As officials confirm that a cow in Alabama had the disease, public-interest groups urge more systematic monitoring.

This week a cow in Alabama became the third confirmed case of mad cow disease in the US since December 2003. But what appears to be a relatively isolated incident points out the difficulty of preventing - or even detecting - such cases under the current voluntary testing regimen. The news also makes it more difficult for US officials to convince overseas consumers that American beef is safe.

The cow in question, discovered on an unidentified farm, initially tested "inconclusive" last week.

On Monday, after a more thorough test at US Department of Agriculture (USDA) labs in Ames, Iowa, officials confirmed that the animal (which had been euthanized by a veterinarian and buried on the farm) had mad cow disease. USDA officials now are trying to trace the cow's herd of origin and offspring.

Mad cow, known scientifically as bovine spongiform encephalopathy (BSE), is believed to be carried by animal feed made from cattle brains or spinal cord. Such feed is now banned in the US and other countries, but cases of BSE have continued to appear around the world.

Although medical authorities attribute 150 deaths in Europe in the 1980s and 1990s to the human form of the disease, no such human cases have been reported in the United States.

About 650,000 cattle are tested

Since the first case of BSE was reported here in 2003, the number of cattle tested for the disease has increased substantially. Still, only about 650,000 of the total US herd (some 35 million slaughtered annually) have been tested - a rate far lower than the percentage tested in Europe or Japan.

Of those tested, two have turned up positive for BSE. That is "evidence that the prevalence of this disease in the United States is extremely low," says Terry Stokes, chief executive officer of the National Cattlemen's Beef Association.

"The bottom line for consumers remains the same," Mr. Stokes says. "Your beef is safe."

That's the same position taken by USDA officials, who are responsible for ensuring the safety of agricultural products as well as promoting the $48 billion beef industry here and abroad.

Cows origin and age difficult to trace

The USDA's inspector general last month found problems in sampling for and tracking of mad cow disease. For example, there was difficulty tracing the origin and age of cattle, which are critical factors for determining patterns of risk. Sen. Tom Harkin (D) of Iowa and others have called for a national animal identification system.

Public-interest groups say this week's news indicates the weakness of the current system of testing and detection.

"We applaud the farmer who did the right thing by turning over the sick cow in question to a veterinarian for testing," says Wenonah Hauter, executive director of Food & Water Watch, a nonprofit consumer group in Washington.

Groups want mandatory system

"But this is still a voluntary system that must be made mandatory for the sake of public health," Mr. Hauter says. Without a mandatory reporting system, who knows what else is out there?"

In 2003, the USDA issued an emergency rule banning so-called "downer" cattle (those too sick or injured to walk on their own) from the human food supply. Public- interest groups are pushing proposals in Congress that would make that ban permanent and extend it to pigs and other livestock.

"There is no reason to play Russian roulette with the food supply, nor is there any reason to torment nonambulatory livestock by dragging or pushing them into slaughterhouses with chains, bulldozers, or forklifts," says Wayne Pacelle, president of the Humane Society of the United States.

Delicate time for mad cow discovery

Cases of mad cow are never good news for the beef industry, but this one came at a particularly delicate time. After the first US case of mad cow disease in 2003, Japan banned US beef.

The ban had been lifted, but in January, Japan (once the largest importer of US beef) again halted imports after veal cuts from the US were found to contain backbone material. With this week's news, South Korea also may delay re-opening its market to US beef imports.

This week's discovery of a cow in Alabama with BSE came just as Agriculture Secretary Mike Johanns was meeting with Japanese officials at a trade conference in London.