Mad cow disease is linked to about 225 cases worldwide of a fatal human brain aliment known as variant Creutzfeldt-Jakob disease.  The first American case of mad cow disease since 2006 was found this week in a dairy cow in California.

“The beef and dairy in the American food supply is safe and USDA remains confident in the health of U.S. cattle,” Agriculture Secretary Tom Vilsack said. “USDA has no reason to believe any other U.S. animals are currently affected, but we will remain vigilant and committed to the safeguards in place.”

Of the millions of cows slaughtered each year, the government tests only 40,000 for the disease, said Michael Hansen, a scientist at Consumers Union. “So we really don’t know if this is an isolated unusual event, or whether there are more cases in U.S. beef,” he said. “Our monitoring program is just too small.”

The symptoms of Mad Cow disease do not appear for years. The animal’s age is being investigated. “There is a lot of information we need to get,” said John Clifford, the USDA’s chief veterinarian.

Old dairy cows are frequently used to make hamburger, but this cow had been collected by a renderer, and the disease was discovered in random testing. Baker Commodities picked up the cow from the dairy after it died.

Agriculture Department officials would not provide the location of the farm or the size of its herd.