vCJD ( Mad Cow )may lurk in more people than realised

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vCJD may lurk in more people than realised

 

* 12:44 19 May 2006

* NewScientist.com news service

* NewScientist.com staff and AFP

 

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The deadly human form of mad cow disease, vCJD, may have infected far

more people than previously thought, suggests a new study.

 

The assumption that most people are genetically shielded from the

devastating disease could be wrong, said the research published on

Friday. But it cautions that the evidence for this remains sketchy.

 

Variant Creutzfelt-Jakob disease (vCJD) is linked to eating meat

infected with bovine spongiform encephalopathy (BSE), also known as

mad-cow disease. A rogue version of a prion protein proliferates in

the brain, leading to distressing mental deterioration, loss of motor

control, and eventually death.

 

After vCJD was first identified in March 1996, some experts calculated

it could inflict a death toll in the tens of thousands, especially in

the UK, where the outbreak began. But these calculations were swiftly

revised downwards to a few hundred or even fewer when it was realised

that the toll was rising far slower than expected.

Key variation

 

At present, the UK has recorded 161 definite and probable cases of

vCJD, six of whom are still alive. One reason for optimism about the

potential extent of the vCJD epidemic has been the assumption that it

is genetic.

 

All of the deaths have occurred among people with a so-called " MM

variation " in part of the prion protein gene, called PRNP, located on

chromosome 20. In the white British population, 42% of people have the

MM variant.

 

The rest of the white population have different types – 47% have the

MV variation while the remaining 11% are VV. The fact that no MV or VV

cases had arisen led many to believe that this was a protection

against the rogue protein.

Two out of three

 

But the new study, which appears in the British Medical Journal,

places a cloud of doubt over this assumption.

 

Researchers led by James Ironside at the University of Edinburgh, UK,

carried out a DNA analysis of three appendix tissue samples found to

carry the mutant prion protein.

 

The tissues were part of a vast earlier study in which UK labs

screened 12,600 appendices and tonsils for the protein in order to get

an idea of the spread of vCJD.

 

Ironside's team say they were extremely surprised to find that two out

of their three samples, which tested positive for vCJD, came from

people with the VV variant. Neither individual, both aged in their

twenties at time of surgery between 1996 and 1999, has the symptoms of

vCJD.

Incubation time

 

But the paper warns against dramatism. It notes that only these two VV

samples have so far been identified, and just because a VV individual

has the protein does not mean that he or she will go on to develop vCJD.

 

On the other hand, no one knows how long it takes for vCJD to

incubate, which raises the possibility that VV individuals may fall

sick years from now. In classic Creutzfeldt-Jakob disease, which

occurs in older people, this can take up to 30 years.

 

And another, as-yet unquantifiable, risk is that VV individuals with

the prion may unwittingly pass it on to others through blood

donations. In 2004, a person with the MV gene variant was found to be

infected with vCJD, but again this individual had no clinical disease.

 

" There are compelling reasons why health officials should take

notice, " said two Canadian specialists, Kumanan Wilson and Maura

Ricketts, in a commentary accompanying the latest paper.

 

" It is conceivable that, having jumped the species barrier (from cows

to humans), transmission of the prion within the species becomes easier. "

 

Journal reference: British Medical Journal (DOI:

10.1136/bmj.38804.511644.55)

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Weblinks

 

* BSE and vCJD special report, New Scientist

* http://www.newscientist.com/channel/health/bse

* British Medical Journal

* http://bmj.bmjjournals.com/