Washington (CNN) -- A quarantine placed on two California farms under investigation for mad cow disease has been lifted, the U.S. Department of Agriculture announced Friday.
The farm where the cow was discovered had been under quarantine since the dairy cow tested positive, agriculture officials said. Officials later placed a second farm under quarantine because that farm was linked to the dairy cow, according to the USDA.
Officials had focused attention on the feed suppliers and animal feed at both farms but now say the feed didn't cause the cow to develop the brain-wasting disease.
After it completed its own testing of the cow, the USDA sent samples to The World Organization for Animal Health in Canada and England where both laboratories confirmed the cow tested positive for atypical Bovine Spongiform Encephalopathy, known commonly as mad cow disease, officials said.
Increasing evidence shows there are different strains of BSE: the typical strain responsible for an outbreak in the United Kingdom in the 1980s and 1990s, and two atypical strains (H and L strains), according to the Centers for Disease Control and Prevention. Officials said the infected cow found in California on April 24 had the atypical "L" strain of the disease. USDA officials said the cow was never presented for human consumption and was not a threat.
The cow gave birth twice. One calf was stillborn and the other was located and tested negative for the disease.
In Friday's announcement, the agency said investigators were focusing on a small number (10-12) of cattle born at the same time and in the same area as the diseased cow which may still be alive and have records that might allow them to be located.
Eating contaminated meat or other animal products from cattle that have BSE is thought to be the cause of Creutzfeldt-Jakob disease. The fatal brain disease was blamed for the deaths of 150 people in Britain, where there was an outbreak in the 1980s and 1990s.
In people, symptoms of the disease include psychiatric and behavioral changes, movement deficits, memory disturbances and cognitive impairments. BSE can cause infected animals to display nervousness or aggression, difficulty in coordination and standing up, decreased milk production or weight loss. It is usually transmitted between cows through the practice of recycling bovine carcasses for meat and bone meal protein, which is fed to other cattle.
Since 2004, the USDA has removed the brain and the spinal column -- the parts suspected of causing the disease in humans -- from the food system. Unlike most other meat-borne illnesses, such as those caused by E.-coli bacteria, cooking does not kill the infectious agent.
Consumers who wish to exercise extra caution can follow the advice presented by the Web-based consumer advocacy group www.consumeraffairs.com, which advises the avoidance of brains, neck bones and beef cheeks, bone marrow, and cuts of beef that are sold on the bone. The group says to choose boneless cuts of meat and ground beef only if it has been ground in the store.
"Evidence shows that our systems and safeguards to prevent BSE are working, as are similar actions taken by countries around the world," said John Clifford, the Agriculture Department's chief veterinarian.
Last year, 29 cases of BSE were reported worldwide, down 99% from the peak of 37,311 cases in 1992.
"This is directly attributable to the impact and effectiveness of feed bans as a primary control measure for the disease," he said last month.