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 | International measures | Additional sources of information

Overview

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

Three infective alphaviruses include Ross River, Barmah Forest and Sindbis viruses.

These all have the capacity to cause a similar disease 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 geographic locations and the differing transmissibility of some strains by different vector mosquitoes.

Victorian statutory requirement

Barmah Forest virus infection (Group B disease) requires notification in writing within five days of diagnosis.

School exclusion is not required.

Infectious agent

Barmah Forest virus (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 southwest 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

Clinical features
Features include fever, arthritis, arthralgia and rash which are clinically indistinguishable from RRV disease. Like RRV 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 six months. Outbreaks of BFV disease sometimes occur concurrently with RRV disease making diagnosis difficult.

Method of diagnosis
Serology shows a significant rise in antibody titre to the BFV. The virus may be isolated from the blood of acutely ill patients. Virological tests are necessary to distinguish BFV disease from other causes of arthritis.

Laboratory evidence requires one of the following:

• isolation of BFV from clinical material
• detection of BFV by nucleic acid testing
• a significant rise in IgG to BFV
• detection of BFV-specific IgM.

Incubation period

The incubation period appears to be seven to ten days.

Public health significance & occurrence

Since 1988, BFV disease has been reported in Western Australia, Queensland, New South Wales, the Northern Territory and Victoria. Outbreaks have been reported in Victoria throughout the Murray Valley and the Gippsland areas.

Reservoir

Like RRV, BFV disease appears after heavy rains encourage 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

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

Period of communicability

There is no evidence of transmission from person to person.

Susceptibility & resistance

Infection with BFV confers lifelong immunity.

Control measures

Preventive measures
BFV 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
Second attacks are unknown. Treatment is symptomatic with rest advisable in the acute stages of the disease. Presently, there is no vaccine available commercially to protect against BFV disease.

Control of contacts
Unreported or undiagnosed cases should be sought in the region where the patient had been staying during the incubation period of their illness. All family members should be questioned about symptoms and evaluated serologically if necessary.

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 through personal protection and education.

Outbreak measures

• Conduct a community survey to determine the species of the vector mosquito involved. Identify their breeding places and promote their elimination.
• Use mosquito repellents for persons exposed to bites because of their occupation, or other reasons.
• Identify the infection among animal reservoirs, for example kangaroos, farm and domestic animals.

International measures

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.

Additional sources of information

Boughton C R 1996, Australian Arboviruses of Medical Importance, A handbook for general practitioners and other clinicians, RACGP Services.