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WASHINGTON (AP) The federal government must allow meatpackers to test their animals for mad cow disease, a federal judge ruled Thursday.

Creekstone Farms Premium Beef, a meatpacker based in Arkansas City, Kan., wants to test all of its cows for the disease, which can be fatal to humans who eat tainted beef. Larger meat companies feared that move because if Creekstone tested its meat and advertised it as safe, they could be forced to do the expensive test, too.

The Agriculture Department currently regulates the test and administers it to less than 1 percent of slaughtered cows. The department threatened Creekstone with prosecution if it tested all its animals.

U.S. District Judge James Robertson ruled that the government does not have the authority to regulate the test. Robertson put his order on hold until the government can appeal. If the government does not appeal by June 1, he said the ruling would take effect.

Mad cow disease, or bovine spongiform encephalopathy, is linked to more than 150 human deaths worldwide, mostly in Britain.

There have been three cases of mad cow disease in the U.S. The first, in December 2003 in Washington state, was in a cow that had been imported from Canada. The second, in 2005, was in a Texas-born cow. The third was confirmed last year in an Alabama cow.

After the first case of mad cow disease heightened concern about the disease, the department increased its testing for the disease to about 1,000 tests each day.

Last July, the department cut its testing by about 90 percent. Agriculture Secretary Mike Johanns said testing should reflect "a very, very low level" of the disease in the United States. Neither the department nor Creekstone immediately responded to a request for comment Thursday evening.

The Agriculture Department argued that widespread testing could lead to a false positive that would harm the meat industry. Robertson said he was concerned by that possibility but noted that Creekstone sought to use the same test the government relies on.

Tests are done on brain tissue from cows, so animals must be killed before they can be tested. Because of this, Robertson rejected the government's stance that it has the authority to regulate the tests because they are used in the treatment of disease.

He said regulation of the tests might be appropriate through the Federal Trade Commission or the Commerce Department but, as the law is written now, the authority does not exist.

Learn more and take action at OCA's Mad Cow Campaign Headquarters here