The research also finds that the "species barrier", which protects humans against BSE and might have prevented a large-scale epidemic, could be far less effective at stopping the transmission of BSE than scientists had thought.
The Anglo-American research team warns in a paper in the Proceedings of the National Academy of Sciences that the findings "raise greater concern that a large section of the UK population may be at considerable risk [of developing nvCJD]".
The comments mirror those last week by Lord Phillips, chairman of the BSE Inquiry, who said the 48 people known to be affected by nvCJD might represent a small fraction of all cases. Scientifically proving that BSE causes nvCJD has been almost impossible because of the difficulty of establishing that eating a meal of infected beef or beef products caused the human disease several years later.
Two years ago, however, scientists published the first hard evidence of cause and effect when they showed that BSE and nvCJD produce exactly the same variation in incubation periods in a panel of genetically different strains of mice, strongly suggesting that BSE and nvCJD are caused by the same infectious agent.
The latest research, by Stanley Prusiner of the University of California at San Francisco, and Robert Will and James Ironside of the National CJD Surveillance Unit in Edinburgh, uses genetically engineered mice to establish an even stronger link between BSE and nvCJD. The mice carry the genes for the cattle prions - the infectious proteins thought to cause the brain disease - andin effect behave as if there is no species barrier between cow and mouse.
When BSE material was injected into the mice, it took 250 days for them to develop the disease. When nvCJD material was injected, the incubation period was exactly the same.
When material from sick mice was injected into healthy mice, the incubation period was again the same, the researchers found. This showed that transmission within a species was virtually the same as between species.
The implication is that with BSE there may be little or no effective species barrier. In human terms this could mean that many of the tens of thousands of people exposed to BSE by eating infected beef in the late 1980s are presently incubating nvCJD.
"The study indicates that this strain is particularly pathogenic [dangerous], therefore we should not underestimate or play down its potential effects on the population as a whole," Dr Ironside said.
The Edinburgh surveillance unit is investigating 10 more cases of suspected nvCJD.
t Organophosphate (OP) sheep dip - blamed by many farmers for causing illness - is to be withdrawn until new containers that reduce exposure to the chemical become available, the Government said yesterday.
Last month the Committee on Toxicity reported that the health effects of prolonged low-level exposure to OPs were unproven. But the Agriculture minister Baroness Hayman said more research would begin into alternatives.Reuse content