What is the Ross River fever?

Ross River virus is one of a group of viruses called arboviruses (or arthropod-borne viruses), which are spread mainly by blood-sucking insects.

Ross River virus is a germ that infects people, particularly in rural areas, sometimes causing a flu-like illness with joint pains, rash and fever.
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Ross River virus is not fatal.

What are the symptoms?

Many people who are infected with the virus will never develop symptoms.

• Some people will have flu-like symptoms that include fever, chills, headache and aches and pains in the muscles and joints.
• Some joints can become swollen, and joint stiffness may be particularly noticeable in the morning.
• Sometimes a rash occurs on the body, arms or legs. The rash usually disappears after seven to 10 days.
• A general feeling of being unwell, tired or weak may also occur at times during the illness. This may affect work performance.

How is the virus spread?

The virus is spread by certain types of female mosquitoes.

• Female mosquitoes feed on animals and people. If they feed on the blood of an infected animal, the mosquito may become infected. The virus then multiplies within the mosquito and is passed to other animals or people when the mosquito feeds again.
• The number of infections tends to peak in the summer and autumn months.
• The virus is not spread directly from one person to another.
• Approximately 30 per cent of people infected with the virus will develop symptoms three to eleven days after being infected.

Who is at risk?

People who are in contact with known mosquito habitats and who live in warm, humid climates near bodies of water will be most at risk of a mosquito bite and as a consequence the Ross River virus.

How is it prevented?

• Avoid being bitten by mosquitoes, especially in the summer and autumn months when infections are more common.
• Various species of mosquitoes bite at different times. Avoid being outside in the late afternoon and dusk. Mosquitoes are usually most active up to one to three hours after sunset and again around dawn.
• When outside wear loose fitting, light coloured clothing that covers your arms and legs, and use an insect repellent that contains the chemical diethyl toluamide (DEET) or picardin.
• Fit fly screens to all windows, doors and chimneys and keep them in good repair.
• Use a knockdown insecticide in bedrooms half an hour before going to bed. Use insecticides according to instructions.
• Ensure open containers of water are removed from around the home to prevent mosquitoes breeding. Cover the openings to water tanks with fine steel mesh to prevent mosquitoes from laying eggs in the tank.
• When camping take precautions such as using flyscreens on caravans and tents and by sleeping under mosquito proof nets. Take particular care while fishing, ensuring that you follow personal precautions to avoid being bitten by mosquitoes.

How is it diagnosed?

Ross River infection is diagnosed by detection of antibodies against the virus in the blood. Blood test taken early in the illness and again two weeks later may be required to confirm the infection.

How is it treated?

There is no specific treatment for Ross River virus infection.

• Your doctor will be able to advise you on medications that will help ease the discomfort of the symptoms.

What is the public health response?

Laboratories are require to notify cases of Ross River, and other mosquito-borne disease to the public health unit. Public health staff monitor the geographic spread of Ross River virus infections and provide information about avoiding mosquito-dorne diseases.

Ross River Fever (RRF) is a mosquito-borne infectious disease caused by infection with the Ross River virus. The illness is typically characterised by an influenza-like illness of and polyarthritis. The virus is endemic to Australia, Papua New Guinea, Fiji, Samoa, the Cook Islands, New Caledonia and several other islands in the South Pacific.[1]

History

The first outbreak of RRF was in 1928 in the Hay and Narrandera region in New South Wales, Australia.[2] The virus was first isolated in 1959 from a mosquito trapped along the Ross River in Townsville, Queensland. Since outbreaks have occurred in all Australian states, including Tasmania, and metropolitan areas.[2] The largest outbreak occurred in 1979-1980 in the Western Pacific, and affected more than 60,000 people.[2]
Before the identification of this infectious agent, the disease was referred to as "epidemic polyarthritis". This term was also used for a similar Australian disease caused by another mosquito-borne virus, Barmah Forest Virus.[3]

