Barmah Forest virus infection is the second most common mosquito-borne infection in Australia (after Ross River virus infection).

First isolated in the Barmah State Forest in the Murray Valley in 1974, it has since been detected in most parts of mainland Australia, with Queensland being the hotspot. On average, some 700 fresh cases are notified each year (over 400 in Queensland), but in epidemic years, numbers have approached 2000. Such outbreaks are related to higher temperatures, high tides and flooding, and are more likely in La Nina years.

In most parts of Australia, cases occur mainly in December to May, with peak incidence in February, but in South-Eastern Western Australia, the peak occurs in Spring. So far, Barmah Forest virus has not been found outside Australia.

Spread

The virus is spread by a number of mosquitoes: Aedes vigilax (northern salt marsh mosquito), Aedes camptorhynchus (southern salt marsh mosquito), Culex annulirostris (common banded mosquito), Aedes normanensis (flood water mosquito), Aedes notoscriptus (backyard mosquito) and Aedes procax (freshwater forested ground pools). These become infected by biting infected marsupials (possums, kangaroos, wallabies) or humans (and possibly birds).

Symptoms

Half to two-thirds of humans infected do not develop symptoms. In those that do, illness suddenly develops between 2 and 10 days (sometimes up to 21 days) after being bitten. Symptoms are similar to those of Ross River virus infection. Flu-like symptoms (fever, chills and headache) are shown by some patients. Nearly all experience pain, swelling and stiffness in several joints (ankles, fingers, knees, wrists), though usually to a lesser extent than with Ross River virus infection. Many also have tendon and muscle pain. Back pain is common and swollen lymph glands, dizziness and light-headedness also occur. A raised red skin rash lasting one to two weeks is common and more obvious than in Ross River virus infection. Almost all patients with symptoms beyond six months have other conditions (e.g. osteoarthritis, rheumatoid arthritis). There are few or no long-term effects, and immunity after an attack is probably life-long.

Diagnosis

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. Reference laboratories can do a nucleic acid test but this is only useful early in the disease. The virus can also be isolated. 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

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. A combination of plenty of rest and gentle exercise is important to keep joints moving and to prevent over tiredness. Emotional stress, physical fatigue and alcohol can make the condition worse.

Prevention

No vaccine is available. Prevention is by preventing mosquito bites 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 (uncovered water containers, small wading pools, old tyres). Particular care should be taken camping or fishing during the mosquito season. Eradication programs are often not acceptable or practical because of breeding areas occurring in environmentally sensitive locations and having an extensive geographical spread and the fact that Aedes vigilax can fly up to 20 kilometres.