The government is depending on a mix of quarantine and detective work to figure out how mad cow disease (search) apparently infected a cow in Washington state.

The nation's mad cow emergency plan — never before used — is to cordon off any cattle that could have come into contact with the infection.

Keeping any possibly infected cow out of the food chain is crucial, as a human version of the disease that literally eats holes in the brain is believed spread by eating meat tainted with infected brain or nerve tissue. The human illness, with the unwieldy name new variant Creutzfeldt-Jacob disease (search), has claimed 143 victims in Britain and 10 elsewhere, but none so far in the United States.

Already, the farm near Yakima, Wash., has been quarantined as the government awaits the last of three tests on the suspect cow's tissue to confirm the presence of BSE, bovine spongiform encephalopathy (search). That final test is performed in Britain, where BSE first erupted in cattle in 1986.

Once confirmed, all the animals in that herd probably will slaughtered so their brains, too, can be tested, said Food and Drug Administration Deputy Commissioner Lester Crawford, a veterinarian who oversees the agency's BSE work.

Key is tracing the source of the first infection to see how the U.S. firewall against this disease was breached.

Beef and cattle imported from countries known to have BSE has long been banned.

But the nation's main defense is a 1997 ban on giving cattle feed made from the protein or bone meal of sheep or other mammals — because that feed is thought to be the way mad cow disease originally spread.

So the first question is whether the cow was illegally imported or ate feed that illegally contained BSE-bearing protein.

Another possibility depends on the cow's age, which wasn't immediately known. If the cow was more than 6 years old, she could have received tainted feed before the FDA's ban began, Crawford noted. The disease's incubation period can be as long as eight years.

A second part of the probe is to trace every cow the ill one — and any others found to be ill — came into contact with, so they also can be quarantined and tested to see if the infection spread.

Canada has had two cases of BSE-infected cows, once in 1993 and once last spring, but never was able to pinpoint the source of infection, Crawford noted. "We would hope to be able to nail it down," he said.

Scientists have long warned it was only a matter of time before BSE reached U.S. cattle, because safeguards can't be foolproof.

Still, "we have in place a system in this country to dramatically reduce the risk of any infected bovine getting into the food system and being consumed," said Michael Osterholm of the University of Minnesota, who has advised the federal government on food-safety issues.

"One case does not make for a crisis," he stressed.

Extra layers of protection come at slaughterhouses, where inspectors prevent sick animals from being killed for human consumption.

In addition, the Agriculture Department had the brains of more than 20,000 cows tested for signs of BSE in the last year, triple the number from previous years.

Any non-ambulatory cow, like the Washington one, as well any cow with signs of neurological disease and all cows over 30 months old — because BSE is more common in old cows — are tested, said the National Cattlemen's Association (search).

Critics say there are loopholes. Europe tests far more cattle for BSE, for example. Also, BSE is caused by rogue proteins called prions that collect in the cow's brain, spinal cord and other nervous system tissue. Processors are supposed to remove the spinal cord to minimize health risks, although a 2002 report found not all processors completely follow that rule. The Agriculture Department last spring began more careful testing to ensure compliance.

"Virtually every aspect of our BSE prevention program will be reevaluated," said FDA's Crawford.