Ross River virus disease

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

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 exclusion for Ross River virus disease

Exclusion is not required.

Infectious agent of Ross River virus disease

RRV was first isolated in 1959 from Aedes vigilax mosquitoes collected near Ross River in Townsville. In 1971 the causative role of RRV disease was confirmed when the virus was isolated in the blood of a human with the disease.

In Victoria, Culex annulirostris is the major vector of RRv infection in inland areas whilst Aedes vigilax and Aedes camptorhynchus are the major vectors in coastal saltwater wetland regions.

Identification of Ross River virus disease

Clinical features

Fever 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.

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.

Diagnosis

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 presence of RRV IgG.

Incubation period for Ross River virus

The incubation period is usually three to nine days but can range up to 21 days.

Public health significance and occurrence of Ross River virus disease

RRV is the most common and widespread arboviral disease in Australia. Major outbreaks have occurred in all parts of Australia, primarily from January to May each year. RRV disease is considered endemic throughout most parts of Victoria, particularly around inland waterways and coastal regions, but not in metropolitan Melbourne. Outbreaks usually follow heavy rains, or high tides that inundate salt marshes or coastal wetlands.

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 Aedes 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 Aedes vigilax in New South Wales and Aedes 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. Transmission requires a mosquito vector.

Susceptibility and resistance to Ross River virus disease

Infection with the RRV confers lifelong immunity.

Control measures for Ross River virus disease

Preventive measures

There is currently no preventive vaccine available.

RRV infection can be prevented by:

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

Control of case

Treatment is supportive only, with rest advisable in the acute stages of the disease. Infection results in lifelong immunity, although symptoms may fluctuate over several months.

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 Ross River virus disease

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

• community education campaign promoting 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.