West Nile Encephalitis

West Nile virus is an arthropod-borne virus (i.e an arbovirus) transmitted between birds by infected mosquitos. People become infected with the virus following the bite of an infected mosquito. The virus is not transmitted from person to person. There is no evidence that a person can get the virus from handling live or dead infected birds. However, it is still recommended that bare-handed contact with dead animals is avoided.

The incubation period 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 results when the virus invades the central nervous system destroying the brain substance with accompanying inflammation. The clinical features range from muscle weakness and paralysis; to mild confusion and behavioural changes (which may be mistaken for hysteria); and to convulsions (fits) and deep coma.

There is no specific treatment for West Nile virus. Current management consists of treating the complications of the disease 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 when the mosquito bites and avoiding areas where stagnant water can be found - mosquito larvae need still water to develop. For further protection use an insect spray containing at least thirty per cent DEET (N,N-diethyl-3methlybenzamide) and sleep under bed-nets.

West Nile virus (genus Flavivirus, family Flaviviridae) was first identified in 1937. It was subsequently shown to have a very wide area of distribution that includes most of Africa, southern Europe, the Middle East, and even parts of the Far East. In its natural cycle, the virus is transmitted primarily between birds by mosquitoes. Classically, West Nile virus causes a non-specific febrile illness, and until recently nervous system manifestations were considered a rarity. However, in recent years the epidemiology has changed with the virus spreading to new areas and causing different disease patterns. In 1996 there was an epidemic of West Nile encephalitis in Romania, and in 1999 the virus reached the United States for the first time with an outbreak in New York. Since then it has spread across the Eastern and Central American states, and in 2002 caused its largest outbreak, with more than six thousand cases and thirty deaths reported by September 2002.

Inheritance patterns
None.

Prenatal diagnosis
None.

Japanese Encephalitis

Japanese encephalitis virus has always been recognised as a killer. Only about 1 in 300 infections results in disease, and there is a wide range of presentations from a simple febrile illness to a severe meningoencephalitis, as well as polio-like acute flaccid paralysis. There are estimated to be fifty thousand cases of Japanese encephalitis annually, with fifteen thousand deaths. The actual numbers may become clearer with the application of new simple rapid diagnostic tests. In addition to the high mortality, approximately half the survivors have severe neuropsychiatric sequelae, with their associated socioeconomic burden.

Over the last fifty years the disease has spread relentlessly across Southeast Asia, India, southern China, and the Pacific reaching Australia in 1998. Epidemics of encephalitis were described in Japan from the 1870's onwards, and Japanese encephalitis virus was first isolated from a fatal case in the 1930's. The virus causing Japanese encephalitis is a member of the Flavivirus group which derives its name from Yellow fever virus (in Latin, 'yellow' is 'flavus'). The flaviviruses are relatively new viruses, thought to derive from a common ancestor ten to twenty thousand years ago, that are rapidly evolving to fill new ecological niches. Japanese encephalitis virus is transmitted in an animal cycle between small birds by Culex mosquitoes, and pigs are important amplifying hosts. Humans become infected by Culex mosquitoes coincidentally, but are not part of the natural cycle. Recent findings in Asia raise important issues about the spread, control and pathogenesis of Japanese encephalitis. It is thought to be spread by birds, but mosquitoes blown between Pacific islands may contribute too. An expensive formalin inactivated and newer live attenuated vaccines against Japanese encephalitis are available, but not for the majority of the two point eight billion people living in affected regions. For them, the factors determining who, of all those infected with Japanese encephalitis virus, develops neurological disease may be critically important. The relative contributions of the human immune response, and viral strain differences are currently being investigated.

Inheritance patterns
None.

Prenatal diagnosis
None.

Tick-borne encephalitis

After one to two weeks incubation the virus causes a sudden onset of fever, headache nausea and photophobia. In mild cases this resolves after a week, but in more severe cases there is a second phase of illness with meningoencephalitis or myelitis. The latter tends to cause flaccid paralysis of the upper limb and shoulder girdle. Respiratory muscle and bulbar (brainstem) involvement lead to respiratory failure and death.

Tick-borne encephalitis virus is a member of the flavivirus genus (group) that circulates in small wild animals, mostly rodents, and is transmitted between them, and to humans, by Ixodes ticks. Humans may also become infected by drinking goat's milk.

It has a wide area of distribution across Europe and the former USSR, and its seasonal incidence is reflected in one of the many pseudonyms 'Russian spring-summer encephalitis.' Genetic sequencing has allowed Western tick-borne encephalitis virus, which is endemic in Germany, Austria and much of Europe, to be distinguished from Far-Eastern tick-borne encephalitis virus which is found across the former Soviet Union.

Far Eastern tick-borne encephalitis has a higher case fatality rate, but the Western form is often associated with sequelae (after effects).

A formalin inactivated vaccine given as two doses four to six weeks apart, has been recommended for those likely to be exposed in the endemic forested areas of Europe and the former USSR. Such vaccines are now used widely in Austria.

Louping ill virus 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 a meningoencephalitis, which can be severe.

Powassan virus is a distantly related tick borne flavivirus found principally among small mammals in Canada that has occasionally caused meningoencephalitis in humans.

Inheritance patterns
None.

Prenatal diagnosis
None.

Medical text written September 2002 by Dr T Solomon. Last updated May 2007 by Dr T Solomon, Hon. Lecturer in Medical Microbiology and Tropical Medicine, University of Liverpool, Liverpool, UK.

Further Online Resources
Medical texts in The Contact a Family Directory are designed to give a short, clear description of specific conditions and rare disorders. More extensive information on this condition can be found on a range of reliable, validated web sites and links to them are included in the CD-ROM version of this Directory. Further information on these resources can be found in our Medical Information on the Internet article.

Information and support on all forms of Encephalitis can be obtained from the Encephalitis Support Group