NEUROLOGICAL ASPECTS OF TROPICAL DISEASE: Japanese encephalitis

Although considered by many in the west to be a rare and exotic infection, Japanese encephalitis is numerically one of the most important causes of viral encephalitis worldwide, with an estimated 50 000 cases and 15 000 deaths annually.1 2 About one third of patients die, and half of the survivors have severe neuropshychiatric sequelae.Most of China, Southeast Asia, and the Indian subcontinent are aVected by the virus, which is spreading at an alarming rate.In these areas, wards full of children and young adults aZicted by Japanese encephalitis attest to its importance. Historical perspectiveEpidemics of encephalitis were described in Japan from the 1870s onwards.Major epidemics were reported about every 10 years, with more than 6000 cases reported in the 1924 epidemic.3 The term type B encephalitis was originally used to distinguish these summer epidemics from von Economo's encephalitis lethargica (sleeping sickness, known as type A), but the B has since been dropped.In 1933 a filterable agent was transmitted from the brain of a fatal case to cause encephalitis in monkeys; the prototype Nakayama strain of Japanese encephalitis virus was isolated from the brain of a fatal case in 1935.The virus was later classed as a member of the genus Flavivirus (family Flaviviridae) named after the prototype yellow fever virus (Latin; yellow=flavi).Although of no taxonomic significance, the ecological term arbovirus is often used to describe the fact that Japanese encephalitis virus is insect (arthropod) borne. EpidemiologyNEUROTROPIC FLAVIVIRUSES: A GLOBAL PERSPECTIVE Japanese encephalitis virus is transmitted between animals by Culex mosquitoes, and occurs across eastern and southern Asia and the Pacific rim.However, related neurotropic flaviviruses are found across the globe (fig 1); they share many virological, epidemiological, and clinical features.2 Molecular virological studies suggest that all flaviviruses derived from a common ancestor some 10-20 000 years ago, and are rapidly evolving to fill ecological niches.4 Examples of mosquito borne neurotropic flaviviruses include Murray Valley encephalitis virus in Australia, and St Louis encephalitis virus in North America.West Nile virus, a flavivirus found in Africa, the Middle East, and parts of Europe, is traditionally associated with a syndrome of fever arthralgia and rash, and with occasional nervous system disease.However, in 1996 West Nile virus caused an outbreak of encephalitis in Romania, 5 and a West Nile-like flavivirus was responsible for an encephalitis outbreak in New York in 1999.6 7 In northern Europe and northern Asia, flaviviruses have evolved to use ticks as vectors because they are more abundant than mosquitoes in cooler climates.Far eastern tick-borne encephalitis virus (also known as Russian spring-summer encephalitis virus) is endemic in the eastern part of the former USSR, and Western tick-borne encephalitis virus occurs in Europe and has caused recent epidemics in Germany and Austria.8 In the United Kingdom the tick borne Louping Ill virus is is enzootic in sheep, and occasionally causes encephalitis in sheep and humans.