Japanese encephalitis is a viral disease that infects animals and humans. It is transmitted by mosquitoes and in humans causes inflammation of the membranes around the brain. Intensification and expansion of irrigated rice production systems in South and South-East Asia over the past 20 years have had an important impact on the disease burden caused by Japanese encephalitis. Where irrigation expands into semi-arid areas, the flooding of the fields at the start of each cropping cycle leads to an explosive build-up of the mosquito population. This may cause the circulation of the virus to spill over from their usual hosts (birds and pigs) into the human population.
The disease and how it affects people
Japanese encephalitis (JE) is a disease caused by a flavivirus that affects the membranes around the brain. Most JE virus infections are mild (fever and headache) or without apparent symptoms, but approximately 1 in 200 infections results in severe disease characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and death. The case fatality rate can be as high as 60% among those with disease symptoms; 30% of those who survive suffer from lasting damage to the central nervous system. In areas where the JE virus is common, encephalitis occurs mainly in young children because older children and adults have already been infected and are immune.
The virus causing Japanese encephalitis is transmitted by mosquitoes belonging to the Culex tritaeniorhynchus and Culex vishnui groups, which breed particularly in flooded rice fields. The virus circulates in ardeid birds (herons and egrets). Pigs are amplifying hosts, in that the virus reproduces in pigs and infects mosquitoes that take blood meals, but does not cause disease. The virus tends to spill over into human populations when infected mosquito populations build up explosively and the human biting rate increases (these culicines are normally zoophilic, i.e. they prefer to take blood meals from animals).
Japanese encephalitis is a leading cause of viral encephalitis in Asia with 30,000-50,000 clinical cases reported annually. 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. Because of the critical role of pigs, its presence in Muslim countries is negligible. JE distribution is very significantly linked to irrigated rice production combined with pig rearing.
Scope of the Problem
Japanese encephalitis is a patchy disease and important outbreaks have occurred in a number of places in the past 15 years, including South India (Arkot district in Tamil Nadu) and in Sri Lanka (Mahaweli System H).
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. An inexpensive live-attenuated vaccine is used in China, but is not available elsewhere. Chemical vector control is not a solution, as the breeding sites (irrigated rice fields) are extensive. In some rice production systems faced with water shortages, however, certain water management measures (alternate wetting and drying) may be applied that reduce vector populations. Personal protection (using repellents and/or mosquito nets) will be effective under certain conditions. Eliminating the pig population is often a measure taken in the wake of outbreaks. Certainly, the introduction of pig rearing as a secondary source of income for rice-growing farmers in receptive areas must never be encouraged.
Prepared for World Water Day 2001. Reviewed by staff and experts from the cluster on Communicable Diseases (CDS), and the Water, Sanitation and Health Unit (WSH), World Health Organization (WHO), Geneva.