Government is investigating 3rd possible case of mad cow
July 28, 2005
The New York Times
Sandra Blakeslee
The Department of Agriculture is investigating another possible case of mad cow disease in a domestic cow, its chief veterinarian said yesterday.
The department would not say where the farm was, other than to say it was remote.
The veterinarian, Dr. John Clifford, said that the 12-year-old cow died in April but that its brain tissue was not tested until last week. Because the initial results were ambiguous, scientists at the National Veterinary Services Laboratory in Ames, Iowa, are conducting more tests to determine whether the cow was infected.
Although the brain tissue was collected in April, the veterinarian forgot to send it in, Dr. Clifford said. "While that time lag is not optimal, it has no implications in terms of the risk to human health," he said.
The carcass was destroyed, Dr. Clifford said, "and therefore there is absolutely no risk to human or animal health from this animal."
Samples are also being sent to an international mad cow laboratory in Weybridge, England. Scientists there have a wide variety of methods for finding the misfolded proteins, or prions, that cause the brain-wasting mad cow disease.
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 was a Texas cow that died in November but whose brain was not tested for the disease until last month. British scientists made the diagnosis after American scientists missed it because of using incomplete testing methods.
The latest cow died of complications in calving, Dr. Clifford said. A private veterinarian took a brain sample from it and preserved it in a chemical called formalin. Because rapid screening methods require fresh tissue, the samples are being examined with a slower method called immunohistochemistry, or IHC.
Last month the Agriculture Department changed its testing protocols to require fresh tissue from animals regarded at high risk for mad cow disease. When rapid tests are inconclusive, the slower IHC test is to be carried out along with a second confirmatory test called the western blot.
In the IHC test, antibodies are added to brain slices. When prions are present, the antibodies stick to them and create a staining pattern. Because different brain slices from an infected animal may or may not contain the prions, many slices need to be tested.
Last week, the Ames laboratory ran the IHC test on a brain slice from the suspect cow and found "nondefinitive results," Dr. Clifford said. The tissue showed some weak staining that could indicate that the disease is present, he said, but the staining pattern was not typical.
After more slices are tested in Iowa and in England, he said, results will be announced next week.
