Key messages

  • West Nile virus and Kunjin virus infection are Group B notifiable diseases.
  • Many people (80 per cent) infected with West Nile virus are asymptomatic and about 20 per cent have mild symptoms. A small proportion develop severe disease.
  • Subclinical infection of Kunjin virus – a subspecies of West Nile virus – is common.
  • Kunjin virus is found in parts of Australia, particularly the Northern Territory and northern Western Australia. West Nile virus has not yet been detected in Australia.
  • Neither disease is vaccine preventable.

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

Notification requirement for West Nile virus and Kunjin virus disease

West Nile and Kunjin virus virus infection (Group B disease) requires notification within 5 days of diagnosis.

This is a Victorian statutory requirement.

Primary school and children’s services centres exclusion for West Nile virus and Kunjin virus disease

School exclusion is not required.

Infectious agent of West Nile virus and Kunjin disease

West Nile virus

West Nile virus is carried by mosquitoes and belongs to a group of viruses called flaviviruses. West Nile virus 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.

Kunjin virus

Kunjin virus is a subspecies of West Nile virus. It 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.

There are at least seven genetic lineages of West Nile virus. Kunjin virus is a small but genetically distinct sublineage, designated as 1b. The epidemics of severe disease in humans and animals during the past decade have been due to West Nile virus sublineage 1a, which has never been found in Australia.

Identification of West Nile virus and Kunjin virus disease

Clinical features

West Nile virus

Approximately 80 per cent of people who become infected have no illness; up to 20 per cent of people who become infected have mild symptoms; and a very small minority experience severe disease. West Nile virus disease can be described as a febrile illness of sudden onset, often accompanied by tiredness, headache, muscle pain, nausea, rash, vomiting, swollen glands and eye pain. Some patients experience gastrointestinal symptoms, including nausea, vomiting, loss of appetite or diarrhoea. Symptoms usually resolve within 7-10 days, although tiredness can last for some weeks.

Kunjin virus

Serological surveys for Kunjin virus indicate that subclinical infection is common. Two main clinical forms of disease have been reported: mild disease and encephalitis. Mild diseases consist of lymphadenopathy, fever, lethargy and rash. Additional muscle weakness and fatigue may also be reported.

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

Diagnosis

Infection is confirmed by a significant rise in antibody titre to the virus in two blood specimens taken 7-10 days apart.

Laboratory evidence requires one of the following:

  • isolation of West Nile virus and Kunjin virus
  • detection of West Nile virus and Kunjin virus by nucleic acid testing
  • IgG seroconversion, a significant increase in antibody level or a fourfold rise in titre of West Nile virus/Kunjin virus
  • West Nile virus and Kunjin virus–specific IgM detected in the cerebrospinal fluid in the absence of IgM to Murray Valley encephalitis (MVE), Japanese encephalitis or dengue viruses
  • West Nile virus and Kunjin virus–specific IgM detected in serum in the absence of IgM to MVE, Japanese encephalitis or dengue viruses. This is only accepted as laboratory evidence for encephalitic illnesses.
  • focal neurological disease or clearly impaired level of consciousness
  • abnormal computed tomography (CT) scan, magnetic resonance imaging (MRI) scan or electroencephalogram
  • presence of pleocytosis in the cerebrospinal fluid.

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 or 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 computed tomography (CT) scan, magnetic resonance imaging (MRI) scan or electroencephalogram
  • presence of pleocytosis in the cerebrospinal fluid.

Asymptomatic disease

Case detected as part of a serosurvey should not be notified.

Incubation period of West Nile virus and Kunjin virus

West Nile virus

The incubation period is 2–14 days.

Kunjin Virus

The incubation period is not well defined, but is typically 2-6 days, ranging from 2-14 days; in immunocompromised individuals, it can be as long as 21 days.

Public health significance and occurrence of West Nile virus and Kunjin virus disease

West Nile virus/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 during periods when weather patterns and other surveillance indicators suggest that an outbreak of MVE virus may be imminent in south-east Australia. MVE has a higher mortality rate and can be more prevalent.

West Nile virus

West Nile virus was first identified in the West Nile district of Uganda in 1937. It is known to lethally infect horses, domestic birds and wild birds, as well as humans. Until quite recently, West Nile virus was confined to eastern Europe, west Asia, the Middle East and Africa. The virus was imported into New York creating a large outbreak, that then spread throughout the United States of America, the virus is now widely spread fom Canada to Venezuela . In areas known to harbour infected mosquitoes, it is estimated that less than 1 per cent of the mosquitoes carry the virus. In a study in New York City in 1999 during the height of virus activity that year, 2.6 per cent of residents were infected.

The National Arbovirus and Malaria Advisory Committee classifies the risk of transmission as potential local transmission through an animal host, causing significant morbidity and mortality. The necessary mosquito vectors and avian species are present to allow the establishment and spread of permanent mosquito–bird transmission cycles. There is potential for high economic impacts should the virus spread to the equine population.

Kunjin virus

Kunjin virus, a subtype of West Nile virus, is found in parts of Australia, particularly the Northern Territory and northern Western Australia. Kunjin virus is less virulent than the current United States strain of West Nile virus. Serological surveys have shown that Kunjin virus infection has occurred over wide areas of Australia. The virus has infected humans, and wild and domestic animals, including cattle, sheep and horses. Similar to MVE virus, one theory is that Kunjin virus occasionally spreads southwards from the tropical north to central and south-eastern Australia after heavy rains.

Kunjin virus has been detected in Victoria on several occasions since 1974, most recently in 2001. There have been three confirmed cases in Victoria since 2001. There have been 15 cases of Kunjin virus reported in Australia.

There are at least seven genetic lineages of West Nile virus. Kunjin virus is a small but genetically distinct sublineage, designated as 1b. The epidemics of severe disease in humans and animals during the past decade have been due to West Nile virus sublineage 1a, which has never been found in Australia. The appearance of another subspecies of West Nile virus would be of concern in Australia.

Reservoir for West Nile virus and Kunjin virus

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

Mode of transmission of West Nile virus and Kunjin virus

Transmission occurs via mosquitoes. In Australia, the most common carrier of Kunjin virus is the freshwater mosquito Culex annulirostris.

Period of communicability of West Nile virus and Kunjin virus disease

There is no evidence of person-to-person transmission.

Susceptibility and resistance to West Nile virus and Kunjin virus disease

People with antibodies to Kunjin virus may be immune to infection with West Nile virus. Infection confers lifelong immunity.

Control measures for West Nile virus/Kunjin virus disease

Preventive measures

There is no preventive vaccine available.

West Nile virus/Kunjin virus infection can be prevented by:

  • mosquito control measures
  • personal protection measures, such as long sleeves
  • using personal repellents containing diethyltoluamide (DEET) or picaridin
  • avoidance of mosquito-prone areas and vector biting times at dusk and dawn.

Control of case

Investigate the source of infection.

The patient with suspected infection, or a friend or relative, should be asked to recall whether, in the month before onset of symptoms, they had:

  • been bitten by mosquitoes
  • visited regions where arboviruses are endemic
  • carried out any outdoor activities at the time when they were likely to have acquired the infection.

Control of contacts

If others are unwell, it is advisable that they see their own doctor for testing.

Control of environment

To reduce or prevent virus transmission, reduction of human–mosquito contact is required by suppression of the vector mosquito population through local control measures.

Outbreak measures for West Nile virus and Kunjin virus disease

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

National Arbovirus and Malaria Advisory Committee 2009, Arboviral diseases and malaria risk assessment framework, draft, July 2009.

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