HANFORD (AP) — Quarantines were lifted on two Central California dairies associated with a case of mad cow disease after investigators found no link between the illness and food the diseased bovine might have consumed, federal officials said Friday.

Tests performed by the World Organization for Animal Health also confirmed what U.S. labs had found: The cow had a random mutation of the illness that was unlikely to affect other cows in the herd.

The tests were part of an investigation begun in April when an examination of a carcass of a nearly 11-year-old cow taken to a Hanford rendering plant tested positive for mad cow disease, the nation's fourth case and the third "atypical" strain to be discovered.

Mad cow disease is a deadly affliction of the central nervous system that can be transmitted to humans who eat meat from infected cows.

The rash of cases that occurred in Great Britain in the 1990s were caused by cattle being fed protein supplements made from the spinal columns and brains of diseased cows, a practice that has since been banned.

The California cow had what is known as atypical L-type bovine spongiform that scientists know happens occasionally.

In the disorder, a protein the body normally harbors folds into an abnormal shape called a prion, setting off a chain reaction that eventually kills brain cells.

Scientists say they do not yet know what causes this strain of the disease. The incubation period is two to eight years.

The USDA tests 40,000 of the 35 million cattle slaughtered annually for BSE, but some public health experts have called for more aggressive testing, especially in light of Friday's announcement.

"If that's true, then it's even more important to increase surveillance since the feed ban could not be expected to prevent future cases," said Dr. Michael Greger, director of public health and animal agriculture with the Humane Society of the United States.

He said adopting the European model of testing all older cattle, or the Japanese model of testing every cow slaughtered for human consumption would add mere pennies per pound of beef sold and lower the risk of human cases of the fatal disease.

As part of its investigation, the FDA and the California Department of Food and Agriculture examined feed records at the affected dairy and identified at least 10 suppliers. They said Friday that all were in compliance with regulations.

The California cow, which came from a still-unnamed Tulare County dairy, had been unable to stand when she was euthanized and hauled away to a plant that renders carcasses into animal food protein and other products.

Dairy operators are not required to report if their cattle contract neurological diseases.

Investigators with the California Department of Food and Agriculture and the USDA's Animal Plant Health Inspection Service were still working to track down at least a dozen other living cows that were raised on a calf ranch with the sick cow. Calves taken from their mothers after birth are fed a protein supplement made from slaughtered cattle blood, and some question whether that blood might carry BSE.

Already investigators have tracked down two offspring of the diseased cow. One that was euthanized for testing turned out to be healthy. Another calf was stillborn within the last two years, but officials have not yet said what happened to the carcass.

Baker Commodities, the rendering plant where the diseased cow was discovered, is a voluntary participant in the USDA testing program.