"Mad cow" disease was a major fear in the United Kingdom following a large outbreak that peaked in 1993 and caused the deaths of 180,000 cattle and more than 150 people.
Now, a new study suggests one in 2,000 Brits may still be harboring signs of the disease that's caused in humans by eating infected cow's meat.
"Will these people develop disease and can they transmit it? There are many questions we still do not know the answers to," said study author Dr. Sebastian Brandner, a professor of neuropathology at University College London, said to the BBC.
Bovine spongiform encephalopathy (BSE) is the medical name for mad cow disease, a deadly infection of the central nervous system that was originally thought to be contracted in cows that ate the remains of other sick cattle which had been processed into protein supplements. It is now-banned a feeding practice.
The infection-causing agent behind mad cow is called a prion, which is a type of protein that changes from its normal state into an abnormal harmful one that damages the central nervous system. Doctors still do not know what causes the change leading to the deadly infection.
Humans that eat meat contaminated with mad cow may develop what's called variant Creutzfeldt-Jakob disease (vCJD), a deadly brain disorder characterized by psychiatric and neurological symptoms like depression, anxiety, dementia, confusion, difficulty walking, pain and involuntary movements until patients become completely immobile and mute before death occurs.
For the new study, British researchers conducted a study of appendix samples from 32,000 anonymous people of all ages who underwent an appendectomy between 2000 and 2012 across England.
The youngest participant was born in 1985, a couple years before the U.K. health scare began taking shape.
Of those, 16 samples tested positive for the abnormal prion proteins, which worked out to about 493 people per one million. Breaking it down further, the researchers estimated one in 2,000 residents are likely carriers of the protein.
Rates did not differ across age, gender or region of the country. The findings were published Oct. 16 in BMJ.
Previous studies have suggested around one in 4,000 people in the U.K. carry vCJD prions.
It's not clear whether these carriers will develop symptom, or if they pose a risk for transmitting the disease to others through surgery or blood transfusions. The researchers noted U.K. health agencies have already taken steps to secure the blood supply and reduce risk.
However, further genetic testing of the positive samples revealed differences in the gene-encoding protein that were not seen in the earlier outbreak of vCJD that killed 177 U.K. residents. Thus, they called for more studies to analyze and monitor citizens, since this type of vCJD may not show as many symptoms or might take longer to develop.
In the United States, only three cases of vCJD have been confirmed historically. Blood donors who have spent five years or more in countries where mad cow is found, such as the U.K., are not eligible to donate blood in the U.S., according to the American Red Cross.
One expert wrote in a commentary that the new study shows the disease is not "yesterday's news" for Europe.
"This shows we need to understand more about the natural history of the prion," Dr. Richard Salmon, a retired neurologist who authored the editorial accompanying the study in BMJ, told New Scientist. "Infections don't lead to disease as readily as we originally feared, but we don't know why, or whether these infections have a sting in their tail."