While official numbers show human cases of mad cow disease have been tapering off, new research published Friday indicates Britain may be facing thousands of new, previously undetected cases.
Using a mathematical extrapolation, scientists determined that up to 3,800 additional cases may be found in the United Kingdom, where the human form of the fatal brain disease terrified the population and hobbled the beef industry following its discovery in the mid-1980s.
The researchers based their findings on the discovery of potentially infectious proteins in appendix and tonsil tissues removed from 12,674 patients between 1995 and 1999, well after many of the 153 cases of variant Creutzfeldt-Jakob disease (vCJD), the human variant of mad cow, were detected.
Among those samples, the researchers found three that contained aberrant prion proteins, thought to be the cause of vCJD. The disease is believed to be transmitted by eating meat from cows infected with bovine spongiform encephalopathy; both ailments create holes in the brain and result in death.
Based on those findings in the British population, the researchers arrived at a potential number of 3,808 people with possible incubating cases of vCJD. Like many diseases, vCJD can take years to manifest itself in a patient, and is thought to grow more dangerous as someone consumes more infected tissue. The disease is most frequently found among patients in their 20s and early 30s, so the majority of samples for the study were taken from people aged 20 to 29.
'Interpreted with caution'
The study's authors urged caution in drawing conclusions from their findings, which were reported in the Journal of Pathology, but they and other researchers noted that the discovery may point to additional cases not yet found, and to a much longer incubation period for vCJD than previously thought.
“Our findings need to be interpreted with caution, but cannot be discounted,” David Hilton, of Derriford Hospital in Plymouth, England, and the report's author, told Reuters.
Hilton tested the samples with scientists from the CJD Surveillance Unit in Edinburgh, Scotland, and Imperial College in London. But two of the three positive samples showed different characteristics than most tissue infected with aberrant prions, protein strands that have been implicated in a range of human and animal diseases.
The researchers acknowledged they weren't sure what the differences in the two samples mean. The accumulation of prions in some of the appendix tissue was unlike that found in most other cases of vCJD, but scientists also haven't found any other disease that shows the same sort of protein accumulation.
As such, the estimate of 3,800 might have to be reduced by as much as two-thirds, said Neil Cashman, a researcher of prion ailments at the University of Toronto's Center for Research in Neurodegenerative Diseases who has been working on vaccines for diseases like vCJD.
While an estimated 1,200 or so additional cases still exceeds earlier projections of 540 future cases, it is also far below other estimates that forecast up to 10,000 new cases. "That doesn’t sound good but it could even be worse," Cashman said.
However, the findings also highlight the difficulty in tracking a disease like vCJD, and the scant knowledge of its true incubation period, which some scientists believe could be 30 years or more. The researchers wrote that their findings could indicate "a future increase in numbers of vCJD cases or a significant number of individuals with a 'carrier state,'" meaning the patients may have some infected tissue but will never get sick with vCJD.
Britain, which confronted mad cow to a far greater degree than any other country, took major steps in the late 1980s and again in the late 1990s to keep its citizens from eating infected meat. Cattle are thought to get bovine spongiform encephalopathy, as mad cow disease is formally known, by eating the tissue of other cattle and similar mammals.
What lies ahead
British authorities banned the use of such protein in cattle feed, a move mirrored by most other countries around the world, and barred the use of some of the riskier parts of cattle, like the spinal cord, in the human food chain. Following the discovery of a single case of mad cow in the United States last December, U.S. officials have implemented many -- but not all -- of the same precautions. And U.S. rules limiting the content of cattle feed have been in place since 1997; more were announced in January by the Food and Drug Administration, but have yet to be implemented.
British authorities have described a tapering number of vCJD cases after taking steps over the past decade to limit the spread of the human form of mad cow, including a partial ban on blood transfusions this March after researchers found a vCJD case last December that apparently was transmitted through blood. But the new research shows that Britain may be dealing with the disease for a long time.
For example, the incubation for a similar prion disease in humans, kuru, can be as long as 40 years. While most vCJD cases are thought to have incubated in about 12 years, generally dating to before the first changes to the British cattle industry, that stretch of time seems to be close to the shortest possible onset of the disease.
"That’s the floor," Cashman said. "We don’t know what the ceiling is."
The researchers said their findings also highlight the need for better diagnostic tests. Currently, tests to confirm the presence of the disease can only be performed after a patient dies, unless -- as was the case with the current study -- some nervous system tissue is removed through surgery. The researchers called for more tests on freshly removed tonsils as a way to check for initial signs of the disease.
Only one U.S. case of vCJD has been found, and the patient had spent long periods of time in Britain. But some U.S. groups have complained that surveillance for the disease in this country is hampered by a lack of autopsies on patients who show signs of CJD, which also comes in a spontaneous form that afflicts about one in 1 million people. Many apparent cases of sporadic CJD, as it is known, are not confirmed because autopsies are never performed.
Florence Kranitz, president of the CJD Foundation, which counsels patients and their families about the illness, said the research highlights a need to confirm whether, and how, the disease is transmitted through blood. Beyond the British blood restrictions, and a U.S. ban on British blood donations, the new estimate underscores the need to ensure the disease isn't transmitted during surgery.
And, Kranitz noted, any increase in the projected number of cases has a stark human toll. "To say that this could translate to 3,800 cases is horrifying," she said. "We're all staggered by it."
Reuters contributed to this report.