patients could contract the human form of Creutzfeldt-Jakob Disease (CJD) from blood transfusions because current tests cannot detect a dormant strain of the virus, a leading scientist has warned.

Professor Marc Turner, medical director for the Scottish National Blood Transfusion Service, said shortfalls in technology mean blood donors are not screened for the passive form of variant (vCJD), otherwise known as mad cow disease.

Some 2500 Scots are estimated to be "silent" carriers of defective proteins that have caused people to develop the deadly brain-wasting illness, which can kill sufferers in 12 to 18 months.

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The lack of understanding about the variant form means it is impossible to know which carriers of the proteins – known as prions – will go on to develop the disease, or if new cases will emerge.

Humans cannot now contract vCJD from eating British beef, following the culling of millions of cows in the late 1980s.

However, UK Government experts fear one in 20,000 Britons carry a dormant form that could be passed on in blood donations.

Mr Turner said: "We know vCJD can pass through blood transfusion... what's unknown is whether what we've done collectively so far in terms of precautionary measures has been enough to mitigate the risk of transmission. The key issue is whether [donors] have any evidence of infection in their blood.

"Unfortunately it has proved very technically demanding to develop a vCJD blood test due to the very low levels of abnormal prions you might find in the blood of such individuals."

Mr Turner said most people over the age of 16 or 17 would have been exposed to BSE in the food chain, especially during the 1980s, and warned that in principle those exposed could have been infected with a form of vCJD.

He said these kind of diseases could have an incubation period of up to 50 years but there was no certainty the dormant form would ever become active.

The comments follow reports that up to 1000 people could die from the disease through infected blood given to them in hospitals, according to a risk assessment by the UK Government's Health Protection Analytical team. The total death toll from vCJD currently stands at 176.

Last month Nick Baxter, 65, the founder and former chief executive of leading Scottish social care charity Cornerstone, died after contracting sporadic CJD, one of four forms of the disease. Mr Baxter did not have the human form of mad cow disease.

Politicians and experts said the findings in the Government's report were worrying and called for nationwide screening of blood donors to be established.

Asked if he wanted to see such a system, Mr Turner said: "If we can get a test which we know is sensitive enough to pick up people who are incubating the illness and specific enough not to falsely identify positive people then yes, that would be clearly a good thing."

A Department of Health spokesman said it continues to encourage everyone to give blood, adding the UK has one of the safest blood supplies in the world.

He added: "There is no evidence of any UK clinical cases of vCJD being linked to a blood transfusion given after 1999. There have been no new cases in the UK for more than two years."