Doctors have raised their estimate of the number of Britons infected with the abnormal protein that causes variant CJD, the human form of "mad cow disease", after a survey of medical samplesaccording to a report in the Guardian.

According to a report in the Guardian, some 24,000 adult Britons are now thought to carry the rogue protein after eating contaminated meat products in the late 1980s and early 1990s, but it is unclear how many of these, if any, will develop the disease. People who carry the BSE (bovine spongiform encephalopathy) protein may also pose a risk to others if they donate infected blood that is used for transfusions or have operations that contaminate surgical instruments.

While most of the British public was exposed to meat products infected with BSE, the UK has so far seen only 177 cases of variant Creutzfeldt-Jakob, a devastating brain disease that is incurable and ultimately fatal. The relatively small number of cases suggests that infection with the BSE prion protein only rarely causes disease in humans, though the condition may develop so slowly in some people that they die for other reasons before symptoms arise. There are no suspected cases of vCJD in Britain today.

Scientists arrived at the latest figure after testing 32,000 anonymous appendix samples from operations that took place from 2000 to 2012. The patients were born between 1941 and 1985, according to a report in the British Medical Journal. The tests found that 16 samples contained the abnormal BSE protein, implying that one in 2,000 people in the age group tested is likely to be a carrier. The figure means around 24,000 over-18s in Britain harbour the rogue protein.

"It is a concern that a proportion of those who are carriers might ultimately develop the disease," said Sebastian Brandner, professor of neuropathology at University College London. "But the other real public health issue is that these people might transmit it through blood transfusions."

The UK has taken steps to reduce the risk of vCJD being spread through blood products, including banning at-risk donors and importing more blood. But there is no test that can be used widely to distinguish between healthy and contaminated blood.

"We are definitely concerned, but we are also reassured by the absence of a large number of blood transmission cases to date," said Noel Gill, lead investigator on the report at Public Health England. If the disease spread easily through donated blood products, more cases should have been seen by now, he added.

Healthy humans make prion proteins that are used throughout the body. But infection with BSE prions can cause these human prions to change shape and take on the abnormal BSE form.
One issue that intrigues scientists is that all 177 vCJD patients in Britain so far have belonged to the 40% of the population that make the so-called MM form of the human prion protein. The remaining 60% make other variants of the prion, called MV and VV. It is unclear whether these people are immune to vCJD or simply develop the disease much more slowly.

According to the BMJ report, people were no less likely to be infected with the abnormal prion if they belonged to the MV or VV groups. "They are not protected from acquiring it, but they may have been more protected from developing the disease," said Brandner.

An earlier survey of appendix tissues reported in 2004 found that only one in 4,000 in the age range tested were likely to be carriers of the BSE protein. That figure suggested that 15,000 to 20,000 people in the UK carried the prion.

"Although the results of this latest study indicate a higher prevalence than the first survey, they are consistent with the previous findings and the range of the prevalence estimates largely overlap," said Gill. "We have looked at a larger number and a wider age group of patients and this new estimate is more precise."

There is a small chance that the prions discovered in the appendix tissues are unrelated to the flood of BSE-infected meat that entered the food chain in the 1980s and 1990s. To address the possibility, another survey is under way to test appendix material collected in the 1970s and earlier, before the BSE crisis struck. The results are due to be published before the end of 2015.