The number of cattle tested for mad cow disease has fallen almost 90 percent since 2005, according to Agriculture Department statistics, a drop that consumer groups say endangers America's food supply.

Agriculture Secretary Tom Vilsack on Wednesday said that animal testing was adequate, a day after his department confirmed the nation's first known case of mad cow disease in six years, a dead dairy animal on its way to a rendering plant in central California.

About 40,000 cattle were tested in the year ended Sept. 30, down from 399,575 in 2005, according to USDA data. The drop-off followed a surge in testing as the agency was trying to understand the risk and create a long-term monitoring system, John Clifford, the USDA's chief veterinarian, said in an interview.

He said testing was only one component of the agency's strategy to combat mad cow disease, which includes limiting the consumption of certain parts of cattle and restricting the content of their feed.

Chris Waldrop, director of the Food Policy Institute of the Consumer Federation of America, a Washington-based safety advocacy group, said the testing decline meant the United States was relying too much on other safeguards, which are not foolproof.

"If you're not going to test as much, your fire walls better be perfect, and there are loopholes in the fire walls," Waldrop said. "After going so long without having a case in the U.S., and now we have one, it warrants another look at the surveillance program and ramping it up, at least temporarily, to see if there is something new going on."

USDA investigations typically include tests of animals that were in the same herd as the diseased cow, as well as feed the animal may have consumed, Clifford said. Any offspring of the infected animal are also checked.

The USDA has not publicly identified the farm where the animal came from, or whether the cow had any calves. Clifford would not comment on the current investigation.

The new case won't spur more tests, Clifford said, because one case of the disease, known as bovine spongiform encephalopathy, in six years doesn't necessarily indicate BSE is becoming more prevalent.