WASHINGTON — Initial tests last week indicating a possible new case of mad cow disease in the United States have proved wrong after subsequent testing, the U.S. Department of Agriculture announced Tuesday.
The potential case of bovine spongiform encephalopathy, or BSE, was found through a rapid screening program initiated by the USDA after the first -- and only -- U.S. case was discovered in Washington state last December.
More than 121,000 cattle have been tested since the program started June 1. Tests on three, including the current case, came back positive in preliminary screening but the cattle were cleared of infection after more definitive tests.
"The main thing is we've taken action aggressively," said Ed Loyd, a spokesman for the Agriculture Department. "We've taken this very seriously and implemented many of the safeguards last year. This is not unexpected."
The screening program tests cows considered at risk for the disease -- older cattle, "downers" too ill to walk, cattle displaying symptoms of neurological ailments -- as well as healthy cows. The initial test is designed to be extremely sensitive and false-positive findings are expected, department officials said.
If the screening test is positive, the USDA conducts a more definitive test that has been called "the gold standard" for BSE identification. In the current case, the USDA said, the samples were tested twice and came back negative both times. The tests were conducted at the department's National Veterinary Services Laboratory in Ames, Iowa.
When the screening test results were announced last week, the USDA suggested that the cow was in one of the groups at higher risk for the disease. No information has been released about the cow's origins. The carcass never reached processing for human food or animal feed, the USDA said.
Tuesday's news relieved meat industry officials, who had been concerned that a new case would hurt beef exports.
The announcement of the case last December caused more than 40 countries -- including Japan, the biggest U.S. beef export market -- to cut off all imports of American beef. The markets have slowly reopened; Japan had agreed only last month to resume imports.
BSE was first identified in 1986 in Britain, infecting about 178,000 cattle and leading to the destruction of 3.7 million animals.
The outbreak caused a nearly worldwide ban on British beef, costing farmers there billions of dollars in lost sales.
Scientists believe there is strong evidence that humans who have eaten meat from animals with BSE can develop variant Creutzfeldt-Jakob disease, a fatal brain-wasting illness.