Incubation period for human BSE infection could exceed 50 years

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A person could possibly be infected with bovine spongiform encephalopathy (BSE) prions for over 50 years before developing variant Creutzfeldt-Jakob disease (vCJD), according to a study on another human prion disease called kuru in this week's issue of The Lancet. The findings suggest that the eventual size of a vCJD epidemic could be much bigger than previously thought, state the authors.

Dietary exposure of the UK population to BSE prions has been widespread. So far, around 160 vCJD patients have been identified in the UK, with cases also reported in France, Italy, Ireland, the Netherlands, Canada, Japan, and the USA. Predictions of the eventual size of a vCJD epidemic have varied widely; recent estimates, based on the current numbers of VCJD patients, have suggested that the total epidemic may be relatively small. However, the incubation period for BSE in people is key in predicting the true extent of an epidemic, and this has been unknown until now.

John Collinge (University College London, UK) and colleagues studied the only example of a major human prion disease epidemic available - a disease called kuru. Kuru is caused by cannibalism and reached epidemic proportions in certain communities in Papa New Guinea, where the consumption of dead relatives, as a mark of respect and mourning, was ritual practise. Its incidence fell steadily after Australian authorities stopped the practise in the 1950s.

In the study the investigators identified 11 patients with kuru from July 1996 to June 2004. As the latest year of birth recorded for a patient with kuru is 1959, the authors assumed that transmission by cannabilism ceased by 1960. They worked out the minimum incubation period for the disease as the time between 1960 and the date of onset of kuru in the patients. They found that the minimum incubation periods ranged from 34 to 41 years. However, in men more accurate assessment was possible and estimated incubation periods ranged from 39 to 56 years and could have been up to 7 years longer. When the researchers analysed the genetic make-up of kuru patients, they found that they had a particular gene varation that is associated with extended periods of incubation and resistance to prion disease.

The authors believe the incubation period for BSE prions in humans could be even longer than that seen in kuru patients because of species-barrier effects, where infection between species typically takes longer than within species.

Professor Collinge states that vCJD patients identified so far "could represent a distinct genetic subpopulation with unusually short incubation periods for BSE."

"…a human BSE epidemic may be multiphasic, and recent estimates of the size of the vCJD epidemic based on uniform genetic susceptibility could be substantial underestimations," he adds.

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See also accompanying Editorial.

Contact: Professor John Collinge via the Medical Research Council press office T) 0207 637 6011 press.office@headoffice.mrc.ac.uk


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