How to test for mad cow.

How to test for mad cow.

How to test for mad cow.

Answers to your questions about the news.
March 16 2004 6:43 PM

How Do You Test for Mad Cow?

First, kill Flossie. Then send her brain to Iowa.

The U.S. Department of Agriculture is planning to test over 221,000 cows for bovine spongiform encephalopathy, better known as mad cow disease. That's a tenfold increase over the number of cattle tested in 2003. How does the USDA figure out which cows have BSE?

Very slowly, at least until the next generation of tests are perfected. There's currently no such thing as an antemortem test for BSE; a cow has to die before an appropriate sample can be obtained. Thin slices are razored off the dead animal's brain, then shipped to the USDA's National Veterinary Services Laboratories in Ames, Iowa. There, researchers examine the brain tissue samples through microscopes. They search for evidence of prions, infectious proteins that cause mad cow. Because this testing regime relies on scientists' judgment calls, samples that are deemed infected are typically sent abroad for further verification. That was the case in December, when a Washington dairy cow tested positive for BSE at the Ames lab. A sample was then sent to the BSE international reference laboratory in Weybridge, England, a nation with far more mad cow expertise.


More than a week can elapse between a cow's death and the USDA's verdict, and experts on mad cow disease are concerned that the backlogged Ames lab will have trouble handling a tenfold increase in samples. Several companies are hoping to capitalize on those concerns by developing quicker tests, such as the one peddled by Bio-Rad Laboratories. The company's BSE Test Kit, the best-selling test in Europe and Japan, automates the detection process, rather than relying on the fallible human eye. Antibodies that adhere to prions are added to the tissue samples, and a computer then scans for evidence of antibody-prion binding. Bio-Rad claims that a lab using its kit can process between 800 and 1,000 samples per day, compared to 50 to 60 samples per day using the USDA's current method. But the Bio-Rad test has yet to receive USDA approval, though trials are underway.

The hope is that a rapid antemortem test can be developed, too, so entire herds needn't be slaughtered simply because a single cow tests positive for BSE. Last September, researchers at the University of California-San Francisco announced that they'd launched trials on a test called conformation-dependent immunoassay (CDI). The novel test, which can detect minute traces of prions, may eventually be sensitive enough to suss out the telltale signs of BSE in muscle tissue or blood. It's already proven effective at detecting prions in living lab mice.

Cost remains a big question mark. Testing every slaughtered cow—which is the policy in Japan—would cost $20 to $25 per head. Down the product stream, that would add 6 cents to the price of a pound of beef—or a penny or two to the price of every McDonald's Quarter Pounder With Cheese.