Barmah Forest virus infection is one of the most common mosquito-borne infections in Australia (along with the Ross River virus infection) (SA DoH 2020a).
First isolated in the Barmah State Forest near the Murray Valley in 1974, the virus has since been detected in most parts of mainland Australia, with Queensland being the main hotspot. On average, between 1500 and 2000 fresh cases are notified each year. Outbreaks are related to higher temperatures, high rainfall and flooding, and are more likely in La Nina years (VIC DoH 2015; Griffin 2015; WA DoH 2020).
Most cases occur in the northern part of Australia between December and February (the wet season). So far, Barmah Forest virus has not been found outside of Australia (Farmer & Suhrbier 2019; QLD DoH 2017).
Spread of Barmah Forest Virus
While the virus was first isolated in Culex annulirostris mosquitoes (common banded mosquito), it has also been found in others species including Aedes vigilax (northern salt marsh mosquito), Aedes camptorhynchus (southern salt marsh mosquito) and Culicoides marksi (Culicoides biting midges). These mosquitoes likely become infected by biting infected marsupials (e.g. possums, kangaroos and wallabies) (VIC DoH 2015).
Symptoms of Barmah Forest Virus
Many people infected with the vius do not develop symptoms. In those that do, the incubation period is on average between 7 and 10 days (sometimes up to 21 days) (SA DH 2020b). Symptoms are similar to those of Ross River virus infection, but tend to be milder (Better Health Channel 2017). They may include:
Flu-like symptoms (fever, chills, headache);
Pain, swelling and stiffness in joints (ankles, fingers, knees, wrists);
Tendon and muscle pain;
Swollen lymph glands; and
Rash on the trunk and limbs (more common in Barmah Forest virus than in Ross River virus).
(SA DoH 2020b; Better Health Channel 2017; Healthy WA 2013)
Most people completely recover within a few weeks. However, in some cases, it may take up to months or even over a year for the symptoms to resolve. In some of these cases, symptoms are prolonged due to another underlying cause. People who have had Barmah Forest Virus virus are likely to develop life-long immunity (SA DoH 2020b; Better Health Channel 2017).
Diagnosis of Barmah Forest Virus
Diagnosis is usually by blood tests measuring a rise in IgG antibody over two weeks. A single IgM test may return a false positive result, so a second test should be performed 14 days later (NSW DoH 2016). Reference laboratories can do a nucleic acid test but this is only useful early in the disease (Lab Tests Online Australasia 2018).
The disease may need to be distinguished from dengue, Ross River virus infection, infectious mononucleosis, rubella, Q fever, rheumatoid arthritis and systemic lupus erythematosus.
Treatment of Barmah Forest Virus
Treatment is non-specific. Non-steroidal anti-inflammatory medicines, such as ibuprofen, appear to provide the most relief. Paracetamol or aspirin (not in children less than 12 years old) may also be used. Rest and gentle exercise are important to prevent overtiredness and ensure joints are kept moving. Emotional stress and physical fatigue can make the condition worse (SA DoH 2020b; QLD DoH 2017; Healthy WA 2013).
Prevention of Barmah Forest Virus
No vaccine is available, however, the condition can be prevented by avoiding mosquito bites (SA DoH 2020b). This can be achieved by:
Using insect repellents;
Wearing protective, light-coloured clothing;
Avoiding being outside during times of heavy mosquito infestation (early evenings in warmer months);
Screening living and sleeping areas; and
Checking homes regularly for potential mosquito breeding areas (e.g. uncovered water containers, small wading pools, old tyres).
(QLD DoH 2017)
Particular care should be taken camping or fishing during the mosquito season. Eradication programs are the most effective way to prevent spread, however, wetland mosquitoes can fly up to 20 kilometres from their breeding grounds, potentially making this difficult (SA DoH 2020a; QLD DoH 2017; Webb 2019).