The mad cow that was recently discovered through routine testing in California had been euthanized after it became lame and started lying down at a dairy, federal officials revealed Thursday.

How worried should we be about mad cow in US?

The U.S. Department of Agriculture also said the cow was 10 years and seven months old in its update on the fourth case of mad cow disease ever discovered in the U.S.

California mad cow 'just a random mutation'

A spokesman for U.S. Rep. Devin Nunes of California had said Wednesday that the sick cow was 5 years old. It came from a dairy farm in Tulare County, the nation's No. 1 dairy-producing county.

The USDA didn't elaborate on the cow's symptoms other than to say it was "humanely euthanized after it developed lameness and became recumbent."

Routine testing at a transfer facility showed the dead Holstein, which was destined for a rendering plant, had mad cow disease, or bovine spongiform encephalopathy. The results were announced Tuesday.

Animals at high risk for the disease include those with symptoms of neurological disease, "downer" animals at slaughterhouses, animals that die at dairies or cattle ranches for unknown reasons, and cows more than 2 1/2 years old, because BSE occurs in older cows.

U.S. health officials say there is no risk to the food supply. The California cow was never destined for the meat market, and it developed "atypical" BSE from a random mutation, something that scientists know happens occasionally. Somehow, a protein the body normally harbors folds into an abnormal shape called a prion, setting off a chain reaction of misfolds that eventually kills brain cells.

In other countries, BSE's spread through herds was blamed on making cattle feed using recycled meat and bone meal from infected cows, so the U.S. has long banned feed containing such material.

The last two cases found in the U.S. were atypical as well.