A recent attempt to determine whether the British sheep flock carries bovine spongiform encephalopathy (BSE), or mad cow disease, culminated in what can only be described as deeply embarrassing failure. The tissue samples under study turned out to come from cows, not sheep. As a result, whether or not the sheep harbor BSEwhich in humans takes the form of variant Creutzfeldt-Jakob disease (vCJD)remains unknown. If they do, however, and if they can pass the disease-causing agent to humans, the public health risk could far surpass that posed by infected cows alone, according to a report published online today by the journal Nature.

To estimate the potential impact of a sheep epidemic, Neil M. Ferguson and his colleagues at Imperial College, London, considered three possible scenarios. In the best case, BSE does not spread within or between flocks and therefore has a negligable impact on the vCJD epidemic. But at worst, BSE spreads wildly both within and between flocks and raises the vCJD toll from a maximum of 50,000 to 100,000 deaths due to infected cattle alone to a combined total of as many as 150,000 deaths.

The good news is that placing on sheep products the same restrictions that are currently levied against cattle could reduce the risks from BSE-infected sheep by up to 90 percent. Still, the authors note that their findings underscore the need for further investigation. "Our estimates are ultimately dependent on the quality and quantity of information that is gathered by other researchers," Ferguson remarks, "and we feel that large-scale testing of the national flock, and additional experimental research are urgent priorities."