Japanese encephalitis

Notification requirement for Japanese encephalitis

JE is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately to the Department of Health upon initial diagnosis (suspected or confirmed) by calling 1300 651 160 (24 hours). Pathology services must follow up with written notification within 5 days.

This is a Victorian statutory requirement.

Primary school and children’s services exclusion for Japanese encephalitis

Exclusion is not required.

Infectious agent of Japanese encephalitis

JE is caused by infection with the Japanese encephalitis virus which is a flavivirus.

Other flaviviruses known to cause similar clinical presentations include Murray Valley encephalitis virus and West Nile/Kunjin virus.

Identification of Japanese encephalitis

Clinical features

Most JE virus infections are asymptomatic. Less than one percent of people with JE virus infection develop clinical disease. While acute encephalitis is the most commonly recognised clinical manifestation of JE virus infection, milder forms of disease, such as aseptic meningitis or undifferentiated febrile illness, can also occur.

Mortality from encephalitis is 20 to 50%. Among survivors, 30 to 50% may have significant neurological complications.

Illness usually begins with:

• fever
• headache
• nausea
• vomiting
• myalgia

People with severe illness can develop encephalitis or meningoencephalitis and have symptoms such as:

• drowsiness, confusion or other mental status changes
• meningism with severe headache, neck stiffness and photophobia
• cranial nerve pathology
• generalised weakness or paresis
• movement disorders such as ataxia and Parkinsonism
• seizures, which are more common in children
• loss of consciousness and coma

Diagnosis

Confirmation of JE is made by isolating or detecting the virus from a clinical sample or by a rising antibody titre in conjunction with compatible clinical evidence.

Bilateral thalamic involvement on CT or MRI Brain is classical. Other areas that may be involved include the basal ganglia and brainstem.

The usual investigations for common causes of encephalitis or meningoencephalitis should be conducted concurrently, including CSF sampling if safe and appropriate. Where CSF is obtained, it should be tested for Herpes Simplex Virus (HSV), varicella-zoster virus (VZV), enteroviruses and other common causes of meningoencephalitis by multiplex PCR and culture.

Flavivirus testing should be considered in the appropriate clinical context. It is especially important to exclude bacterial meningitis and HSV as they are treatable conditions.

Recommended laboratory testing for JE includes all of the following:

Collect acute and convalescent (2 to 4 weeks post symptom onset) serology samples. This is essential as antibodies may not be detectable at symptom onset.

Cross reaction of antibodies to other flaviviruses is possible.

Samples should be sent urgently to the Victorian Infectious Diseases Reference Laboratory (VIDRL) which performs testing for MVE virus and other flaviviruses in Victoria. Request forms should be appropriately labelled and include relevant clinical and epidemiological history including symptom onset, vaccination, travel history and country of birth, to guide laboratory interpretation.

The on-call lab manager at VIDRL should be contacted to provide information on samples being sent. Samples should be transported at 4 degrees Celsius.

Incubation period of Japanese encephalitis virus

The incubation period of JE virus is usually 5 to 15 days.

Public health significance and occurrence of Japanese encephalitis

JE is endemic throughout most of Asia and parts of the Western Pacific region with more than 65,000 cases occurring each year.

JE virus was introduced into the Torres Strait islands in 1995, with 2 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.

In early 2022 JE virus detections in humans, pigs and mosquitoes were reported for the first time in south-eastern Australia (Victoria, New South Wales, southern Queensland and South Australia), much further south than where the virus had previously been detected.

JE virus was detected in Victoria for the first time in February 2022. JE virus was initially detected in pigs, and locally acquired human cases of JE, and virus detections in mosquitoes were subsequently identified. A comprehensive response across human and animal health sectors has been implemented following the first detections of the virus in Victoria and is ongoing.

Reservoir for Japanese encephalitis virus

The JE virus 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 JE virus.

Mode of transmission of Japanese encephalitis virus

JE virus is transmitted to humans through the bite of an infected mosquito, primarily the Culex species.

People cannot be infected by eating meat. Pork or pork products are safe to consume.

Period of communicability of Japanese encephalitis

There is no evidence of transmission of JE virus from person to person.

Susceptibility and resistance to Japanese encephalitis

Infection with JE virus confers lifelong immunity.

Prevention and control measures for Japanese encephalitis

Treatment of JE is supportive. Suspected cases should be discussed with the local Infectious Disease service.

JE virus is transmitted to humans through the bite of an infected mosquito. Avoiding mosquito bites is the most important way to prevent JE.

There are 2 safe and effective vaccines available to protect against Japanese encephalitis.

A mosquito surveillance and control program is in place in Victoria to monitor the number and species of mosquitoes, presence of viruses in mosquitoes (including Japanese encephalitis virus) and support local councils to manage the risk of mosquitoes in their local areas.

Vaccination

There is significant global demand for the JE vaccine. Victoria has a limited supply and therefore access is restricted to specific priority groups, targeted to those most at risk, including those not eligible for Medicare. Eligibility criteria will continue to be monitored.

Immunisation providers should assess a person's eligibility (as listed below) and offer the free JE vaccine.

Resources for health professionals

• Japanese encephalitis virus (JEV) – Know the facts about JEV and stay safe fact sheet(opens in a new window)
• The Australian Immunisation Handbook - Japanese encephalitis