Scientists have raised the alarm over the recent diagnosis of a cow with BSE, as experts admit they are at a loss to explain how the animal contracted the disease. It was born in 1997 after measures to combat mad cow were fully enforced, and its mother has shown no signs of the disease, ruling out the possibility of maternal transmission.

Since the fatal disease was first detected in a UK herd in 1986, more than 179,682 cases of BSE have been reported in the country. Only two have been reported in Britain (and one in Northern Ireland) in animals born after August 1996 however, at which time the practice of using rendered animals in animal feeds became outlawed.

Scientists believe that the transmission of BSE is largely down to the consumption of contaminated feeds, but during the late 1990s they revealed that in rare cases animals might also contract the disease via maternal transmission, although the mechanics of this were unknown.

Dr Stephen Dealler, a microbiologist who has studied BSE since 1988, explained to Reuters that there has been an over-emphasis on these two types of transmission, a fact highlighted by the latest case. "There are going to be lots of arguments about this one," he said, pointing out, "the thing about this case is that the mother is 11 years old and hasn't got BSE." The government should have tested more widely and encouraged research into other possible means of BSE transmission, he added.