USA: A Second Test Shows Animal Did Not Have Mad Cow Disease
The New York Times
July 1, 2004
A Second Test Shows Animal Did Not Have Mad Cow Disease
By DONALD G. McNEIL Jr. and SANDRA BLAKESLEE
A dead cow that had a positive preliminary test for mad cow disease
on Friday tested negative on a confirmatory test, the Agriculture
Department said yesterday.
The second test, done at the department's National Veterinary
Laboratory in Ames, Iowa, is considered definitive, said Dr. John T.
Clifford, the deputy administrator for animal and plant health
inspection. The animal is considered not to have the disease, he
said, and no other laboratory, including the British one that
confirmed the United States' first case of mad cow disease last
December, will be asked to confirm it.
Dr. Clifford would not give any information about the cow's age,
breed, sex or place of slaughter. The carcass was held after the
preliminary test and did not enter the human food chain, he said.
A second cow tested positive on a preliminary test on Wednesday, and
the department is still waiting for the results of its second test,
called an immunohistochemistry test.
More than 8,500 cattle have been tested since the department revamped
its testing on June 1, Dr. Clifford said, and all but two have been
negative on preliminary tests that take less than three hours. The
test, called Elisa, looks for misfolded prions, the abnormal proteins
that cause mad cow and diseases like it.
Late last year, the disease was found in an old dairy cow from a
Washington State farm, leading to a widespread ban on exports of
American beef and costing the cattle industry hundreds of millions of
dollars. Japan, the biggest foreign market, still wants every animal
tested before it will reopen its doors to American beef.
Andrew Kimbrell, executive director of the Center for Food Safety, a
Washington advocacy group, said the department should have
independent laboratories to confirm its tests.
"The U.S.D.A. has a secretive monopoly on these tests, and it's led
to widespread suspicions that there's more out there than they admit
to," he said.
In the Elisa test, in this case from Bio-Rad Laboratories of
Hercules, Calif., a bit of cow brain stem is ground into mush.
Enzymes are added that digest normal prions, leaving only the
abnormal misfolded ones. Then antibodies that cling to prions are
added. But false positives can result when some normal prions are
In the immunohistochemistry test, which agriculture officials
routinely call the "gold standard," slices of brain stem are treated
to increase the concentration of enzyme-resistant prions, then
scanned and examined under a microscope. The accuracy of the test
depends on the expertise of the examiner.
In Japan and some European countries, a third test that takes up to
three days, a Western blot, is used to double-check suspicious
results or tests that disagree. That led to a surprising finding that
the disease can be found in animals younger than 30 months.
Copyright 2004 The New York Times Company