LONDON - Preliminary results of a study to gauge how many Britons could be infected with the human form of mad cow disease have provided little insight into the extent of the threat, government scientists said Friday.
The deaths of 53 people in Britain have been attributed to a variant of the brain-wasting Creutzfeldt-Jakob disease. Experts believe it is a human version of bovine spongiform encephalopathy and that people get it by eating contaminated beef.
Researchers working for the Department of Health analyzed 3,170 preserved samples of tonsil or appendix tissue for traces of an infectious protein called prion, which has been found in the appendixes and tonsils of people who have the disease.
None of the samples showed the protein, they said.
''The fact that no positives have been found is welcome news, but these early results should not be taken as an indication of an 'all clear,''' said Britain's chief medical officer, Dr. Liam Donaldson.
Dr. Les Borysiewicz, a professor of medicine at the University of Wales who chaired the committee reviewing the data, said it would be inappropriate to draw any conclusions from the study's negative results.
''The first thing we need is a better diagnostic test,'' he said.
The test used, while the best available at the moment, has limitations, he said.
''We're quite a way away from being able to give reassurance,'' Borysiewicz said. ''It really does not take us further in estimating the size of the epidemic.''
Scientists believe most of the cases of infection with the human form of mad cow disease occurred in the late 1980s, before the introduction of controls to prevent contaminated meat getting to the table.
How many people eventually succumb to the disease, which emerged in 1995, largely depends on the length of its incubation period, which scientists say may be up to 20 years or more. Estimates of the number of people who could be infected range from about 100 to hundreds of thousands.
The scientists intend to test a total of 18,000 samples, including freshly removed tonsils and appendices.
Dr. Graham Medley, an expert on Creutzfeldt-Jakob disease at Warwick University, in England, said it is difficult to anticipate what is to come.
''If the incubation period is 10 years, then we are in the middle of the epidemic,'' he said. ''If the incubation time is 30 years, then we are only at the beginning. We have never before seen this disease in humans and we simply do not know at the moment.''
British beef was banned in Europe for two years following an outbreak of mad cow disease and the emergence of Creutzfeldt-Jakob disease. The ban was lifted in August 1999 everywhere but France, which has been taken to court by the European Commission.