"Get used to seeing these," said University of Missouri Extension veterinarian Robert Larson. "There will be lots of them."

Larson was referring to statements issued by the U.S. Department of Agriculture on June 25 and 29 announcing inconclusive results from rapid tests for bovine spongiform encephalopathy, or BSE, in suspect cattle.

As of press time, two inconclusive results had been reported. Both test results were confirmed negative following more precise testing, John Clifford, deputy administrator for the USDA Animal and Plant Health Inspection Service, said during two press conference.

"The rapid screening test, called ELISA, used in the enhanced BSE surveillance program is very sensitive," said Larson, a veterinary beef extension specialist with the MU Commercial Agriculture program. "It's not likely to say a positive sample is negative."

Using ELISA speeds surveillance and allows carcasses that test negative to be returned to processing quicker, he said. "If an inconclusive result is returned, then a more time-consuming and accurate immunohistochemistry test is performed at the USDA national lab in Ames, Iowa."

Larson likened the difference between the two tests for BSE to the difference between a home pregnancy test and the test administered by a physician.

"Just like a home pregnancy test provides a result quicker than a test at the doctor's office, ELISA provides results quicker than immunohistochemistry," he explained. "However, ELISA will sometimes say a negative sample is positive, just like a home pregnancy test will sometimes say a woman is pregnant when in fact she's not."

Larson added that when testing for diseases with a low prevalence such as BSE, "most of the test results are false positives rather than true positives. This is a result of mathematics, not biology. The general public may not have dealt with diagnostic test interpretation that isn't immediately black and white before, but cattle producers who've dealt with disease eradication programs for brucellosis and tuberculosis are familiar with it.”

NCBA President Jan Lyons believes the follow up testing shows the system is working. “This result is not unexpected. As part of USDA’s expanded BSE surveillance program, a rapid screening test is used as the first step in a two-part testing process. USDA expected some inconclusive results from this initial step. Because the rapid tests are sensitive, they are subject to occasional inconclusive results that later prove to be negative. It is a little like going through the airport metal detector. We all have had the detector beep on us at least once, but it didn’t mean we were carrying a prohibited item. It simply meant more testing was needed.”

The USDA was criticized following the release of information on the first two inconclusive test results. The information will continue to be released, but according to the Texas Cattle Feeders Association, the USDA will no longer hold technical briefings when releasing the information. Instead, inconclusive results will be posted on the Web.

Lyons supports open reporting of inconclusive BSE test results to assure consumer confidence.
“We realize the process is affecting the cattle markets...however, failure to announce inconclusive test results could create rumors, speculation and increased market volatility,” Lyons said.

The USDA expanded its BSE surveillance following the discovery of an infected dairy cow in Washington state last December. At current screening levels, the program should find the disease if it occurs in as few as 1 in 10 million adult cattle, according to USDA statements.

BSE is one of several brain-wasting diseases, including scrapie in sheep and goats, chronic wasting disease in deer and elk, and Creutzfeldt-Jacob disease in humans, believed to be caused by abnormal proteins called prions. The build-up of prions in an animal’s central nervous tissue eventually kills nerve cells, leaving behind lesions and sponge-like holes in the brain.

Though primarily a cattle disease, BSE has been documented to pass from cattle to humans. When contracted by humans, it is referred to as variant Creutzfeldt-Jacob disease.

As of June 28, the USDA reported 8,585 negative tests under the enhanced BSE surveillance program, which was implemented June 1. For more information about the program, visit http://www.aphis.usda.gov/. ©