EARLY in October 1991 graziers Malcolm and Alister Fraser were going about their usual station work when they discovered dead cows and calves in a paddock beside the Darling River. There were no signs of injury or any other clues as to what had killed them. Not far away they saw several of their merino rams lying dead in the yellowing grass.

Something strange was happening on their river block. There had been hardly any rain for the past nine months, and everyone knew drought was setting in, but conditions were hardly dire. The previous year the region had seen its largest floods in nearly twenty years. In all, the Frasers counted thirty dead Brahman cows along with their calves, and about 140 merinos. The surviving cows were so weak that many had to be helped to their feet. A week ago the animals had been fine, and now they were dead or distressed. What was killing them? Why had they died so suddenly?

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