Infectious diseasesepidemiology and surveillance

Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures

Victorian statutory requirement

Japanese encephalitis (Group A disease) must be notified immediately by telephone or fax followed by written notification within five days.

School exclusion: case should be isolated until the fever subsides to prevent further mosquito bites.

Infectious agent

Japanese encephalitis virus (JEV) was first isolated in Japan in 1935. However, the disease Japanese encephalitis 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 on to 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.

Identification

Clinical features
Over 90% of Japanese encephalitis virus infections are subclinical. Encephalitis is its serious manifestation. This is clinically indistinguishable from other viral encephalilitides and has a mortality of 20–50%. 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 requires one of the following:

• isolation of JEV from clinical material
• detection of JEV viral RNA in clinical material
• 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
• JEV specific IgM in the CSF, in the absence of IgM to Murray Valley encephalitis, Kunjin and dengue viruses
• 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 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

The incubation period is usually six to sixteen days.

Public health significance & occurrence

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 water birds are widespread across the mainland. There are also many wild pigs in north eastern Australia to 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

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 important amplifier hosts. Pigs do not show signs of infection other than abortion and stillbirth, but have continuing viremia allowing transmission to man 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 Culex tritaeniorhynchus, usually transmit JEV. In the Torres Strait Islands outbreak virus was isolated from Culex annulirostris mosquitoes which were considered to be the main vector involved. Culex gelidus is a new potential vector in Australia if introduced from Asia.

Period of communicability

There is no evidence of transmission from person to person.

Susceptibility & resistance

Infection with JEV confers lifelong immunity.

Control measures

Preventive measures
There is an effective vaccine available. It requires three doses on days zero, seven and 28 with a booster every three years.

Control of case

• Isolate patient and prevent mosquito access until fever subsides.
• Investigate source of infection.

Control of contacts
Not applicable.

Control of environment
Search for and eliminate breeding sites of mosquito vectors in the urban area.
Use mosquito repellents, mosquito nets and other methods of personal protection.

Outbreak measures

Not applicable.