Around one in every 2,000 people in the UK carry proteins linked to the human form of mad cow disease, new research shows.
The large study set out to estimates the number of people carrying the variant Creutzfeldt-Jakob disease (vCJD) prion proteins based on tests on tissue samples.
The findings, published in the British Medical Journal, raise the prospect that people with the proteins may develop the condition in the longer term, or possibly be able to spread it to others through blood or organ donation.
VCJD is a degenerative brain disease which emerged after exposure to contaminated meat products in the late 1980s and early 1990s due to bovine spongiform encephalopathy (BSE) - mad cow disease - in cattle.
While there have only been 177 clinical cases of vCJD to date in the UK, previous research has suggested one in 4,000 people may carry vCJD prions. However, it remains unclear how many will eventually develop the disease.
For the latest study experts, including Professor James Ironside at the National Creutzfeldt-Jakob Disease Research and Surveillance Unit in Edinburgh, examined over 32,000 anonymous appendix samples removed between 2000 and 2012 at 41 hospitals across England.
They found 16 samples tested positive for abnormal prion protein, leading to estimates that one in 2,000 people in the UK are likely to be carriers.
The researchers also found a higher proportion of the positive cases were from patients of a specific genetic make-up - or genotype - than previously suspected.
A quarter had the VV genotype, while all 177 patients with clinical vCJD were of the MM type.
The researchers said the concern was that people with the VV genotype may be susceptible to developing the condition over a longer incubation period, or may not show clinical signs of the disease.
They stressed that the number of patients with clinical vCJD was still well below the number suggested by the prevalence of the abnormal proteins.
But the experts said it was essential to continue research into tests to detect prion proteins in the blood, and to examine tissue from the 1970s and earlier before BSE appeared.
Lead researcher Professor Noel Gill said the findings did not indicate that twice as many people are carrying vCJD as previously thought, despite the earlier studies.
“The range of uncertainty overlaps with previous studies,” he said. “Ours is a more robust figure.
“This new survey is a larger survey so we have more confidence in the result.
“In the earlier survey, it was smaller and they only had three positive samples, we have 16.
“The phenomenon is more real in our study. It is a more robust estimate.
“Technically, the estimate is the same as the previous study.”
Dr Graham Jackson, MRC Prion Unit at University College London, said: “Given the high levels of infection indicated by this research it is now crucial we establish how many people in the UK harbour that infection in their bloodstream in order to adequately assess the risks of transmission through contaminated blood donations.
“Studies to develop new blood tests for CJD must remain a priority to assist with screening and protecting the UK blood supply.”