The first possible case of mad cow disease detected through a new nationwide screening program was confirmed negative after further screening, the Agriculture Department announced Wednesday during a press conference.
The USDA disclosed June 25 that testing had uncovered an initial positive result in one cow. But additional testing at the government's National Veterinary Services Laboratories in Ames, Iowa, revealed that the brain sample of the cow in question did not contain the abnormal proteins that can cause the fatal disease.
"That particular result is negative," said USDA chief veterinarian John Clifford. Clifford said he was notified of the result at 3:45 p.m. ET Wednesday, though by late morning, the agency had scheduled a late-afternoon news briefing.
The Ames lab is conducting a similar confirmation on a second possible case, which the government announced late Tuesday. The final determination on that cow is not expected for at least another two to three days.
Expanded mad cow testing program
The government began an expanded mad cow testing program June 1 and intends to test somewhere between 201,000 and 268,000 older cattle in the next year to 18 months, although it won't set a firm target. It unveiled the program in March, three months after the nation's first case of mad cow disease was found in a Washington state dairy cow.
By the end of last week, a handful of public state labs across the country had performed 8,585 tests, with nearly 3,000 performed last week alone. At that rate, the government could reach the low end of its possible goal in about 16 months. But, Clifford said, "We would expect [the rate of testing] to continue to increase for quite a bit of time."
Initial screenings for bovine spongiform encephalopathy, as mad cow is formally known, can be completed in as little as four hours, but are highly sensitive and so can occasionally return a false positive result, as appears to have happened with the animal whose results were announced last week.
The USDA said it intends to report all initially positive results as soon as it finds them, but will not disclose the location or other details about the suspect animal.
The agency has approved 12 state university labs to conduct testing. It is not clear how many are up and running, though one official familiar with the government's plans believed about five of the 12 are currently conducting tests.
How results compare
To verify the findings, all samples are sent to the Ames lab, where a highly accurate test protocol known as immunohistochemistry is used to arrive at a final determination whether a cow has become infected with BSE.
However, the initial screenings are conducted using a rapid protocol known as Elisa, which knocks away normal prions — certain proteins found in nervous tissue — and looks for aberrant ones, which are a key sign of the disease. Because the Elisa tests are so sensitive, the USDA has said it will not release any information about suspect animals until they receive final results from the confirmatory tests.
After about 9,000 tests, USDA officials refused to draw comparisons between other countries' testing results and their discovery of a single, ultimately negative result (occasionally called "false positives," though some scientists frown on that term).
While the U.S. testing system will cover just a tiny fraction of the 37 million cattle slaughtered in this country each year, its initial results are not unlike the early stages of Japan's testing program, which checks every head of cattle entering the human food chain for BSE. Japan's early results included about 1 in 10,000 initial positives that turned out negative. European labs also had a higher rate of initially positive cases in the early stages of testing.
But those results drop to just one in 127,000 tests last year in Japan, and one in 302,000 worldwide, according to data provided by California firm Bio-Rad, which manufactures the Elisa test being used in U.S. and many international labs.
While the USDA mandates that all initially positive samples be sent directly for confirmation after a single Elisa test, many European labs conduct two or three passes with the rapid test before sending a sample for confirmation.
"It would be exactly what a laboratory would do to test a patient's blood sample," said Brad Crutchfield, a Bio-Rad vice president. "If you got a result you would repeat that result before sending it off."
While Clifford said scientists intend to conduct additional Elisa tests on brain material that initially tests positive, or "inconclusive," in the agency's parlance, the tests will be used simply for quality control. Results will not be released, he added.
Beef industry officials were relieved by Wednesday's announcement and pointed out that any initially positive result should be viewed skeptically until confirmed.
"It is a little like going through the airport metal detector," Jan Lyons, president of the National Cattlemen's Beef Association, said in a statement. "We all have had the detector beep on us at least once, but it didn't mean we were carrying a prohibited item."