| Ross River fever | |
|---|---|
| Specialty | Infectious diseases  |
Ross River fever is amosquito-borne disease caused by infection with theRoss River virus. The illness is typically characterised byflu like symptoms combined withpolyarthritis and a rash. The virus isendemic to mainlandAustralia andTasmania, the island ofNew Guinea,Fiji,Samoa, theCook Islands,New Caledonia and several other islands in the South Pacific.[1] The illness is Queensland's most prolific mosquito-borne disease.[2]
Symptoms of the disease vary widely in severity, but major indicators arearthralgia,arthritis,fever, andrash.[3] The incubation period is 7–9 days. About a third of infections are asymptomatic, particularly in children.[4][3]
About 95% of symptomatic cases report joint pain.[4] This is typically symmetrical and with acute onset, affecting the fingers, toes, ankles, wrists, back, knees and elbows.[3] Fatigue occurs in 90% and fever,myalgia and headache occur in 50–60%.[4]
A rash occurs in 50% of patients and is widespread andmaculopapular.Lymphadenopathy occurs commonly;pharyngitis andrhinorrhea less frequently.Diarrhea is rare. About 50% of people report needing time off work with the acute illness.[4] If the rash is unnoticed, these symptoms are quite easily mistaken for more common illnesses likeinfluenza or thecommon cold. Recovery from the flu symptoms is expected within a month, but, because the virus currently cannot be removed once infection has occurred secondary symptoms of joint and muscle inflammation, pain and stiffness can last for many years. Less common manifestations includesplenomegaly,hematuria andglomerulonephritis. Headache, neck stiffness, andphotophobia may occur. There have been three case reports suggesting meningitis or encephalitis.[citation needed]
Reports from the 1980s and 1990s suggested RRV infection was associated witharthralgia, 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% at 6 months and 9% at 12 months, similar toEpstein–Barr virus andQ fever.[4] The only significant predictor of the likelihood of developing chronic symptoms is the severity of the acute illness itself. No other aspects of the patient's medical or psychiatric history have been found to be predictive. However, in those with the most persisting symptoms (12 months or more),comorbid rheumatologic conditions and/or depression are frequently observed.[4]
The virus can only be spread bymosquitoes. The mainreservoir hosts arekangaroos andwallabies, although horses,possums and possibly birds andflying foxes play a role. Over 30 species have been implicated as possiblevectors, but the major species for Ross River fever areCulex annulirostris in inland areas,Aedes vigilax in northern coastal regions andAe. camptorhynchus in southern coastal regions.[4][3]
Ablood 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]
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 (e.g. removingBromeliads, plant pots, garden ponds).[4] 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.[citation needed]
Patients are usually managed with simpleanalgesics,anti-inflammatories,anti-pyretics and rest while the illness runs its course.[1][3]Pentosan polysulfate has also shown recent promise.[5]
Most notifications are from Queensland, tropical Western Australia and theNorthern Territory. Geographical risk factors include areas of higher rainfall and higher maximal tides.[4] In the tropics, Ross River fever is more prevalent during the summer/autumn "wet season", particularly January—March, whenmosquito populations numbers are high. In southern parts of Australia, this time period may shift to earlier in the year during spring/summer. Areas noted of common place contraction of the virus include townships and along the River Murray areas. Backwaters and Lagoons are breeding grounds for mosquitos and local medical treating facilities report higher cases than cities away from the river around the riverina areas.[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 affected.[4] Ross River fever is on the AustralianDepartment of Health and Ageing's list ofnotifiable diseases.[6]
The first outbreak of RRF was in 1928 in theHay andNarrandera region inNew South Wales, Australia.[4] The virus was first isolated in 1959 from a mosquito trapped along theRoss River inTownsville, Queensland. Since then, outbreaks have occurred in all Australian states, including Tasmania, and metropolitan areas.[4] The largest outbreak occurred in 1979–1980 in the Western Pacific, and affected more than 60,000 people.[4]
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 anothermosquito-borne virus,Barmah Forest virus.[3]
The study of RRF has been recently facilitated by the development of amouse 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 theC3 protein do not develop severe disease following infection.[7] This indicates that an aberrant innate immune response is responsible for severe disease following RRV infection.