USDA chief veterinary officer John Clifford said the case was atypical and that there was no cause for alarm from the animal.
The USDA has begun informing authorities at the World Organization for Animal Health (OIE) as well as US trading partners, but the incident should not affect the US beef exports, Clifford added.
USDA noted that the carcass of the infected cow is under quarantine and would be destroyed.
Cows can contract the mad cow disease spontaneously in rare cases; however, the disease cannot be transmitted unless the brain or spinal tissue is consumed by humans, according to scientists.
Mad cow, or Bovine Spongiform Encephalopathy, is considered to cause Creutzfeldt-Jakob - a deadly brain disease variant, in humans who consume infected parts from animals with the disease.
The first case of mad cow disease in the US was in late 2003, which caused the nation's beef exports to fall by nearly $3bn in 2004.
