Japanese Encephalitis
Infectious Agent
Japanese encephalitis virus (JEV) was first isolated in Japan in1935. However, the disease Japanese encephalitis (JE) had been first described in Japan as early as 1871, and since then has been found in Russia, most of the Far East and South-East Asia, and more recently it has spread to the Indian subcontinent and Nepal. It is the principal cause of epidemic viral encephalitis in the world, resulting in of the order of 50,000 clinical cases annually. Of great concern to Australia was the introduction of the JEV into the Torres Strait islands [1995] with two fatal cases of encephalitis and onto the mainland of Australia [Cape York] in 1998. Sero-positive pigs were also detected on the mainland. The most likely source of the outbreak in the Torres Strait islands was Papua New Guinea, where the first human cases were detected in 1997.
There is an effective vaccine available. It requires three shots on day 0, 7 and 28 days along with a booster every 3 years.
Identification
Clinical features
Over 90% of infections are subclinical. Encephalitis is its serious manifestation, which is indistinguishable clinically from other viral encephalities; it has a mortality of 20-50% and up to 50% of patients have serious sequelae.
Method of diagnosis
Confirmation of JEV infection is made by either isolating the virus or by a rising antibody titre.
Laboratory evidence
Isolation of Japanese encephalitis virus from clinical material
OR
Detection of JEV RNA in clinical material
OR
IgG seroconversion or a significant increase in antibody level or a fourfold rise in titre of JEV specific IgG proven by neutralisation or another specific test, with no history of recent JE or Yellow Fever vaccination
OR
JEV specific IgM in the CSF, in the absence of IgM to Murray Valley encephalitis, Kunjin and Dengue viruses
OR
JEV specific IgM detected in serum in the absence of IgM to Murray Valley encephalitis, Kunjin and Dengue viruses, with no history of recent JEV or Yellow Fever virus vaccination.
Confirmation by a second arbovirus reference laboratory is required if the case appears to have been acquired in Australia.
Clinical evidence
Febrile illness of variable severity associated with neurological symptoms ranging from headache to meningitis or encephalitis. Symptoms may include headache, fever, meningeal signs, stupor, disorientation, coma, tremors, generalised paresis, hypertonia, and loss of coordination. (The encephalitis cannot be distinguished clinically from other central nervous system infections.)
Incubation period
Usually 6 to 16 days
Health Significance and Occurrence
The occurrence of JEV disease in Papua New Guinea and Timor and probable spread into the Torres Strait islands or northern Australia, poses a significant threat to Australia. Suitable vector mosquitoes (Cx annulirostris) and vertebrate hosts in the form of water birds are widespread across the mainland, plus there many wild pigs in northern Australia to act as amplifiers for the virus. There is a concern that migratory birds could also carry the virus further south in Australia.
Reservoir
Infection is maintained in enzootic cycles between birds and pigs: water birds (herons and egrets) are the main reservoir for disseminating the virus whilst pigs are the important amplifier hosts. Pigs do not show signs of infection, other than abortion and stillbirth, but have continuing viraemia allowing transmission to humans via mosquitoes. Humans and other large vertebrates, such as horses, are not efficient amplifying hosts, and are therefore "dead-end' hosts for the JEV.
Mode of Transmission
In Asia the rice field breeding mosquitoes, mainly Cx. tritaeniorhynchus, usually transmit the JEV. In the outbreak in the Torres Strait islands [see above], virus was isolated from Culex annulirostris mosquitoes, which were considered to be the main vector involved. Culex gelidus, introduced from Asia, is now a new potential vector in Australia.
Period of Communicability
There is no evidence of transmission from person to person.
Susceptibility and Resistance
Infection with JEV confers life-long immunity.
Control Measures
Preventive measures
There is an effective vaccine available. It requires three shots on day 0, 7 and 28 days along with a booster every 3 years.
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