Also, a calf born to the infected cow was found and tested negative for the disease.

Cattle records at the two dairies are being matched to determine if any at-risk cattle are on the farms, said the Agriculture Department.

USDA said the infected cow was a rare "atypical" case of the disease, meaning it arose spontaneously rather than through the feed supply. However, it is USDA's standard procedure to search for other cattle, offspring or herd mates, that might be exposed to the fatal disease, even though mad cow disease is not contagious.

The brain-wasting mad cow disease was confirmed on April 24 in an elderly cow that died at a dairy farm in Tulare County, about 175 miles north of Los Angeles. The farm and an associated dairy are under quarantine and a hold order has been placed on cattle at the second farm, said USDA.

"In addition, a calf ranch where the initial positive cow was raised 10 years ago is being investigated," said USDA.

Two calves were born to the infected cow in the past two years. One was stillborn. The other was found in another state, was killed and tested negative for mad cow disease, said USDA.

Scientists say mad cow disease, formally named bovine spongiform encephalopathy, is spread when cattle eat feed that contains parts of infected cattle. As a safeguard, feed mills are barred from using cattle parts in cattle feed.

The Food and Drug Administration and California agriculture officials are reviewing feed records at the dairy that was home to the infected cow, a rendering plant and the calf ranch. Ten feed companies have been identified as suppliers to the dairy.

Investigators confirmed the rendering plant followed federal rules on labeling of meat and bone meal.

Officials did not identify the dairy farm, the rendering plant or the calf ranch. Tulare County is the largest milk-producing county in California, the No 1 dairy state.

The latest case was the fourth U.S. case since 2003 and the first in six years. Mad cow disease has a long incubation period and generally is a disease of older cattle.

It is believed people can contract a similar fatal brain disease by eating infected parts of affected cattle.

A delegation of South Korean government officials, academics and consumer group representatives was in the United States this week to review U.S. practices to prevent mad cow disease.

(Reporting By Charles Abbott; Editing by Bob Burgdorfer)

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