Infectious diseasesepidemiology and surveillance

Page content: Overview | 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

Overview

Arboviruses are viruses which are spread by the bite of arthropods, particularly mosquitoes. They are divided into alphaviruses and flaviviruses.

Victorian statutory requirement

Kunjin virus infection (Group B disease) requires notification within five days of diagnosis.

School exclusion is not required.

Infectious agent

Kunjin virus (KUNV) is a flavivirus and was first isolated from Culex annulirostris mosquitoes collected in north Queensland in 1960 and given the name of a nearby aboriginal clan living on the Mitchell River. It is closely related to the West Nile virus which was probably exported from the Middle East to New York in 1999 where it caused thousands of deaths in birds and horses and human disease including fatal encephalitis.

Identification

Clinical features
Serological surveys indicate that subclinical infection is common. Two main clinical forms of disease have been reported: mild disease and encephalitis. Mild disease consisting of lymphadenopathy, fever, lethargy and rash was first noted when two laboratory workers acquired the infection in Queensland in 1963. A few other similar cases have been described in Australia including some with additional muscle weakness and fatigue. There has been a comparatively small number (about six) of reported cases of encephalitis due to Kunjin virus but one source quoted that a total of 15 cases occurred prior to 2000.

Fatalities are rare or absent. Very few epidemiological studies have been carried out to determine the life cycle, nature and frequency of Kunjin virus infection in Australia.

Method of diagnosis
Infection is confirmed by a significant rise in antibody titre to the virus in two blood specimens taken seven to ten days apart.

Laboratory evidence requires one of the following:

• isolation of Kunjin virus from clinical material
• detection of Kunjin virus RNA in clinical material
• IgG seroconversion or a significant increase in antibody level or a fourfold rise in titre of Kunjin virus specific IgG proven by neutralisation or another specific test
• Kunjin virus specific IgM detected in the CSF
• Kunjin virus specific IgM detected in serum in the absence of IgM to Murray Valley encephalitis, Japanese encephalitis or dengue viruses. This is only accepted as laboratory evidence for encephalitic illnesses.

Confirmation of laboratory results by a second arbovirus reference laboratory is required if the case occurs in areas of Australia not known to have established enzootic, endemic or regular epidemic activity.

Clinical evidence
Clinical evidence may present as non-encephalitic, encephalitic and asymptomatic disease.

Non-encephalitic illness
Acute febrile illness with headache, myalgia and/or rash.

Encephalitic disease
Acute febrile meningoencephalitis characterised by one or more of the following:

• focal neurological disease or clearly impaired level of consciousness
• abnormal CT, MRI scan or EEG
• presence of pleocytosis in the CSF.

Incubation period

The incubation period is probably similar to Murray Valley encephalitis virus (MVEV) disease.

Public health significance & occurrence

Kunjin virus has many similarities to MVE virus and disease due to these two viruses can only be distinguished by virological tests. This distinction is important in periods when weather patterns and other portends suggest that an outbreak of MVE virus may be imminent in southeast Australia. This has a higher mortality rate and can be more prevalent.

Serological surveys have shown that Kunjin virus infection has occurred over wide areas of Australia infecting humans, and wild and domestic animals including cattle, sheep and horses. Similarly to MVE virus, Kunjin virus occasionally spreads southward from the tropical north to central and southeastern Australia after heavy rains. Kunjin virus has been detected in Victoria on several occasions since 1974, most recently in 2001.

Reservoir

The virus is endemic in the tropical north of Australia and Sarawak where it has cycles of infection between birds and mosquitoes in enzootic foci.

Mode of transmission

Transmission occurs via mosquitoes, particularly Culex annulirostris.

Period of communicability

There is no evidence of person to person transmission.

Susceptibility & resistance

Infection confers lifelong immunity.

Control measures

Preventive measures
There is no vaccine available.

Kunjin virus infection can be prevented by:

• mosquito control measures
• personal protection measures such as long sleeves and mosquito repellents
• avoidance of mosquito-prone areas and vector biting times at dusk and dawn.

Control of case
Investigate the source of infection. Search for unreported or undiagnosed cases of encephalitis from the Murray-Darling drainage basin.

The patient with suspected infection or friend or relative, should be asked to recall if in the month prior to onset of symptoms he or she had:

• been bitten by mosquitoes
• visited regions where arboviruses are endemic
• participated in recreational or other activities involving exposure to bushland or other mosquito habitat such as gardening, bushwalking, camping and picnicking.

Control of contacts
Not applicable.

Control of environment
To reduce or prevent virus transmission, interruption of human-mosquito contact is required by:

• suppression of the vector mosquito population
• avoidance of vector contact and biting times at dusk and dawn
• applying mosquito control measures in local municipalities
• using personal protection measures such as long sleeves, long trousers, mosquito repellents
• avoiding mosquito-prone areas.

Outbreak measures

Search for unreported or undiagnosed cases of encephalitis from the Murray-Darling drainage basin.