A 2004 report found that about one in 4,000 U.K. citizens were carriers of the bovine spongiform encephalopathy (BSE) prion, a misshapen protein that causes the human form of “mad cow disease,” variant Creutzfeldt-Jakob (vCJD). A new study has found, however, that the actual number of people carrying the abnormal prion could be twice as high.

About 24,000 British adults, or one in 2,000, may be carrying the prion in their bodies. It doesn’t always develop into full-blown disease, and can incubate in the body for up to 50 years before symptoms begin appearing. The disease develops as the abnormal prions cause naturally occurring prions in the human body to fold, affecting their ability to function.

The disease spread throughout the population during the 1980s and 1990s when the population consumed meat infected with the prion, causing widespread concern as some people who consumed the meat developed vCJD, a fatal degenerative neurological disorder that causes sponge-like lesions in the brain. The U.K. has only seen 177 cases of vCJD to date.

“We always suspected that there’d be a very large number of individuals who were exposed, have developed the infection, but for various reasons have never developed the full-blown disease,” Colin Masters, a professor of neurology at the Institute of Neuroscience and Mental Health at the University of Melbourne, who wasn’t involved in the study, told ABC Australia. “In certain individuals, in most individuals, it probably will never get to the brain and remains in the gut.”

The researchers found that out of 32,000 appendix samples from patients born between 1941 and 1985, who had them removed between 2000 and 2012, 16 contained the prion protein. These results were consistent regardless of sex or geographic living area. With that knowledge, some medical experts have expressed concern that the prion could be passed from one person to another through blood.

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

The U.K. has already been taking steps to counter this though, by restricting any residents between 1983 and 1996 from donating blood, and by importing blood. Still, there’s no test that can be used broadly to identify contaminated blood. The researchers noted that “the absence of a large number of blood transmission cases to date” is reassuring, saying that more cases would have emerged had preventative measures no been in place.

The researchers also noted that the prions they found in the samples could have been unrelated to the outbreak of “mad cow disease,” and are conducting another study of appendix samples from the 1970s and earlier — before the outbreak — to find out.

Source: Gill O, Spencer Y, Brandner S, et al. Prevalent abnormal prion protein in human appendixes after bovine spongiform encephalopathy epizootic: large scale survey. BMJ. 2013.