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From: TSS ()
Subject: Re: Latest test for mad cow is negative ??? NYT
Date: August 4, 2005 at 6:20 am PST

In Reply to: Latest test for mad cow is negative HOUSTON CHRONICLE posted by TSS on August 4, 2005 at 6:09 am:


Results Negative in 3rd Possible Case of Mad Cow
By SANDRA BLAKESLEE
Published: August 4, 2005
A third cow suspected of having mad cow disease does not on closer examination appear to be infected, the Department of Agriculture said yesterday.

After tests conducted last week yielded ambiguous results about whether the 12-year-old cow was infected with the brain-wasting disease, tissue samples from the animal were retested by the department's laboratory in Ames, Iowa, and by experts at the Central Reference Laboratory for mad cow disease in Weybridge, England.

Both tests came back negative for the disease, bovine spongiform encephalopathy or B.S.E., said Dr. John Clifford, the department's chief veterinarian.

"Needless to say, we are very pleased with these results," Dr. Clifford said in a statement. "Our enhanced surveillance program is designed to provide information about the level of prevalence of B.S.E. in the United States, which by any measure is extremely low."

Two cases of mad cow disease have been confirmed in the United States. The first, involving a cow born in Canada, was discovered in Washington State in December 2003. The second occurred in a cow born in Texas that died in November. The cow's brain was not tested for the disease until June, when tests in England confirmed the diagnosis. Tests conducted earlier in the United States had been negative.

Several types of tests are used to detect the presence of the misfolded proteins, prions, that scientists believe cause mad cow disease. One such test, an immunohistochemistry test, or IHC, was the source of the ambiguous results for the third cow.

In the test, scientists examine extremely thin slices of tissue taken from the brainstem or from other parts of the nervous system where the prions are thought to accumulate.

The slices are treated with antibodies that bind to abnormal proteins. The antibodies are labeled with dyes so they can be seen with the naked eye, and the dyes then show up in a typical staining pattern if the prions are present.

The ambiguous test last week produced some staining but it did not resemble mad cow disease, said Jim Rogers, a spokesman for the Agriculture Department. He said the staining could have been caused by a contaminant in the test or in the tissues, but said, "The fact there was staining made us want to redo the tests."

The IHC test can pick up infectious prions only if the particular slices of tissue taken from the cow's brain happen to contain them, said Dr. Jiri Safar, a neurologist at the University of California, San Francisco, and an expert on prions. This can be a hit-or-miss process, he said, and even if prions are present, certain antibodies can fail to bind if the level of infection is extremely low.

Moreover, cases of so-called atypical bovine spongiform encephalopathy have been showing up in a small number of European and Japanese cows, Dr. Safar said. The antibodies used in conventional IHC tests can fail to identify those abnormal proteins that tend to accumulate in tissues that are not usually examined under the test's protocols.

The suspect cow died on the farm where it was raised after complications from giving birth. The farm's location has not been released. The cow, which was destroyed, died in April, but the veterinarian who took its brain tissue forgot to send the sample for testing until last month.

http://www.nytimes.com/2005/08/04/health/04cow.html?oref=login

TSS




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