Two farms were under quarantine Thursday as officials continued to investigate a case of mad cow disease discovered in California last week, the US Department of Agriculture (USDA) said.

The first US case of mad cow disease -- also known as bovine spongiform encephalopathy, or BSE -- since 2006, was found in an animal at a central California rendering facility, the USDA announced last Tuesday.

The farm the infected cow originated from was quarantined immediately.

Now a second dairy, closely linked to the original farm, also is under quarantine.

"A hold order has been placed on all cattle at a second dairy (dairy 2) that is associated with the dairy of the initial positive cow (also called the index dairy)," the USDA said in a statement late Wednesday.

It added, "Both dairies remain under quarantine. Inventories of both the index dairy and dairy 2 have been completed by CDFA [California Department of Food and Agriculture]. Records are still being matched and validated to determine if any at-risk cattle may be present."

The calf ranch where the diseased cow was raised 10 years ago also is being investigated, the statement said.

The sick cow was euthanized "humanely" after becoming lame. Officials said the animal never presented any risk to the food supply or human health since it was never presented for slaughter.

Investigators have discovered the animal had two offspring over the past two years. One was stillborn, and one was found in another state, where it was euthanized. It tested negative for BSE, the USDA said.

Indonesia has temporarily banned beef imports from the US in response to the incident, while two major South Korean retailers initially pulled US beef from their shelves, though one has since resumed sales.