Communicable disease factsheets

Ross River virus infection - Fact Sheet

Ross River virus (RRV) infection is the most common and most widespread arboviral disease in Australia, with sometimes thousands of clinical cases occurring in epidemics. Disease notifications in Australia average about 4,800 per year.

Infectious Agent

First isolated from Ochlerotatus vigilax (previously called Aedes vigilax) mosquitoes collected in 1959 near the Ross River in Townsville, the cause of Ross River virus (RRV) disease was confirmed in 1971 by its isolation from the blood of an Aboriginal boy with the disease.

Identification

Clinical Features

Pyrexia and other constitutional symptoms are usually slight. A rash can occur up to two weeks before, or after, other symptoms. 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. Rheumatic symptoms are present in most patients except for the few who present with rash alone: these 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 up to a year. Symptoms persisting longer than a year may be due to other reasons. Cervical lymphadenopathy occurs frequently, and paraesthesiae and tenderness of the palms and soles are present in a small percentage of cases.

Method of diagnosis

Serology showing a significant rise in antibody titre to RRV. The virus may be isolated from the blood of acutely ill patients in the early period of the disease. Virological tests are necessary to distinguish RRV disease from other causes of arthritis. In the event of a local outbreak clinical diagnosis may be sufficient, but Barmah Forest virus disease should be considered.

Laboratory evidence

• isolation of RRV from clinical material; or
• detection of RRV by NAT; or
• a significant rise in IgG to RRV; or
• detection of RRV specific IgM
• Incubation period
• Usually three to 14 days.

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Public Health Significance and Occurrence

The incidence of clinical infection varies dramatically between endemic areas and epidemic areas. 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 two to three months. About one-quarter of patients have rheumatic symptoms that persist for up to a year but rarely more.

RRV disease is the commonest and most widespread arboviral disease in Australia, with sometimes thousands of clinical cases occurring in epidemics. Disease notifications in Australia average about 4,800 per year. Major outbreaks have occurred in all parts of Australia, chiefly in the period from January to May. RRV disease cases in the SW of WA generally occur from September to May and in the north of WA the risk is greatest during or just after the wet season. RRV has been detected (and probably transmitted to humans) in most major metropolitan areas of Australia, including Perth, Brisbane, Sydney and Melbourne. Epidemics usually follow heavy rains or after high tides which inundate salt marshes or coastal wetlands. Sporadic cases occur in mainland and coastal regions of Australia and Papua New Guinea at other times. In 1979, a major outbreak of RRV disease (probably exported from Australia) occurred in Fiji and spread to other Pacific islands, including Tonga, the Cook Islands and Samoa.

Reservoir

The virus is maintained in a primary mosquito-mammal cycle involving macropods (kangaroos and wallabies), possibly other marsupials (eg possums), flying fox and native rodents. A human-mosquito cycle may occur in explosive outbreaks. Horses, which may act as amplifier hosts, 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, explaining the rapid appearance of cases of RRV disease after heavy rains. RRV is endemic throughout Australia, Papua New Guinea, East Timor, adjacent islands of Indonesia and the Solomon islands

Mode of transmission

It is transmitted by a number of different mosquitoes, with Culex annulirostris being the major vector in inland areas, whilst Oc. vigilax, Ve. funerea and

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Oc. camptorhynchus are the major vectors in coastal regions.

Period of communicability

There is no evidence of transmission from person to person in the absence of a mosquito vector.

Susceptibility and resistance

Infection with the RRV probably confers life-long immunity.

Control Measures

Preventive measures

Ross River virus infection can be prevented by:

• Mosquito control measures.
• Personal protection measures (long sleeves and pants, mosquito repellents and mosquito coils).
• Avoidance of mosquito-prone areas. Vectors usually bite between dusk and dawn.

Control of case

Treatment is symptomatic with rest advisable in the acute stages of the disease. Symptoms may recur but this is not thought to be due to re-infection. Presently, there is no vaccine available commercially to protect against RRV 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/prevent virus transmission, interruption of human/mosquito contact is required by:

• suppression of the vector mosquito population
• avoidance of vector contact (personal protection/education)

Outbreak Measures

• Conduct a survey to determine the species of the vector mosquito involved. Identify their breeding places and promote their control.
• Promote the use of mosquito repellents and other mosquito avoidance measures for persons exposed to bites because of their occupation, or other reasons.
• Identify the infection among animal reservoirs, for example, kangaroos and small marsupials.

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 (for example, Aedes polynesiensis) may transmit the disease.

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