Two farms have been quarantined by the U.S. Department of Agriculture as the agency continues to investigate last month's discovery of mad cow disease at a California dairy farm.

Authorities also have launched an investigation at a calf ranch where the initial infected cow was raised 10 years ago, according to a statement released late Wednesday by the USDA.

Last week, the USDA documented the fourth confirmed U.S. case of Bovine Spongiform Encephalopathy (BSE) - a brain wasting disease affecting cattle - known commonly as mad cow disease, at a rendering facility in central California. USDA officials said the cow was never presented for human consumption and was not a threat.

The farm where the cow was initially discovered has been under quarantine since the discovery, agriculture officials said. Wednesday's announcement of a second quarantine involves a farm closely associated with the dairy where the sick cow was discovered last month, the USDA said. The agency is still trying to determine if any at-risk cattle are present at either of the two farms.

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