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Aug. 31--The federal government concluded its investigation into the nation's second case of mad cow disease Tuesday, announcing it couldn't determine how a 12-year-old Texas cow became infected.
Food and agricultural officials cited a lack of detailed feeding records at a Lubbock-area cattle operation where the infection occurred and their inability to find most of the infected animal's herd mates and offspring.
Following a two-month investigation, officials of the U.S. Department of Agriculture and Food and Drug Administration speculated the cow most likely got the disease before 1997 by eating feed containing the remains of other cattle infected with the disease.
"We can't be absolutely positive," said Dr. Stephen Sundlof, director of the Center for Veterinary Medicine at the U.S. Food and Drug Administration.
USDA investigators reported that while tracking 200 former herd mates and having to trace 213 calves while searching for the animal's two offspring, they found most of the animals had already been slaughtered and their meat fed to humans and pets.
Thirty-four former herd mates were presumed dead and 20 more proved untraceable, they said, making them unavailable for testing.
That spurred some critics to question the safety of the nation's meat supply and renew calls for testing all cows at the slaughterhouse.
"This really begs the question: How much mad cow is there? And the only way to know for sure is to test millions of cattle a year," said John Stauber, author of the 2004 book, "Mad Cow U.S.A."
But in a nation where beef consumption has been increasing, USDA and FDA officials again downplayed risks of eating red meat.
"We feel the risk is extremely small. We do not feel the public or the pet food industry should have any concern as a result of this issue," said Dr. John Clifford, chief USDA veterinarian, during a telephone conference call.
A human form of mad cow disease, variant Creutzfeldt-Jakob disease, which scientists believe is contracted by eating parts of infected cows, has killed more than 160 people globally, but none has contracted the disease in North America.
The USDA announced June 24 that the Brahma cross cow had been diagnosed with bovine spongiform encephalopathy, making it the nation's first domestic case of the always fatal disease in cattle.
A December 2003 diagnosis of the disease in Mabton, Wash., involved a cow that came from Canada. That case triggered a 53-nation ban on U.S. beef exports, which has since declined to 26 nations.
The sickly Texas cow died last November while being transported to a slaughterhouse and was then shipped to a pet food plant near Waco. It never entered the food chain at either location.
Sixty-eight of its Texas herd mates, later killed and tested for mad cow disease, were found to be free of infection. The government killed 704 herd mates of the infected Washington cow after it was found to have entered the food supply. They also tested negative.
Federal agricultural officials said their two-month investigation pointed toward contaminated feed likely eaten by the cow before a 1997 government ban on feed containing ground-up cattle remains.
"All the establishments in that area (Texas) that would sell feed to that facility have been in compliance with the feed rule since 1997," said Sundlof.
He said government investigators identified 21 kinds of feed used on the farm since 1991 and investigated three retailers and nine manufacturers before concluding that the cattle operation complied with the feed ban.
But the FDA's report also acknowledged that the ranch kept "minimal feed records" and called it "difficult to determine what feeds were in use at specific times and what the formulation of those feeds were at the time they were fed."
Scientists believe animal feed containing infected cattle or sheep parts is the prime conduit for a disease that has infected more than 183,000 cows in Great Britain and thousands more in 20 other countries.
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