People infected with the human equivalent of "mad cow" disease who have yet to show symptoms could pass on the infection through blood transfusions, a study published today has concluded.

People infected with the human equivalent of "mad cow" disease who have yet to show symptoms could pass on the infection through blood transfusions, a study published today has concluded.

Scientists have shown it is possible to transmit bovine spongiform encephalopathy (BSE) from sheep incubating the disease to healthy sheep.

In Britain, an unknown number of people who are seemingly healthy are infected with the agent that causes variant Creutzfeldt-Jakob disease (vCJD), which scientists have described as "human BSE".

A study published in The Lancet, led by Christopher Bostock, the head of the Institute for Animal Health, found that one sheep out of 19 that had received blood from sheep deliberately fed BSE material developed the symptoms of mad cow disease.

"This report suggests that blood donated by symptom-free vCJD-infected human beings may represent a risk of spread of vCJD infection among the human population in the UK," the scientists say.

These findings have been rushed into print to allay fears of a cover-up at the end of the experiment in three years.

Dr Bostock said it was not known which part of the blood contained the infectious agent, but all the evidence pointed to the leucocytes, the white blood cells that are now filtered out during human transfusions.

"When vCJD was first identified and people started thinking about the consequences for human health, and in particular blood transfusions, the procedures put in place to reduce the risk assumed a theoretical possibility that blood would be infectious during the incubation period," said Dr Bostock. "This work shows that the measures that were taken were entirely appropriate."

The Department of Health said yesterday that all blood transfusion products were made from plasma imported from countries with no BSE.

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