'Inconclusive' BSE test confirmed negative;
false positives a product of math, not biology

COLUMBIA, Mo. - "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.

The first inconclusive result was confirmed negative following more precise testing, John Clifford, deputy administrator for the USDA Animal and Plant Health Inspection Service, said during a June 30 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."

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/.