SPOKANE, Wash. - When the nation's first case of mad cow disease was discovered on a Washington farm, it took the slaughter of more than 700 healthy cattle to prove the disease had not spread.

That's because there's no test for mad cow that can be done on live animals, and there may not be one for some time.

"I don't know how far away we are," said Don Knowles, who runs a U.S. Department of Agriculture laboratory in Pullman that is working jointly with Washington State University scientists to develop such a test.

"Data and announcements are coming out all the time," Knowles said. "At this moment, none of these tests has enough validation data behind them."

Being able to diagnose an infection quickly could help keep contaminated beef out of the food chain and also cut the economic loss that comes from slaughtering healthy animals, according to a 2003 report by the National Research Council. The animals killed in Washington state, for example, would have been worth well over half a million dollars at market.

Mad cow disease is a public health concern because scientists believe humans who eat infected beef products can develop a brain-wasting disease called variant Creutzfeldt-Jakob disease, which has killed 153 people worldwide.

Quick detection also could prevent contaminated human blood from entering the blood supply, the report said.

The misshapen proteins - called prions - thought to cause mad cow disease concentrate in the brain and central nervous system, and the best test for the disease involves killing the animal and analyzing a cross section of its brain.

Finding a test on living animals should be a top priority, but it does not appear to be imminent, according to the NRC report. "Major breakthroughs are needed to achieve the levels of sensitivity and specificity required to test live animal and human tissues," the report said.

Scientists are following a theory that prions may move through the blood supply, and are looking for a way to detect them there, Knowles said. They are also looking for a genetic marker that would reveal when prions are present, he said.

The Pullman researchers were the first to develop a test to detect a similar disease in sheep called scrapie, said Charlie Powell, a spokesman for the university's College of Veterinary Medicine.

That was the first and only test that can detect the class of diseases known as transmissible spongiform encephalopathies - which includes mad cow disease - in living animals, Knowles said. But that test does not work on cattle, he said.

Efforts to assure the public that the Washington case did not spread across the country prompted officials to euthanize 704 cows at various Northwest farms. It took more than a month to track many of the animals that may have had contact with the infected cow, and post-mortem tests revealed that none of those animals had mad cow disease.