Japanese investigators searching on Sunday for the cause of an outbreak of mad cow disease zeroed in on a milk substitute which may have been fed to all the animals involved.
The cows that have tested positive for the brain-wasting disorder, formally known as bovine spongiform encephalopathy (BSE), were all given a milk substitute produced at a factory in northern Japan, the media said.
A Health Ministry official said that an expert panel of the ministry officially confirmed on Sunday that a Holstein dairy cow raised in Gunma prefecture had tested positive for BSE, taking to three the confirmed cases of the disease.
Scientists believe that eating beef infected with BSE can infect humans with a form of the illness, the fatal brain disease variant Creutzfeldt-Jakob disease (vCJD).
A local government official at Gunma confirmed to Reuters that the three cows were fed a milk substitute. But he said they had yet to confirm if it was produced at the same factory.
"We are currently investigating the case," he said. "The milk substitute is an important issue in uncovering the cause of the disease, but it is still too early to say that it is the cause."
The milk substitute is mainly made of skim milk powder and contains plasma proteins made from pigs and beef tallow, Kyodo news agency quoted the maker as saying.
"We are not sure if there are any substances in the milk substitute that could have been the cause," the Gunma official said.
It is not known how the Japanese cows became infected, but scientists have linked the disease to meat-and-bone meal (MBM), a protein feed made from the crushed internal organs, skin and bones of cows and other animals.
The Agriculture Ministry said last week it would slaughter all of the 5,129 cows that had been fed MBM.
Japan banned all MBM imports and the use of MBM as feed early last month after suspecting infected meal as the source of the disease.
In Europe, vCJD has killed about 100 people. No one has died or fallen sick since Japan's first BSE case--the first outside Europe--was reported on September 22, but beef sales in the country have been halved since then.
The government has been testing all cattle slaughtered in Japan since October 18.
Japan's first mad cow case was discovered on a farm in Chiba Prefecture, east of Tokyo. The second, confirmed on November 21, was discovered on the northern island of Hokkaido.