BEEF CRISIS: Experts fear spread of infection to sheep

Experts are considering the possibility that some of Britain's sheep may have become infected with the "mad cow disease" which has been linked to 10 human cases of the degenerative brain condition Creutzfeldt-Jakob disease.

The Government's scientific advisers are considering whether it is necessary to ban sheep offal as well as cattle offal from entering the human food chain, following experiments in which sheep fed with BSE-contaminated material went on to develop the disease.

Scientists are concerned that sheep infected with this potentially more dangerous strain may be going undetected because the outward symptoms are the same as the naturally occurring sheep disease, scrapie, which is believed to be harmless to humans.

Experts now believe that cows developed BSE after being fed protein concentrate containing the ground-up remains of scrapie-diseased sheep and that after sheep-scrapie "passaged" through cattle it altered its characteristics and virulence so as to become more hazardous to humans. If it has "passaged" back into sheep, then these could be just as hazardous to human health as contaminated cattle, but whereas BSE is easily recognisable in cattle, in sheep it would be confused with ordinary scrapie.

The "fingerprint" of BSE shows up only when the infectious agent is injected into laboratory mice - an expensive business. Fewer than 10 samples taken from sheep with naturally occurring scrapie have been tested in this way to see if they had BSE. None of them did.

Scrapie is endemic in British sheep flocks and the Ministry of Agriculture, Fisheries and Food was unable last night to put a figure on the number of scrapie cases or even to estimate the percentage of sheep affected. An offal ban would include sheep intestines - used to make haggis and some sausages - thymus, tonsils, spleen, brain and eyes.

Kevin Taylor, the ministry's deputy chief veterinary officer said "no evidence exists that BSE has entered the national sheep flock but the possibility cannot be ruled out".

However, Mr Taylor pointed out that sheep-farming practices are very different from cattle farming. "Concentrate feeding of sheep is uncommon because lambs suckle from their mothers rather than being artificially fed." In addition, he said, "lambs are killed young, limiting any opportunity for agent multiplication in the tissues".

Britain's flocks vary seasonally, from a maximum of 43 million sheep last June to 29 million in December. Many lambs are taken direct to the slaughter from their mothers and those kept longer are fed grass, turnips and grain rather than protein concentrates, according to Colin Smith of the Meat and Livestock Commission. Most are sold by the time they are six months of age and almost all by the age of ten months, he said.

In addition, there is relatively little mechanically recovered meat from sheep - one of the possible sources of BSE in cattle - because most sheep meat is sold on the bone and so there is little of the carcass left.