Australian encephalitis (AE) is a severe, often fatal, viral illness transmitted to humans by mosquitoes and influenced by the ENSO (Nicholls, 1986). Since 1917 when the clinical symptoms of the illness were first diagnosed, there have been only 7 years when cases of AE were observed in southeastern Australia. Cases occur between January and April and follow widespread flooding over several seasons. Flooding leads to increased mosquito numbers by increasing the numbers of breeding sites and host populations (birds, marsupials). The probability that AE will be diagnosed in southeastern Australia is predictable from the SOI in the previous spring (September-November). The relationship is sufficiently strong to allow health authorities to increase surveillance and prophylactic action, in years when the SOI, during spring, is very high.
4 ECOLOGICAL IMPACTS OF EL NiÑO-SOUTHERN OSCILLATION 811
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