Key messages

  • Japanese encephalitis is an ‘urgent’ notifiable condition that must be notified immediately to the department by medical practitioners and pathology services. 
  • Most Japanese encephalitis virus (JEV) infections are asymptomatic; less than 1 per cent of people infected with JEV develop clinical disease.
  • JEV is transmitted to humans through infected mosquitoes, primarily Culex spp.
  • A vaccine, which is very efficacious, is available, but Japanese encephalitis vaccination is not part of the National Immunisation Program schedule.
  • JEV is present in the Torres Strait islands.

Notification requirement for Japanese encephalitis

Japanese encephalitis is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately by telephone upon initial diagnosis (presumptive or confirmed). Pathology services must follow up with written notification within 5 days. 

This is a Victorian statutory requirement.

Primary school and children’s services centre exclusion for Japanese encephalitis

Exclusion is not applicable.

Infectious agent of Japanese encephalitis

Japanese encephalitis virus (JEV) is a single-stranded RNA virus that belongs to the genus Flavivirus, and is closely related to West Nile and St Louis encephalitis viruses.

Identification of Japanese encephalitis

Clinical features

More than 90 per cent of JEV infections are subclinical and asymptomatic. Less than 1 per cent of people infected with JEV develop clinical disease.

Encephalitis is its serious manifestation. This is clinically indistinguishable from other viral encephalitides and has a mortality of 20–50 per cent. Among survivors, 30–50 per cent may still have significant neurological or psychiatric sequelae, even years after their acute illness.

Illness usually begins with sudden onset of fever, headache and vomiting. Mental status changes, focal neurological deficits, generalised weakness and movement disorders may develop over the next few days. 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.

Acute encephalitis is the most commonly recognised clinical manifestation of JEV infection. Milder forms of disease, such as aseptic meningitis or undifferentiated febrile illness, can also occur.

The following are other recognised presentations:

  • A parkinsonian syndrome resulting from extrapyramidal involvement is a very distinctive clinical presentation of JEV infection.
  • Acute flaccid paralysis, with clinical and pathological features similar to poliomyelitis, has also been associated with JEV infection.
  • Seizures are very common, especially among children.

Diagnosis

Confirmation of JEV infection is made by either isolating the virus or by a rising antibody titre.

Laboratory evidence requires one of the following:

  • isolation of JEV
  • detection of JEV by nucleic acid testing
  • IgG seroconversion, 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 Japanese encephalitis or yellow fever vaccination
  • detection of JEV-specific IgM in serum, in the absence of IgM to Murray Valley encephalitis, Kunjin and dengue viruses, with no history of recent Japanese encephalitis or yellow fever vaccination.

Confirmation by a second arbovirus reference laboratory is required if the case appears to have been acquired in Australia.

Incubation period of Japanese encephalitis virus

The incubation period is usually 6–16 days.

Public health significance and occurrence of Japanese encephalitis

JEV was first isolated in Japan in 1935. However, the disease Japanese encephalitis had been described in Japan as early as 1871, and since then has been found in Russia, most of the Far East and South-East Asia. 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 around 50,000 clinical cases annually.

Of great concern to Australia was the introduction of JEV into the Torres Strait islands in 1995, with two fatal cases of encephalitis, and onto the mainland of Australia (Cape York) in 1998. Seropositive 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.

The occurrence of JEV disease in Papua New Guinea and probable spread from there to cause disease in the Torres Strait islands poses a significant threat to Australia. Suitable vector mosquitoes such as Culex annulirostris and vertebrate hosts in the form of waterbirds are widespread across the mainland. There are also many wild pigs in north-eastern Australia that act as amplifiers for the virus. There is a theoretical concern that migratory birds could carry the virus southwards in Australia, even as far as Victoria.

Reservoir for Japanese encephalitis virus

Infection is maintained in enzootic (particular to animals in a geographic area) cycles between birds and pigs; waterbirds (herons and egrets) are the main reservoir for disseminating the virus, while pigs are 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 JEV.

Mode of transmission of Japanese encephalitis virus

JEV is transmitted to humans through the bite of an infected mosquito, primarily Culex species. In Asia, the rice-field breeding mosquitoes, mainly C. tritaeniorhynchus, usually transmit JEV. In the Torres Strait islands outbreak, virus was isolated from C. annulirostris mosquitoes, which were considered to be the main vector involved. C. gelidus is a new potential vector in Australia if introduced from Asia.

Period of communicability of Japanese encephalitis

There is no evidence of transmission from person to person.

Susceptibility and resistance to Japanese encephalitis

Infection with JEV confers lifelong immunity.

Control measures for Japanese encephalitis

Preventive measures

JEV is transmitted to humans through the bite of an infected mosquito, primarily Culex species. With few exceptions, vaccination is recommended for all travellers to countries or areas where there is a risk of JEV transmission, including the Torres Strait islands. It requires three doses on days 0, 7 and 28, with a booster every 3 years. Refer to the Australian immunisation handbook to identify recommendations for Japanese encephalitis vaccination, including booster doses. Japanese encephalitis vaccination is not part of the National Immunisation Program schedule.

Control of case

The case should be protected from exposure to mosquitoes until fever has subsided, to prevent further mosquito bites. This is to prevent local mosquitoes becoming vectors for the disease.

Investigate the source of infection.

Control of contacts

The aim of identifying contacts is to:

  • •alert them to the possibility that they could develop disease
  • recommend that a subset be offered preventive treatment, if appropriate.

Contacts are those who may have been exposed to mosquitoes at the same time and place the case is believed to have been infected (for example, fellow traveller). Any unimmunised person who has travelled through a Japanese encephalitis endemic country with the case should be screened for illness and potentially placed in isolation to avoid possible contact with mosquitoes.

Control of environment

Control measures may include:

  • searching for and eliminating breeding sites of mosquito vectors
  • avoiding having domestic pigs near residential areas
  • using mosquito repellents, mosquito nets and other methods of personal protection.

Outbreak measures for Japanese encephalitis

The department’s role is predominantly human case surveillance (enhancing arrangements to help identify new cases) and assisting in the provision of public health messages to the general public (human avoidance of mosquitoes and encouraging people with symptoms to present to a medical practitioner).

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