Barmah Forest virus disease

Arboviruses are viruses that are spread by the bites of arthropods, particularly mosquitoes. They are divided into alphaviruses and flaviviruses. Barmah Forest virus is a member of the alphavirus genus, which also includes (but not limited to) Ross River, Sindbis and Chikungunya virus. All four have the capacity to cause similar diseases in humans, characterised by fever, joint involvement and a rash

Notification requirement for Barmah Forest virus disease

Barmah Forest virus infection is a ‘routine’ notifiable condition and must be notified by pathology services in writing within 5 days of diagnosis. Medical practitioners are not required to notify cases of BFV disease.

This is a Victorian statutory requirement.

Primary school and children’s services exclusion for Barmah Forest virus disease

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

Clinical features

Features of BFV disease include fever, arthralgia and rash that are clinically indistinguishable from those caused by Ross River virus disease. Similar to 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

• Laboratory evidence requires one of the following: 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 presence of BFV IgG.

Incubation period of Barmah Forest virus

The incubation period is usually seven to ten days but can range from three to 21 days.

Public health significance and occurrence of Barmah Forest virus disease

BFV disease is considered endemic throughout Victoria. It has also been reported in Western Australia, Queensland, New South Wales and the Northern Territory. Like Ross River virus disease, BFV disease appears after heavy rains that facilitate the breeding of mosquito vectors. 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 climatic and other conditions. There were between five and 18 cases notified each year during the five years from 2015 to 2019, The 5-year median (2015–2019) was 6 notifications per year.

Reservoir of Barmah Forest virus

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.

Mode of transmission of Barmah Forest virus

BFV is transmitted by mosquitoes.Culex annulirostris is the major vector in inland areas throughout Australia. While, Ochlerotatus camptorhynchus (southern parts of Victoria and Tasmania) and Ochlerotatus vigilax (New South Wales) are the major vectors in coastal regions.

Period of communicability of Barmah Forest virus disease

There is no evidence of transmission from person to person. Transmission requires a mosquito vector.

Susceptibility and resistance to Barmah Forest virus disease

Infection with BFV confers lifelong immunity.

Control measures for Barmah Forest virus disease

Preventive measures

There is no preventive vaccine available.

BFV infection can be prevented by:

• personal protection measures, such as wearing long, loose-fitting, light coloured clothing
• using personal repellents containing diethyl toluamide (DEET) or picaridin
• avoidance of mosquito-prone areas especially at dusk and dawn, which are peak vector biting times.
• mosquito control measures

Control of case

Treatment is supportive only, and rest is advisable in the acute stages of the 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:

• avoiding mosquito-prone areas.
• preventing mosquitoes from entering the home or accommodation
• suppressing the mosquito population, through removal of stagnant water, using knockdown sprays or long acting surface sprays if mosquitoes are particularly bad

Outbreak measures for Barmah Forest virus disease

In addition to prevention activities, outbreak measures for BFV may include:

• community education campaign to promote the use of personal protection measures
• conducting a survey to determine the species of the vector mosquito involved, identify their breeding places and promote their elimination
• enhanced human surveillance through increased testing and notification