Southern Oscillation Index and transmission of the Barmah Forest virus infection in Queensland, Australia.

El Niño-Southern Oscillation (ENSO) events are usually accompanied by changes in the trade winds, cloud amounts and rainfall over the tropical Pacific and Australian regions, and seem to be related to many climatic anomalies around the globe. A measure of ENSO is the Southern Oscillation Index (SOI), which is the normalised atmospheric pressure diVerence between Tahiti in the south Pacific and Darwin in northern Australia. The SOI is closely related to variations in temperature and rainfall across the Pacific and in eastern Australia. A positive index (low pressure at Darwin, greater rainfall, higher sea levels) means that the south eastern trade winds feed moisture across the Pacific towards the Australian region. Hence positive SOI values would tend to favour salt marsh mosquito breeding, and as a consequence might impact on the transmission of some mosquito borne diseases such as the Ross River virus infection. Barmah Forest virus (BFV) infection, characterised by polyarthritis, myalgia, rash, fever, lethargy and malaise, is caused by an alphavirus, with Aedes and Cules mosquitoes as major vectors and marsupials as suspected host. The incubation period may be 7–9 days, the rash lasts an average of seven days and BFV infection may also lead to chronic illness in some patients. It has been regarded as one of the most important arbovirus diseases in the Australasian region and recently emerged as an increasing cause of human disease in mainland, Australia. In Australia, most BFV infections occur in Queensland with a tropical and sub-tropical climate. In 1996, for example, 68.8% of the BFV cases were developed in Queensland and the incidence was as high as 15.93/100 000.