Arboviruses are viruses that are spread by the bite of arthropods, particularly mosquitoes. They are divided into alphaviruses and flaviviruses.
Four infective alphaviruses are Ross River, Barmah Forest, Sindbis and Chikungunya 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 differences in isolates from different areas (distinct topotypes). This may explain varying disease patterns that sometimes occur in certain geographic locations, and the differing transmissibility of some strains by different vector mosquitoes.
Notification requirement for Ross River virus disease
Ross River 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 Ross River virus infection.
This is a Victorian statutory requirement.
Primary school and children’s services centres exclusion for Ross River virus disease
Exclusion is not required.
Infectious agent of Ross River virus disease
Ross River virus (RRV) is a member of the Alphavirus genus, which also includes Barmah Forest virus and Chikungunya virus. These three notifiable alphaviruses are of concern in Victoria.
First isolated in 1959 from Aedes vigilax mosquitoes collected near Ross River in Townsville, the causative role of RRV disease was confirmed in 1971 by isolation of the virus from the blood of an Indigenous child with the disease. Some Aedes species of mosquitoes have recently been renamed Ochlerotatus spp.
Identification of Ross River virus disease
Pyrexia and other constitutional symptoms are usually slight. A rash can occur up to 2 weeks before or after other symptoms. It can be absent in about one-third of cases. The rash is variable in distribution, character and duration, and may be associated with buccal and palatal enanthems (spots on mucous membranes). Rheumatic symptoms are present in most patients, except for the few who present with rash alone. Rheumatic symptoms consist of arthritis or arthralgia, primarily affecting the wrist, knee, ankle and small joints of the extremities. Prolonged symptoms are common. In some cases, there may be remissions and exacerbations of decreasing intensity for years. Cervical lymphadenopathy occurs frequently, and paraesthesiae and tenderness of the palms and soles are present in a small percentage of cases.
Laboratory evidence requires one of the following:
- isolation of RRV
- detection of RRV by nucleic acid testing
- IgG seroconversion, a significant increase in antibody level, or a fourfold or greater rise in titre to RRV
- detection of RRV-specific IgM in the absence of IgM to Barmah Forest virus unless RRV IgG is also detected
- detection of RRV-specific IgM in the presence of RRV IgG.
Incubation period for Ross River virus
The incubation period is usually 3–11 days.
Public health significance and occurrence of Ross River virus disease
Infection is subclinical in up to 60 per cent of cases. Clinical features of infection are rare before puberty, after which the disease has a similar pattern at all ages. The disease can cause incapacity and inability to work for 2–3 months. About one-quarter of patients have rheumatic symptoms that persist for a year or more.
RRV is the commonest and most widespread arboviral disease in Australia. In Australia, the five-year (2005–09) average number of notifications was 4,781. Major outbreaks have occurred in all parts of Australia, primarily from January to May each year. RRV disease has been detected and possibly transmitted to humans in most major metropolitan areas of Australia, including Perth, Brisbane, Sydney and Melbourne. RRV disease is considered endemic throughout most parts of Victoria, particularly around inland waterways and coastal regions, but not in metropolitan Melbourne. Epidemics usually follow heavy rains, or high tides that inundate salt marshes or coastal wetlands. Following increased Victorian rainfall and floods, there were about 400 notified cases in 2010, compared with 80 in 2009.
Reservoir of Ross River virus
The virus is maintained in a primary mosquito–mammal cycle involving macropods (particularly the western grey kangaroo), and possibly other marsupials and wild rodents. A human–mosquito cycle may occur in explosive outbreaks. Horses can act as amplifier hosts, and appear to develop joint and nervous system disease after infection with RRV. Fruit bats might act as vertebrate hosts in some areas. Vertical transmission in desiccation-resistant eggs of Ochlerotatus spp. mosquitoes may be a mechanism to enable the virus to persist in the environment for long periods. This could explain the rapid appearance of cases of RRV disease after heavy rains. RRV is endemic throughout Australia, Papua New Guinea, adjacent Indonesia and Solomon Islands.
Mode of transmission of Ross River virus disease
RRV is transmitted by mosquitoes. Culex annulirostris is the major vector in inland areas, whereas Ochlerotatus vigilax in New South Wales and O. camptorhynchus in southern parts of Victoria and Tasmania are the vectors in coastal regions. These vectors are well established in Australia.
Period of communicability of Ross River virus disease
There is no evidence of transmission from person to person.
Susceptibility and resistance to Ross River virus disease
Infection with the RRV confers lifelong immunity.
Control measures for Ross River virus disease
RRV infection can be prevented by:
- mosquito control measures
- personal protection measures, such as long sleeves
- using personal insect repellents containing diethyltoluamide (DEET) or picaridin
- avoidance of mosquito-prone areas and vector biting times at dusk and dawn.
Control of case
Second attacks are unknown, although symptoms may fluctuate over several months. Treatment is symptomatic, with rest advisable in the acute stages of the disease. There is no vaccine currently available commercially to protect against RRV 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, reduction of human–mosquito contact is required by suppression of the vector mosquito population through local control measures.
Outbreak measures for Ross River virus disease
Outbreak measures for RRV disease include:
- conducting a community survey to determine the species of the vector mosquito involved, identify their breeding places and promote their elimination
- avoidance of vector contact through personal protection and education
- use of mosquito repellents for people exposed to bites because of their occupation, or other reasons
- enhanced human surveillance through increased testing and notification
- identifying infection among animal reservoirs, such as kangaroos, small marsupials, and farm and domestic animals.
Airport vector control in Australia and Papua New Guinea may be necessary to prevent spread from endemic areas to countries where local vectors such as Aedes polynesiensis may transmit the disease.