The government is checking a possible case of mad cow disease in the United States in an animal previously cleared of being infected, the Agriculture Department (search) said Friday.

Agriculture Secretary Mike Johanns (search) said more tests were needed to confirm the disease, but he emphasized that the animal did not enter the food supply.

Only one case of mad cow disease has been confirmed in the United States, in a dairy cow in Washington state in December 2003. Since then, preliminary tests have indicated the existence of the disease in three cows, but further testing had ruled out any infection.

The department decided this week to perform additional tests, and one of those three — a beef cow — turned up positive. Johanns said the department's inspector general had recommended the additional testing, but the secretary did not say why.

"It's going to require additional testing before we can confirm one way or another whether this is BSE (search) (mad cow disease)," said Dr. John Clifford, chief veterinary officer of the department's Animal and Plant Health Inspection Service.

A sample from the animal was being sent to an internationally recognized laboratory in Weybridge, England, which provided independent confirmation of the first U.S. case of mad cow disease. The Agriculture Department will also conduct further tests.

Johanns said he received results of the latest testing 10 minutes before making the announcement Friday night.

Johanns, former governor beef-producing Nebraska, said that there was no health risk and that he intended "to enjoy a good steak."

"Consumer confidence, I am very confident, will remain," he said. "This is a situation where the firewalls worked. We do not have a human health risk. This animal did not enter the food chain. This animal never got near the food or feed chain."

Cattle must be killed to be tested for the mad cow disease. The government has tested more than 375,000 cows for the disease since the Washington state case.

The officials could not say whether the cow was born in the United States. The Washington state animal had been imported from Canada, which has had three other cases of mad cow disease. Investigators never located all 80 of the cattle that crossed the border into the United States with the infected animal.

Commonly called mad cow disease, bovine spongiform encephalopathy, or BSE, is a brain-wasting ailment. In humans, it can cause a variant of the degenerative, fatal brain disorder known as Creutzfeldt-Jakob disease. It has killed more than 150 people, mostly in Britain, where there was an outbreak in the 1990s.

Johanns said the new test result should not interfere with efforts to persuade foreign beef customers to reopen their markets. Dozens of countries banned U.S. beef imports after the Washington state case. Japan, once the biggest customer of U.S. beef, has not yet lifted its ban despite agreeing to do so last fall.

"I don't anticipate problems with our trading partners," Johanns said. "They'll want to know what the issues are and what we have done. We want to assure them, and to assure the public, that what we are doing here is transparent."

The department has been trying to lift a U.S. ban on Canadian cattle that was imposed after Canada's first BSE case in May 2003. The border was scheduled to reopen in March, but a federal judge in Billings, Mont., ordered it be kept closed at the request of ranchers suing to block Canadian cattle imports.