japanese%20encephalitis%20virus
JAPANESE ENCEPHALITIS VIRUS

Japanese encephalitis virus is an RNA flavivirus that causes virus encephalitis across Asia, the western Pacific region & parts of Australia.

It is transmitted in an enzootic cycle and the virus is transmitted to humans through the bite of infected Culex tritaeniorhynchus mosquitoes.

There is no specific antiviral treatment for Japanese encephalitis virus and management is mainly symptomatic treatment and supportive care.

Personal protection from mosquito bites in endemic areas and obtaining vaccination are the primary strategies to control Japanese encephalitis virus infection due to lack of specific antiviral therapy, high case fatality, & substantial morbidity.

Etiology

  • Japanese encephalitis virus is a single stranded RNA mosquito-borne flavivirus
  • Important cause of viral encephalitis across Asia, the western Pacific region, and parts of Australia due to its frequency and severity
  • Japanese encephalitis virus was first isolated in Japan in 1935 but its first case was first documented in 1871

Pathophysiology

Transmission

  • Japanese encephalitis virus is transmitted in an enzootic cycle involving mosquitoes and vertebrae amplifying hosts (eg pigs and wading birds)
    • Transmission occurs usually in agricultural areas with rice production and flooding irrigation where large numbers of vector mosquitoes breed in close proximity to animal reservoir
  • The flavivirus is transmitted to humans through the bite of infected Culex tritaeniorhynchus mosquitoes
    • Culex tritaeniorhynchus mosquitoes bite during evening and nighttime mainly outdoors with preference for large animals, birds and rarely for humans
    • Humans are incidental and dead-end host in the enzootic cycle as they do not develop sufficiently high viremia to infect feeding mosquitoes
    • Direct person-to-person spread of Japanese encephalitis virus does not occur except rarely through intrauterine transmission
    • On the basis of experience with similar flaviviruses, blood transfusion and organ transplantation are also considered modes of Japanese encephalitis virus transmission
  • Patterns of transmission in endemic areas:
    • Most cases appear when the weather is warmest in areas with temperate climates (eg China, Japan, South Korea, Nepal, northern Vietnam and northern India)
    • There is year-round transmission in areas with tropical climates (eg Cambodia, Indonesia, southern Vietnam and southern Thailand) often intensifying during the rainy season and pre-harvest period in rice-cultivating regions
    • Peak months and the length of the season vary and substantial epidemics can occur

Epidemiology

  • Leading cause of neurological viral infection and disability among children in Asia
  • About 24 countries in the South East Asia and Western Pacific regions have Japanese encephalitis virus transmission risk that includes >3 billion people
  • It causes an estimated 68,000 cases of viral encephalitis and 13,000 to 20,000 deaths annually worldwide
  • In endemic areas, children <15 years of age are typically affected
    • By early adulthood, the majority of the population has protective immunity following natural exposure to Japanese encephalitis virus as a result of ongoing environmental transmission
    • But, when Japanese encephalitis virus enters new geographic areas where there is no immunity, Japanese encephalitis virus affects both adults and children
  • There is very low risk for Japanese encephalitis virus infection for most travellers to Asia but varies on the basis of destination, duration, season and activities
    • Overall incidence is estimated to be less than one case per 1 million travellers
    • Susceptible visitors to endemic areas may be at risk for infection because Japanese encephalitis virus is maintained in an enzootic cycle between animals and mosquitoes where few human cases occur among residents as a result of vaccination or natural immunity
  • Age distribution of the disease shifts to older ages in regions where childhood immunization programs have been introduced
  • Major outbreaks of Japanese encephalitis virus occurs every 2-15 years

 

Signs and Symptoms

  • Incubation period is usually 5-15 days in patients who develop symptoms
  • Clinical illness develops in <1% of patients infected with Japanese encephalitis virus
  • Symptomatic patients initially develop sudden onset febrile illness, followed by headache, vomiting and other neurologic symptoms such as altered consciousness and seizures in 85% of children and 10% of adults
    • In children, gastrointestinal pain and vomiting may be the dominant initial symptoms
  • Mental status changes, focal neurological deficits, generalized weakness and movement disorders may develop over the next few days
  • In some patients, may present as poliomyelitis-like acute flaccid paralysis due to damage to the anterior horn cell, without any alteration in consciousness
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