Japanese encephalitis

Japanese encephalitis is a viral infection carried by mosquitoes across Asia and the western Pacific. Infections are numerous, but the great majority pass unnoticed: either nothing happens at all, or it amounts to a few days of feeling unwell. What makes it dangerous is that in a small proportion of those infected the virus reaches the brain and sets off a severe inflammation for which there is no medicine. Travellers rarely fall ill, but it is precisely the absence of any treatment that puts all the weight on preparation: vaccination and bite protection belong before the trip, not after the first symptoms.

Where it is caught

The virus circulates in nature between water birds, pigs and mosquitoes. Pigs act as amplifiers: the virus multiplies in them particularly readily, and near a pig farm there are noticeably more infected mosquitoes. People enter that circle by accident and do not become a source of infection themselves — Japanese encephalitis does not pass from person to person, not through nursing someone, not sexually, not by coughing.

Infection happens in rural parts of southern, south-eastern and eastern Asia, and in some areas of Oceania: wherever there are flooded rice paddies, irrigation channels, marshy lowlands and pig keeping. The central districts of big cities are not usually reckoned risky; outskirts and villages with fields alongside them are.

The carriers are mosquitoes of the Culex group. Two of their habits matter for protection: they breed in standing water, flooded paddy fields included, and they bite at dusk and through the night — unlike the mosquito that spreads dengue, which is active by day. Evening and night-time protection therefore counts for more here than daytime cover.

The season depends on the climate. In the cooler northern parts of Asia transmission flares up in the warm season and the rains; nearer the equator it runs all year with a rise in the wet months. Check the season for the particular country and region shortly before you go.

Who is genuinely at risk

For the ordinary tourist spending a week in a city and staying in an air-conditioned hotel, the risk is very small. It grows as the trip becomes a rural one:

• a long stay in an area of transmission — a month or more, especially living and working there;
• sleeping in the countryside, in a tent or in a house without window screens, next to paddy fields, channels or pig sheds;
• activities that keep you outdoors for hours in the evening: cycling and walking routes, fieldwork and farm work, fishing, birdwatching, working with livestock;
• travelling at the height of the transmission season, even on a short trip;
• visits to relatives in rural areas — people going "home" often do not think of themselves as travellers and give the risk no thought at all.

The illness runs hardest in small children and in older people.

How it shows itself

Between the bite and the first signs there are roughly five days to a fortnight. Most infected people notice nothing whatever. Some develop non-specific complaints, on which Japanese encephalitis cannot be told apart from any other infection:

• a high temperature and chills;
• headache;
• nausea and vomiting, tummy pain, diarrhoea;
• aching limbs and weakness.

Usually that is where it ends. But in fewer than one in a hundred of those infected the illness moves to the nervous system within a few days, and the picture changes sharply:

• a very severe headache and a stiff neck;
• confusion, strange behaviour, delirium, drowsiness deepening until the person cannot be roused;
• fits — in children often the very first and most obvious sign;
• weakness or loss of feeling in an arm, a leg or one side of the body;
• tremor, stiffness, slowed movement, disturbed speech;
• in babies, a bulging fontanelle, refusing feeds, a monotonous cry.

That switch is the line beyond which time is counted in hours.

When to call an ambulance

Call an ambulance (in Spain, Italy, Portugal, Poland and Ukraine that is the single European number 112) if somebody who has come back from an area where Japanese encephalitis occurs, or who is still there, develops:

• a temperature together with a stiff neck and severe headache;
• confusion, odd behaviour, an inability to wake them;
• fits;
• weakness or numbness in the limbs, difficulty speaking;
• repeated vomiting alongside a severe headache;
• a rash of pinpricks and bruises that does not fade when pressed.

Do not drive yourself, and do not take a confused patient in your own car: a fit may start on the way. Be sure to tell the medical team which countries you were in and when, because the range of possibilities hangs on it; without that sentence the work-up goes down a different road. Malaria, dengue, rabies and bacterial meningitis all begin in a similar way and call for entirely different action, and some of them are treatable, and treatable urgently.

What a hospital can do

There is no medicine that kills this virus, and no antiviral drug has shown any benefit in Japanese encephalitis. Hospital treatment is supportive, and the outcome depends on it: fits are brought under control, breathing is watched and a ventilator used if needed, brain swelling and raised pressure inside the skull are fought, fluids are replaced, the temperature is brought down, feeding is given by tube, and someone who cannot move is nursed. Steroids, once prescribed in this illness, did not improve survival in trials, so they are not something to count on.

The outlook deserves plain speaking, because the decision about vaccination rests on it. Once brain inflammation has set in, roughly one patient in four or five dies. Among the survivors a substantial share — between a third and a half by various estimates — is left with lasting effects: weakness in the limbs, seizures, speech difficulties, poorer memory and attention, changes in behaviour. In children this may tell on later development. Recovery takes months and calls for rehabilitation.

The vaccine

Vaccination is the only measure that protects against the disease itself rather than merely against bites. The vaccine is inactivated, meaning it contains no live virus, so it also suits people with weakened immunity. Both adults and children from an early age can be vaccinated.

The course is two doses several weeks apart; adults also have an accelerated schedule available. Because of that gap you need to see a travel health service or a doctor well in advance — about six weeks before flying, not a couple of days. Protection does not appear the moment the first dose is given. If you keep travelling to areas of risk, a booster will be needed some time after the primary course; the timing depends on the vaccine used, and your doctor will tell you.

The vaccine is discussed rather than handed to everyone: the doctor weighs the season, the length of the trip, the kind of accommodation and activities, your age and your state of health. It is considered separately in pregnancy and in serious illness, where the decision comes down to the risk of the trip against the risk of the vaccine. Side effects are usually limited to soreness at the injection site, headache and aching muscles over the first days.

And the essential point: a vaccinated person still guards against bites. The same mosquito activity in the same places brings dengue, chikungunya and, in many areas, malaria, against which this vaccine gives no protection at all.

How to avoid being bitten

• Put repellent on exposed skin, using products based on DEET or icaridin. Apply it over sunscreen rather than under it, and reapply as the label says, particularly after swimming or heavy sweating.
• Sleep under an insecticide-treated net if the room has no window screens or air conditioning. Check it for holes and tuck it under the mattress.
• From dusk until dawn wear a long-sleeved shirt, long trousers, socks and closed shoes; a close-woven, loose-fitting fabric is better, because a mosquito bites straight through thin clothing stretched against the skin. Clothing can be treated with permethrin.
• When choosing rural accommodation, prefer rooms with air conditioning and window screens, and try not to settle right beside paddy fields, standing water and pig sheds.
• Clear standing water around the house: buckets, plant saucers, tyres, forgotten containers — these are where mosquitoes breed.

Online consultation

An online consultation is most useful before the trip. The doctor will go through your itinerary region by region and season by season, weigh up the nature of the journey and the accommodation, and tell you whether you need the Japanese encephalitis vaccine or whether the risk is too small to bother. They will look at the trip as a whole at the same time: which other vaccines are due, whether malaria prevention is needed, what to pack in the medical kit and when to start so the course is finished in time. After you return, an online appointment makes sense if a fever and malaise appear: the doctor will go over the dates, match them against your route and say which tests are needed and how urgently. But with confusion, fits, a stiff neck or weakness in the limbs nobody waits for an online consultation — you call an ambulance.

This material is for information only and does not replace medical advice.