Experts discover first signs of BSE in sheep

Scientists have detected the first signs that bovine spongiform encephalopathy (BSE) may have crossed into sheep in a study that is likely to rekindle anxieties over the safety of lamb and mutton.

One of three tests used to determine whether sheep that had seemingly died from scrapie were in fact infected with BSE (also known as "mad cow" disease) has produced positive results. The four-year-old animal was thought to have developed scrapie, a brain disorder that affects sheep and is believed to be harmless to humans.

It died in January and its brain was tested by the Veterinary Laboratories Agency (VLA) in Weybridge, Surrey, as part of a national programme to determine whether BSE has become endemic in sheep. Two of the three tests were negative for BSE but the third gave "some characteristics" that were similar to experimental BSE in sheep, the Department for the Environment, Food and Rural Affairs (Defra) said.

Scientific experts who carried out the tests have nevertheless concluded that the case "could not be considered to be" BSE, although they have not ruled out the possibility that this is the first case of BSE disease in sheep. "Some characteristics [of the test] were similar to experimental BSE in sheep and also to [an] experimental strain of sheep scrapie," Defra said.

It was always theoretically possible for sheep to be infected with BSE because they were once fed the same infected material that had spread the brain disease in cattle during the 1980s and early 1990s. But there is no simple test to distinguish BSE from sheep scrapie, and sheep experimentally infected with BSE show the same symptoms as scrapie, which means that scrapie in the field could be masking a hidden BSE epidemic in sheep.

Scientists at the VLA are now testing the sheep's brain with more a sophisticated method based on long-term mouse experiments, which should within two years determine whether the animal really did have BSE or a simply a new strain of scrapie. Defra said that in the meantime the lack of further evidence to suggest that the animal had BSE as opposed to scrapie meant that the existing rules governing the sale of lamb and sheep offal remained unchanged.

Professor Howard Dalton, chief scientific adviser to Defra, said: "As we continue to assess more samples with these improved methods it is likely that we will continue to find samples, such as this, which fall outside our current knowledge of the disease."

The Food Standards Agency said that until there is firm evidence that BSE is present in the national sheep flock there is no need to change existing rules governing the sale of lamb products, such as intestines, which could carry a higher risk of being infected with BSE.

Experiments suggest that BSE could be easily transmitted to sheep, and it could, like scrapie, be passed down the generations. This would mean that BSE could still be infecting the national flock today, more than a decade after meat and bonemeal was banned.