Japanese encephalitis virus (JEV) (Flavivirus: Flaviviridae) is a leading cause of encephalitis in eastern and southern Asia. The virus is maintained in a zoonotic cycle between ardeid wading birds and/or pigs and Culex mosquitoes. The primary mosquito vector of JEV is Culex tritaeniorhynchus, although species such as Cx. gelidus, Cx. fuscocephala, and Cx. annulirostris are important secondary or regional vectors. Control of JEV is achieved through human and/or swine vaccination, changes in animal husbandry, mosquito control, or a combination of these strategies. This review outlines the ecology of JEV and examines the recent expansion of its geographical range, before assessing its ability to emerge in new regions, using the hypothetical establishment in the United States as a case study.

Ecology and Geographical Expansion of Japanese Encephalitis Virus

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. 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.

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NEUROLOGICAL ASPECTS OF TROPICAL DISEASE: Japanese encephalitis

Japanese encephalitis vaccines

Japanese encephalitis (JE) virus is a mosquito-borne zoonotic flavivirus that infects a wide range of vertebrate species in an enzootic cycle primarily of large waterfowl birds and swine. Horses and humans are considered bystanders to this enzootic cycle and, once infected, dead-end hosts. JE infection in humans can manifest in a spectrum of disease from asymptomatic infection to a mildly febrile symptomatic illness to a life-threatening disease affecting the central nervous system (CNS). The latter is associated with a high morbidity and mortality as well as long-term neurologic sequelae. The pathogenesis and disease severity of JE is discussed elsewhere in this volume. JE is the most common cause of encephalitis in most of Asia and causes an estimated 35,000 cases of encephalitis annually (Igarashi 1992). By all accounts this is an underestimate of the true disease burden of this virus and evidence suggests that this virus is endemic over a far wider region today than 50 years ago, despite the use of an effective vaccine for humans and animals.

Japanese encephalitis virus: ecology and epidemiology.

Japanese encephalitis (JE) virus is the most important cause of epidemic encephalitis worldwide with an estimated 30,000–50,000 cases annually (Solomon 2000). The geographical area affected by the virus is expanding, and despite the availability of vaccines, JE is a growing public health problem (Tsai 2000; Vaughn and Hoke 1992). The closely related West Nile (WN) virus is the most widely distributed member of the JE virus complex of flaviviruses. Until recently it was considered to be a relatively benign virus, causing epidemics of a fever—arthralgia—rash syndrome, and only occasional central nervous system (CNS) disease (Solomon and Cardosa 2000). However, its importance has grown with recent outbreaks of West Nile encephalitis (WNE) in Romania and New York (Anonymous 1999; Tsai et al. 1998). For both JE and WN viruses the number of people who present with clinical features represents only a small proportion of those infected. In addition, both diseases have a broad spectrum of clinical manifestations, ranging from a nonspecific febrile illness to a severe meningoencephalomyelitis (inflammation of the meninges, brain matter, and spinal cord). In this chapter, we will look at factors that determine who, of all those infected with JE or WN virus, develops disease. Secondly we will describe the clinical features of infection with JE and WN viruses and consider the extent to which pathophysiological and pathological studies explain these features. Finally we will consider the diagnosis and management of JE and WNE and the prospects for future therapies.

Pien Chiewanich

Papers

Study of Japanese encephalitis virus in Chiangmai valley, Thailand. II. Human clinical infections.