CNN says the U.S. Department of Agriculture (USDA) has quarantined two dairy farms in California as of Thursday, local time, following an outbreak of mad cow disease late last month.

CNN said the farm where the infected cow was initially discovered has been under quarantine since the disease was confirmed in late April. It said a second farm closely associated with the dairy farm where the sick cow was discovered was quarantined more recently.

The USDA is still trying to determine if any at-risk cattle are still present at the two farms.

Meanwhile, CNN reports that authorities also have launched an investigation at a calf ranch where the initial infected cow was raised 10 years ago.