Japanese encephalitis virus (JEV)

introduction

mosquito-born flaviviral infection present in most parts of Asia extending south to the tip of Australia (Outer Torres Strait Islands)
- HOWEVER was detected for the 1st time in Australia in Feb 2022 in Echuca, southern and western NSW and in Qld associated with piggeries - 21 piggeries across 4 states have been infected
- in March 2022, a Victorian man in his 60s from the north parts of Vic becomes 1st death to JEV in Victoria, while a NSW man in his 70's became NSW's 1st death while it is thought a death in SA was due to JEV
travelers have low risk if visiting urban areas
risk is higher if in rural or periurban areas, especially in warmer months although occurs all year round in tropical and subtropical regions
major outbreaks of JE occur every 2-15 years
mosquitoes do not get the virus from infected humans as viraemia is not high enough, but get it from infected pigs, horses or water birds and thus human infections are usually acquired in non-urban areas
infection with JE virus confers lifelong immunity

water birds such as herons and egrets
pigs are important amplifier hosts
- in Victoria:
  - infected mosquitoes have now been detected as far south as Nagambie region suggesting there may be potential for further spread
  - there are commercial sized piggeries which may be at risk of becoming reservoirs at:
    - Trafalgar in Gippsland
    - Berrybank Farm near Ballarat
    - EcoPiggery in the north-central Victoria
    - Western Plains Pork in western Victoria
NB. humans, horses and other large vertebrates are dead-end hosts in the JEV transmission cycle producing not a high enough viremia to be infectious for mosquitoes

1st recognised as a disease in Japan in 1871
virus first isolated in Japan in 1935
it is endemic mainly in SE Asia and Southern Asia as far south as the tip of Australia
introduced into the Torres Strait islands in 1995, Papua New Guinea human cases in 1997, and onto the mainland of northern Australia (Cape York) in 1998
in 2022, cases were found in inland Australia for the 1st time, particularly along the Murray River in association with piggeries

99% are asymptomatic or have only mild symptoms
in children, gastrointestinal pain and vomiting may be the dominant initial symptoms
in endemic regions, most cases are children with no past exposure and 1 in 250 develop severe encephalitis illness:
- rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and ultimately death in 30%
- of those who survive, 20%–30% suffer permanent intellectual, behavioural or neurological problems such as paralysis, recurrent seizures or the inability to speak
- children aged under five and older people who are infected with JE are at a higher risk of developing more severe illness

general mosquito precautions such as insect repellents and long sleeved clothes if visiting at risk zones
consider vaccination if either:
- planning extended time in endemic zones
- work around or with pigs in Australia
- camping in endemic areas (eg. northern Victoria)
vaccine options in Australia³⁾:
- Imojev®:
  - live attenuated vaccine given by subcutaneous injection. Registered for use in people aged ≥9 months.
  - primary course is one dose.
  - Imojev is contraindicated in people who are immunocompromised and in pregnant women.
  - should give at least 5 years coverag
- JEspect®:
  - inactivated JE vaccine given by intramuscular injection.
  - Available for use in infants and children aged ≥2 months, including people who are immunocompromised, and pregnant women.
  - primary course is 2 doses.

testing for JEV-specific IgM antibody in CSF (preferred as has lower false positive rates from prior infection or vaccination than in serum) or serum, using an IgM-capture ELISA
PCR testing of blood or CSF

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