Incidence and epidemiology

Most notifications are from Queensland, tropical Western Australia and the Northern Territory. Geographical risk factors include areas of higher rainfall and higher maximal tides.[2] In the tropics, Ross River Fever is more prevalent during the summer/autumn "wet season", particularly January - March, when mosquito populations numbers are high. In southern parts of Australia, this time period may shift to earlier in the year during spring/summer.[1][3] Areas near suitable mosquito breeding grounds — marshes, wetlands, waterways and farms with irrigation systems — are high risk areas for outbreaks. As such, the disease is more characteristic of rural and regional areas.[3] Infection is most common in adults aged 25–44 years old, with males and females equally effected.[2] Ross River Fever is on the Australian Department of Health and Ageing's list of notifiable diseases.[4]

Transmission

Borne by mosquitos, both person-person and animal-person transmission occurs. The main reservoir hosts are kangaroos and wallabies, although horses, possums and possibly birds and flying foxes play a role. Over 30 species have been implicated as possible vectors, but the major species for Ross River Fever are Culex annulirostris in inland areas, Aedes vigilax in northern coastal regions and Ae. camptorhynchus in southern coastal regions.[2][3]

Symptoms

Symptoms of the disease may vary widely in severity, but major indicators are arthralgia, arthritis fever, and rash.[3] The incubation period is 7–9 days. About a third of infections are asymptomatic, particularly in children.[2][3]

Acute illness

About 95% of symptomatic cases report joint pain.[2] This is typically symmetrical and with acute onset, affecting the fingers, toes, ankles, wrists, knees and elbows.[3] Fatigue occurs in 90%, and fever, myalgia and headache occur in 50-60%.[2] The rash occurs in 50% and is widespread and maculopapular. About 50% of people report needing time off work with the acute illness.[2] If the rash is unnoticed, these symptoms are quite easily mistaken for more common illnesses like influenza or the common cold. Recovery is expected within a month.

Chronic illness

Reports from the 1980s and 1990s suggested RRV infection was associated with athralgia, fatigue and depression lasting for years.[3] More recent prospective studies have reported a steady improvement in symptoms over the first few months, with 15-66% of patients having ongoing arthralgia at 3 months. Arthralgias have resolved in the majority by 5–7 months. The incidence of chronic fatigue is 12% and 6 months and 9% at 12 months, similar to Epstein-Barr virus and Q fever.[2] The presence of a pre-morbid diagnosis of depression strongly influences the chance of significant illness at 12 months.[2]

Diagnosis

A blood test is the only way to confirm a case of Ross River Fever. Several types of blood tests may be used to examine antibody levels in the blood. Tests may either look for simply elevated antibodies (which indicate some sort of infection), or specific antibodies to the virus.[3]

Treatment and prevention

There are no specific treatments, for example, antiviral drugs, that target infections with RRV. Patients are usually managed with simple analgesics, anti-inflammatories, anti-pyretics and rest while the illness runs its course.[1][3] Infection usually affords lifelong immunity and repeated infection with the virus is very rare.[1]

Prevention

There is currently no vaccine available. The primary method of disease prevention is minimizing mosquito bites, as the disease is only transmitted by mosquitoes. Typical advice includes use of mosquito repellent and mosquito screens, wearing light coloured clothing, and minimising standing water around homes (eg. removing Bromeliads, plant pots, garden ponds).[2] Staying indoors during dusk/dawn hours when mosquitos are most active may also be effective. Bush camping is a common precipitant of infection so particular care is required.

Research

The study of RRF has been recently facilitated by the development of a mouse model. Mice infected with RRV develop hind-limb arthritis/arthralgia which is similar to human disease. The disease in mice is characterized by an inflammatory infiltrate including macrophages which are immunopathogenic and exacerbate disease. Furthermore, mice deficient in the C3 protein do not suffer from severe disease following infection.[5] This indicates that an aberrant innate immune response is responsible for severe disease following RRV infection.