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Arboviral Encephalitides

Also known as: West Nile Encephalitis, Japanese Encephalitis, Tick-borne Encephalitis

Background

Encephalitis is inflammation of the brain. In the case of arboviral encephalitides, inflammation is caused by a virus invading the brain. Viruses that are transmitted from one animal host to the next by insects (arthropods) are known as 'arboviruses' (arthropod-borne).

Three types of arboviral encephalitides are described in this entry:

  • West Nile encephalitis
  • Japanese encephalitis
  • tick-borne encephalitis.

Credits

Medical text written September 2002 by Dr T Solomon. Last reviewed December 2012 by Dr T Solomon, Honorary Lecturer in Medical Microbiology and Tropical Medicine, University of Liverpool, Liverpool, UK.

West Nile encephalitis

People become infected with West Nile encephalitis from the bite of an infected mosquito. The virus is not transmitted from person to person. West Nile virus has been described in Africa, Europe, the Middle East, west and central Asia, Oceania, and most recently, North America. The incubation period (time it takes from getting the virus until symptoms are noticed) of a West Nile virus infection is usually five to fifteen days. Mild infections are common and include fever, headache and body aches, often with skin rash and swollen lymph glands.

Encephalitis occurs when the virus invades the central nervous system destroying the brain with accompanying inflammation. The symptoms include muscle weakness and paralysis, mild confusion and behavioural changes (which may be mistaken for hysteria), convulsions (fits) and deep coma.

There is no specific treatment for West Nile virus. Current management consists of treating the complications of the condition such as high fever and aches, some patients are left with severe paralysis, convulsions or raised intracranial pressure.

The simplest preventative measure is to avoid bites from the mosquitoes that carry the virus. This involves wearing long sleeves and trousers, especially during the evening. For further protection use an insect spray containing at least 30 per cent DEET (N,N-diethyl-3methlybenzamide) and sleep under bed-nets.

Inheritance patterns
None.

Prenatal diagnosis
None.

Japanese encephalitis

Japanese encephalitis virus is transmitted in an animal cycle between small birds by culex mosquitoes, and pigs. Humans become infected by a bite from a mosquito. It occurs from the islands of the Western Pacific in the east to the Pakistani border in the west, and from Korea in the north to Papua New Guinea in the south.

Japanese encephalitis is a condition caused by a flavivirus that affects the membranes around the brain causing encephalitis. Most Japanese encephalitis virus infections are mild (fever and headache) or without apparent symptoms. More serve infection result in severe disease characterised by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis (paralysis in which the part of the nervous system that controls coordinated movement of the voluntary muscles is disabled) and sometimes can be fatal.

An effective killed vaccine is available for Japanese encephalitis, but it is expensive and requires one primary vaccination followed by two boosters. This is an adequate intervention for travellers, but has limited public health value in areas where health services have limited resources. A live vaccine is used in China. Personal protection (using repellents and/or mosquito nets) will be effective under certain conditions.

Inheritance patterns 
None.

Prenatal diagnosis
None.

Tick-borne encephalitis

Tick-borne encephalitis (TBE) has a wide area of distribution across Europe and Russia. It is caused by a virus that is spread by ticks. Ticks are small parasites that survive by sucking blood from animals - including humans. It is also found in small rodents and some larger animals such as sheep and goats.

There are three different types of TBE: the European, the Siberian and the Far Eastern types. The European type is mainly found in central, eastern and northern Europe. The main times in the year when people get TBEare spring and summer (it is also sometimes called Russian Spring-Summer Encephalitis).

After one to two weeks' incubation the virus causes symptoms. In the European type of TBE there are normally 2 phases to the illness. The first phase occurs when the virus is in the blood stream. This phase consists of flu-like illness with symptoms such as fever, headache and generalised body aches. It lasts around 5 days. There is then about a week with no symptoms which is followed by the second phase of the illness. The second phase is where the brain or nerves are involved. The severity of the second phase varies between patients. Some people will have a very mild illness whilst in others may be serious.

The symptoms in the second stage are similar to other causes of encephalitis and meningoencephalitis (inflammation of the lining of the brain). These include neck stiffness, headache, drowsiness, poor coordination and tremor. Some patients will also get limb weakness (most often in the arms). In severe cases, the muscles that control the breathing are affected and this may be fatal.

A vaccine given as two doses four to six weeks apart, has been recommended for those likely to be exposed in the forested areas of Europe and Russia where TBE exists. Such vaccines are now used widely in Austria.

Louping ill virus: This is a closely related tick-borne virus notable for being the only flavivirus found naturally in the British Isles (as well as Scandinavia). It occurs naturally among small mammals (hares, wood-lice and shrews), but is also transmitted to highland sheep, which develop encephalitis. The disease is named after the leaping (or louping) demonstrated by the encephalitic sheep. Very occasionally the virus infects humans causing meningoencephalitis, which can be severe.

Powassan virus: This is a distantly related tick-borne flavivirus found principally among small mammals in Canada that occasionally causes meningoencephalitis in humans.

Inheritance patterns
None.

Prenatal diagnosis
None.

Is there support?

Information and support in the UK for arboviral encephalitides is provided by the Encephalitis Society (see entry Encephalitis).