LONDON - People could be infected with the human form of mad cow disease for more than 50 years without developing the illness, which means the size of a potential epidemic may be underestimated, UK scientists said on Friday.

So far about 160 people have been diagnosed with variant Creutzfeldt-Jakob disease (vCJD). Cases of the fatal disease have also been reported in France, Italy, Ireland, the Netherlands, Canada, Japan and the United States.

Estimates have varied widely of how many people are likely to develop the brain illness caused by eating meat products contaminated with bovine spongiform encephalopathy (BSE).

It has been difficult to predict due to the long incubation period, which scientists had thought could be up to 20 years.

But Professor John Collinge and researchers at University College, London believe it could be longer and that an eventual epidemic could be bigger.

“Recent estimates of the size of the vCJD epidemic ... could be substantial underestimations,” he said in a report in The Lancet medical journal.

The scientistsÂ’ findings are based on a study of another human disease called kuru, which like vCJD is caused by a mutated prion brain protein.

Kuru reached epidemic proportions in some parts of Papua New Guinea where cannibalism had been practiced up to the 1950s in a ritual where natives ate dead relatives as a mark of respect.

Collinge and his team calculated the minimum incubation period for kuru based on when cannibalism was stopped and the year of birth of the last recorded patient.

According to their calculations, the minimum incubation time ranged from 34-41 years but in men rose to more than 50 years.

They suspect it could be longer for vCJD because the infection was transmitted between species, from cows to humans, which usually takes longer than within the same species.

They also identified a genetic variation in the kuru patients that was linked with extended incubation and resistance to prion disease.

Patients already diagnosed with vCJD could represent a distinct genetic sub group of the population with an unusually short incubation period, according to Collinge.

The Lancet described the study in an editorial as a innovative piece of research.

“Any belief that vCJD incidence has peaked and that we are through the worst of this sinister disease must now be treated with extreme skepticism,” it said.

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