WebMD Newsroom

by Daniel J. DeNoon

Mad cow disease has been found in a California dairy cow, the U.S. Department of Agriculture announced today.

It’s the fourth time the deadly disease has been detected in U.S. cattle. Mad cow disease — bovine spongiform encephalopathy or BSE — can cause a fatal, incurable brain disease in humans who eat infected animals.

But the California cow never posed a threat to humans, according to USDA Chief Veterinary Officer John Clifford.

“It was never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health,” Clifford says in a news release. “Additionally, milk does not transmit BSE.”

The animal is being held by California authorities and will be destroyed.

In 2011, there were only 29 cases of BSE worldwide. Efforts to control the disease in the U.S. and abroad have been successful. The disease peaked in 1992, when there were over 37,000 infected animals.

Cows and other grazing animals can get BSE and similar diseases from eating feed made with meat from other grazing animals. The diseases are caused by proteins called prions. These prions trigger a chain reaction in the nervous system, resulting in brain damage and death.

It does not appear that the new case of BSE in the California dairy cow was caused by meat-contaminated feed. Such feed is banned by the FDA. And tests of the cow’s BSE show that it was a rare, atypical form of BSE that arises spontaneously from time to time. Atypical BSE is not linked to contaminated feed.

The human version of BSE is called Creutzfeldt-Jakob disease or CJD. Humans can get a variant form of CJD by eating meat contaminated with brain or spinal-cord tissues from BSE-infected cows. The disease, vCJD, is always fatal.

Like atypical BSE, there have been rare, sporadic cases of atypical CJD in humans.