FRESNO, Calif. (AP) — The U.S. Department of Agriculture says a California Holstein discovered in April with mad cow disease was an isolated case and posed no threat to the food supply.

A report issued Friday looked at the movements of the infected dairy cow, her offspring and the food eaten by the herd.

The three-month investigation turned up no other cases of the disease known as bovine spongiform encephalopathy.

USDA veterinarian John Clifford says the investigation confirms the U.S. food supply was not at risk.

Some scientists believe the cow developed atypical BSE from a random mutation, something that happens occasionally.

The 10-year-old dairy cow, only the fourth with the sickness ever discovered in the United States, was found as part of a USDA program that tests for the fatal brain disease in about 40,000 cows of the 35 million slaughtered each year.