Penn State scientists say BSE testing not perfect, but effective

University Park, Pa. -- As the U.S. Department of Agriculture investigates the origins of a recently confirmed case of bovine spongiform encephalopathy or "mad cow disease" in Texas -- the second case discovered on American soil -- some observers have raised questions about how the USDA tests cattle for the disease.

A pair of specialists in Penn State's College of Agricultural Sciences say the protocol may not be perfect, but it is the best currently available. And, they say, the testing and safeguards put in place after the first BSE case in late 2003 have been effective in screening for the disease and reducing the risk for American consumers.

"It's important to note that neither animal that tested positive for BSE entered the food or feed chain," says William Henning, professor of animal science. "The firewalls are in place to identify at-risk animals and keep them out of our beef supply."

BSE is caused by an abnormal protein called a prion. The disease is thought to be spread when cattle eat feed containing rendered byproducts from infected cattle. Since 1997, the United States has banned the use of ruminant byproducts in animal feed. The most recent BSE case was found in a 12-year-old cow born long before the ruminant-to-ruminant feed ban went into effect.

After the 2003 BSE case in a Washington state cow that had been imported from Canada, USDA banned non-ambulatory cows -- regardless of why they can't walk -- from going to market. Also banned was the processing of so-called specified risk materials, such as brain and spinal cord tissues, where the disease-causing prions are found in infected animals. Scientists say there is no evidence that prions occur in meat, milk or blood.

The BSE testing protocol targets non-ambulatory and other at-risk animals, such as those that are greater than 30 months old with abnormal symptoms. BSE is not found in animals younger than 30 months, and most cattle are processed for meat before they reach that age.

"The USDA uses a rapid screening test that errs on the side of finding false positives, and we've had a few of those," Henning explains. "A positive screening test is followed by a confirmation test, which is much more accurate. Some have criticized USDA for its use of the immunohistochemistry (IHC) test, which in the most recent case initially produced a negative result. But at the time the decision was made to use the IHC test in the protocol, most scientists considered it the gold standard.

"To remove all doubt in the future, USDA has announced it now will follow inconclusive screening tests with both the IHC test and the Western blot test, which was used by a lab in Great Britain to confirm the most recent case."

The IHC test is an internationally recognized procedure, according to David Wolfgang, Penn State extension veterinarian and field studies director in veterinary science. "As scientists have refined and improved these tests, we are now able to detect infected animals at the sub-clinical level," he says. "The tests are so sensitive that we are finding and diverting animals that would have been considered 'negatives' in the past. This most recent infected cow was a tough diagnostic case, but USDA still was able to detect it and keep it out of the food supply.

"There have been only about 150 cases of human illness linked to BSE worldwide -- most of those in England -- and as we've learned more about the disease, the rate has been falling steadily," Wolfgang says.

"Since the 2003 BSE case in Washington state, USDA has tested nearly 400,000 at-risk animals for the disease," Henning adds. "The fact that only one additional case has been found -- and in a cow that was born before the 1997 feed ban -- indicates that the incidence of BSE in the United States is extremely low, and the safeguards that are in place to keep infected cows out of our beef supply are working as they should."