately 10,000 of those die, mostly children. However, by far the majority of cases of Japanese encephalitis do not have either brain symptoms or serious consequences.

The mosquito bites at night and loves warmth and humidity. From May to October, for instance, there are five times as many cases in Thailand and Vietnam as throughout the rest of the year.

What are the symptoms of the disease?

The incubation period, from infection to developing Japanese encephalitis, is 4 to 16 days.

At most, 1 person in 100 develops serious encephalitis. The mortality rate in this group is as high as 10 per cent.

In serious cases the following symptoms are observed:

high fever

headache

exaggerated reflexes

reduced levels of consciousness and possibly coma.

Even if the patient recovers, there is a high risk of permanent brain damage.

What can you do yourself?

Vaccination There is a vaccine against Japanese encephalitis virus. Vaccination is recommended if you intend to spend more than three to four weeks in a region where the disease is prevalent. There are two different schedules for vaccination. The first involves three injections over a period of 28 days and the second involves two injections given one to four weeks apart. Both provide protection for a year. A booster vaccination administered one year later extends protection to three years.

People who have the vaccine should stay in areas where they have access to medical care for 10 days following a dose because it can occasionally cause a severe allergic reaction.

The vaccine does not give 100 per cent protection, so it is no alternative to ordinary protection against mosquito bites. There are also vaccines against Russian Spring Summer virus, EEE and WEE. They are used in special circumstances for travellers to Western and Central Europe.

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