Japanese Encephalitis

Japanese Encephalitis

Japanese encephalitis (JE) is a mosquito-borne, viral infection that can result in a febrile illness of variable severity, associated with neurological symptoms ranging from headache to encephalitis or meningitis. Only about one in 250–500 infected individuals manifest clinical disease. The high case-fatality ratio (CFR) (20–30%) and frequent residual neuropsychiatric damage in survivors (50–70%) make JE a major public health concern.

Most fatalities and residual sequel occur in children under 10 years of age.

Symptoms

JE is mostly characterized by the sudden onset of fever, chills, aches and headaches. Some individuals progress rapidly to severe encephalitis with mental disturbances, motor abnormalities and progressive coma.

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Infectious agents

The infectious agent is Japanese encephalitis virus (JE virus), which is a group B arbovirus (Flaviviridae).

Mode of Transmission

JE is transmitted by the bite of an infected mosquito. The vector is the Culex species of mosquito, belonging to the C. tritaeniorhynchus and the C. vishnui groups, which breed particularly in flooded rice fields. Larvae are found in many temporary, semipermanent and permanent groundwater habitats that are sunlit and contain vegetation. Habitats include ground pools, streams, swamps and low-salinity tidal marshes. The female mosquitoes bite mainly cattle and pigs.

 Culex mosquito. Photo Courtesy of Medical News Today.

Humans are incidental hosts; they become infected with JE virus coincidentally when living with or travelling close to animals and birds infected with JE.

Prevention

Health centres comprise an alert network for early diagnosis and symptomatic treatment of suspected cases. Laboratory diagnostic capacity is essential for case confirmation.

Health education should involve:

  • simple information on JE, including its cause and transmission, and the pre- vention of mosquito bites
  • community action in reducing mosquito breeding places; for example, by fill- ing pools, weekly drainage of accumulated water and lowering of water levels in rice fields.

Vector control

Insecticide spraying is not considered a major control strategy. However, for the immediate suppression of infective vectors, ULV (ultra-low volume) or thermal fogging with malathion (an insecticide) may be employed, even though it is not very cost effective. Environmental measures are recommended.

Short-term measures

Larvicide is impractical for widespread breeding habitats.

Long-term measures

Long-term measures include:

  • water management (particularly in irrigated rice fields), for periodic drying of fields
  • selection of rice varieties with minimum water requirements
  • use of larvivorous fish
  • manipulations of the environment (e.g. reduction of drainage, filling of pools and weeding).

Pig vaccination is costly, difficult and time consuming. Also, pig control by segregation or slaughtering is difficult, and the economic losses are high.

References

WHO vaccine-preventable diseases: monitoring system — 2010 global summary. World Health Organization (http://www.who.int/vaccines/globalsummary/immunization/countryprofile select.cfm).

Further reading

WHO — recommended standards for surveillance of selected vaccine-preventable diseases. Geneva, WHO, 2008 (WHO/V&B/03.01) (http://whqlibdoc.who.int/hq/2003/WHO_V&B_03.01.pdf, accessed 11 August 2010).