MILLIONS of people thought to be less vulnerable to the human form of “mad cow” disease may be at risk of developing a different strain, according to research.

Work by the Medical Research Council suggests that Britain could face new outbreaks of variant Creutzfeldt-Jakob disease (vCJD) because some strains may take longer to develop in the human body. The findings, reported in the journal Science, raise the prospect of a second epidemic of the brain-wasting disease.

All the 147 people who have died of vCJD in Britain have been of a genetic type known as MM. The latest work suggests that gene types MV and VV may also be vulnerable after longer incubation.

Variant CJD is thought to be caused by eating food contaminated with rogue proteins, called prions, that cause BSE.

In experiments spanning more than a decade, the research council team studied mice that were genetically modified so that they made the human prion protein, which comes in two common forms: 1M and V. Because everyone has two copies of this gene, there are genetic types MM, MV and VV.

Tests showed that mice with the MM genetic type, present in 38 per cent of people, contracted a vCJD-like disease. Mice with the VV make-up were also found to become infected when given BSE or vCJD prions, but the strain was quite different from vCJD.

John Collinge, director of the research council’s Prion Unit, at University College London, said that the research suggested that a much larger proportion of the population was likely to be susceptible to vCJD.

“These mouse studies give us vital clues about the behaviour of prions and how they appear to modify and adapt, depending on the genetic makeup of the individual they are infecting,” Professor Collinge said.

“We always have to be cautious about making direct comparison to the human condition, but our work strongly suggests that we can not assume only those with one genetic profile are vulnerable to BSE infection.”

Professor Collinge said that while it was not possible to say how the findings might alter estimates of those likely to become ill, they indicated the need for a more sophisticated system for categorising vCJD. To date, the best guess as to the eventual size of the epidemic has been 3,800 people.

He said that the mouse tests suggested that BSE in cattle had caused more than one type of brain disease, including a sporadic CJD not previously associated with eating infected meat.

The disease, which has no cure, first emerged in 1995. It causes personality change, loss of body function and eventually death.

This week the first case of vCJD was identified in the Irish Republic after a young man, believed to be in his 20s, was given a tonsillar biopsy.

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