Jul 28, 2005 (CIDRAP News) Federal officials said yesterday that testing of a 12-year-old cow yielded possible signs of bovine spongiform encephalopathy (BSE) and that further tests are being conducted to clarify whether the disease was present.
The carcass was destroyed and did not enter the human food or animal feed chain, said Dr. John Clifford, chief veterinarian for the US Department of Agriculture (USDA). He did not disclose where the cow lived.
The cow died of calving complications on a farm in April, Clifford said. A private veterinarian took a sample of brain tissue but forgot to send it to the USDA for testing until last week, he explained.
"The carcass of this animal was destroyed, therefore there is absolutely no risk to human or animal health from this animal," Clifford said.
He said the veterinarian preserved the sample in formalin, which is suitable for the immunohistochemistry (IHC) test for BSE but rules out the use of rapid screening tests and the Western blot test. Use of the preservative was permitted under USDA guidelines at the time, but the protocols were changed in June to require that samples be sent in while still fresh. Since June, USDA policy calls for using both IHC and Western blot testing to confirm BSE when screening tests are inconclusive.
Calling the original IHC test on the sample "nondefinitive," Clifford said it revealed some staining indicating the abnormal prion protein associated with BSE, but it "did not match a normal pattern" for BSE. He said the USDA decided to run additional IHC tests at its national laboratory in Ames, Iowa, and also send samples to the BSE reference lab in Weybridge, England. Results are expected next week.
"As we have previously experienced, it is possible for an IHC test to yield differing results depending on the 'slice' of tissue that is tested," Clifford stated. "Therefore, scientists at our laboratory and at Weybridge will run the IHC test on additional 'slices' of tissue from this animal to determine whether or not it was infected with BSE."
He said it appears that the cow was born in the United States. Because BSE has not been confirmed, officials have not imposed a quarantine on the farm.
Given the cow's age, it was born years before the government banned the use of cattle protein in cattle feed in 1997. Cattle contract the disease by consuming protein from infected animals.
If tests confirm the disease, it would be the third BSE case found in the United States. The first case surfaced in a Canadian-born cow in Washington state in 2003, the second in a Texas cow that died last November, though the disease was not confirmed until last month.
Most cattle tested in the USDA's BSE surveillance program are animals that can't walk or that show signs of disease when they arrive at a slaughterhouse; they are diverted and brain samples are sent to laboratories for screening tests. Clifford explained that as an extension of the surveillance program, "accredited private veterinarians, who often visit farms in remote areas, collect samples when warranted."
Statement by Dr. John Clifford of USDA
Transcript of Jul 27 USDA news briefing