Arboviruses are viruses that are spread by the bites of arthropods, particularly mosquitoes. They are divided into alphaviruses and flaviviruses.
Ross River, Barmah Forest, Sindbis and Chikungunya viruses are four infective alphaviruses.
All four have the capacity to cause similar diseases in humans, characterised by fever, joint involvement and a rash. Molecular studies of epidemiologically distinct isolates of Ross River and Sindbis viruses have shown changes in isolates from different areas (distinct topotypes). This may explain varying disease patterns, which sometimes occur in certain geographical locations, and the differing transmissibility of some strains by different vector mosquitoes.
Notification requirement for Barmah Forest virus disease
Barmah Forest virus (BFV) and other arbovirus infections (Group B diseases) require notification in writing within 5 days of diagnosis.
Primary school and children’s services centre exclusion for Barmah Forest virus disease
School exclusion is not required.
Infectious agent of Barmah Forest virus disease
BFV was first isolated in 1974 from Culex annulirostris mosquitoes collected in the Barmah Forest near the Murray River in northern Victoria, and simultaneously from mosquitoes collected in south-west Queensland.
It has also been isolated from numerous other mosquitoes, including the coastal species Ochlerotatus vigilax (New South Wales) and Ochlerotatus camptorhynchus (Victoria), which enjoy a salt marsh habitat, and from the midge Culicoides marksi in the Northern Territory. Subsequently, BFV has been detected in most parts of mainland Australia, and serological surveys indicate that it causes widespread human infection.
Identification of Barmah Forest virus disease
Features of BFV disease include fever, arthralgia and rash that are clinically indistinguishable from those caused by Ross River virus disease. As in Ross River virus disease, there is a high subclinical rate of infection and a low disease rate in children. Recovery usually occurs within several weeks, but lethargy, arthralgia and myalgia can persist for over 6 months. Outbreaks of BFV disease sometimes occur concurrently with Ross River virus disease, making diagnosis difficult.
Diagnosis may be determined with the following tests:
- isolation of BFV
- detection of BFV by nucleic acid testing
- IgG seroconversion or a significant increase in antibody level or a fourfold or greater rise in titre to BFV
- detection of BFV-specific IgM in the absence of Ross River virus IgM, unless BFV IgG is also detected
- detection of BFV-specific IgM in the presence of BFV IgG.
Incubation period of Barmah Forest virus
The incubation period appears to be 7–10 days.
Public health significance and occurrence of Barmah Forest virus disease
BFV disease is considered endemic throughout Victoria. Since 1988, it has been reported in Western Australia, Queensland, New South Wales and the Northern Territory. Outbreaks have been reported in Victoria throughout the Murray Valley and the Gippsland area.
In Victoria, the number of notified cases per year varies widely depending on seasonal and other conditions. The 5-year median (2005–2009) is 26 notifications per year.
Reservoir of Barmah Forest virus
Like Ross River virus disease, BFV disease appears after heavy rains that facilitate the breeding of mosquito vectors. It is not established, but it is likely, that macropods and other marsupials are the principal hosts for the virus. BFV antibodies have been found in kangaroos, cattle, horses and sheep on the south coast of New South Wales.
Mode of transmission of Barmah Forest virus
BFV is transmitted by mosquitoes. Culex annulirostris is the major vector in inland areas; in coastal regions, Ochlerotatus vigilax (New South Wales) and Ochlerotatus camptorhynchus (southern parts of Victoria and Tasmania) are the vectors.
Period of communicability of Barmah Forest virus disease
There is no evidence of transmission from person to person.
Susceptibility and resistance to Barmah Forest virus disease
Infection with BFV confers lifelong immunity.
Control measures for Barmah Forest virus disease
BFV infection can be prevented by:
- mosquito control measures
- personal protection measures, such as wearing long sleeves
- using personal repellents containing diethyl toluamide (DEET) or picaridin
- avoidance of mosquito-prone areas and vector biting times at dusk and dawn.
Control of case
Although symptoms may fluctuate, second infections of BFV disease are unknown. Treatment is symptomatic, and rest is advisable in the acute stages of the disease. Currently, there is no commercially available vaccine to protect against BFV disease.
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, it is necessary to interrupt human–mosquito contact by:
- suppressing the vector mosquito population
- avoiding vector contact through personal protection and education
- avoiding mosquito-prone areas.
Outbreak measures for Barmah Forest virus disease
Outbreak measures include the following:
- conduct a community survey to determine the species of the vector mosquito involved. Identify its breeding places and promote its elimination
- use mosquito repellents for people exposed to bites because of their occupation or other reasons
- enhance human surveillance through increased testing and notification
- identify the infection among animal reservoirs (for example, kangaroos, farm animals and domestic animals).
International measures for Barmah Forest virus disease
Airport vector control in Australia and Papua New Guinea may be necessary to prevent spread from endemic areas to other countries where local vectors such as Aedes polynesiensis may transmit the disease.