Ross River virus disease

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

Ross River virus infection (Group B disease) requires notification within five days of diagnosis.

School exclusion is not required.

Infectious agent

First isolated in 1959 from Aedes vigilax mosquitoes collected near the Ross River in Townsville, the causative role of Ross River virus (RRV) was confirmed in 1971 by its isolation from the blood of an indigenous child with the disease. Some Aedes species have recently been renamed Ochlerotatus spp. mosquitoes.

Identification

Clinical features
Pyrexia and other constitutional symptoms are usually slight. A rash can occur up to two 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. 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 years. 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 shows a significant rise in antibody titre to RRV. The virus may be isolated from the blood of acutely ill patients. 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 outbreaks of RRV disease sometimes occur concurrently with BFV disease making diagnosis difficult.

Laboratory evidence requires one of the following:

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

Incubation period

The incubation period is usually three to eleven days.

Public health significance & occurrence

Infection is subclinical in up to 60% 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 two to three months. About one quarter of patients have rheumatic symptoms which persist for a year or more.

RRV disease is the commonest and most widespread arboviral disease in Australia, sometimes thousands of clinical cases occur in epidemics. Disease notifications in Australia average about 4800 per year. Major outbreaks have occurred in all parts of Australia. These occur chiefly in the period from January to May. RRV has been detected and probably transmitted to humans in most major metropolitan areas of Australia including Perth, Brisbane, Sydney and Melbourne. In 1993, 1216 cases of RRV disease were notified in Victoria. 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 of the year. In 1979, a major outbreak of RRV disease which was probably exported from Australia occurred in Fiji and spread to other Pacific islands, including Tonga and the Cook Islands.

Reservoir

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 man–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 the Solomon islands

Mode of transmission

RRV is transmitted by mosquitoes. Culex annulirostris is the major vector in inland areas whilst Ochlerotatus vigilax in 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 the RRV confers lifelong immunity.

Control measures

Preventive measures
RRV infection can be prevented by:

mosquito control measures
personal protection measures such as wearing long sleeves and mosquito repellents
avoidance of mosquito-prone areas and exposure during 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. There is no vaccine currently 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 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, small marsupials, 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.

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