BBC Monday, 14 May, 2001, 21:20 GMT 22:20 UK

CJD scientists warn of 'second wave'

By BBC News Online's Helen Briggs

Scientists are warning that the predicted size of the variant CJD epidemic may have been underestimated.

Research in mice suggests that only people with the shortest incubation periods for the disease are showing symptoms of the human form of BSE.

If confirmed, the findings would mean that the current cases are just the tip of the iceberg and that a "second wave" of cases will emerge.

Projections of the scale of the epidemic are based on the theory that some people are unlikely to contract CJD from infected meat because of their genetic make-up.

But this idea is contradicted by research reported in the scientific journal Proceedings of the National Academy of Sciences.

"Those patients we have seen so far with vCJD may be those genetically disposed to have the shortest incubation periods," says John Collinge, director of the Medical Research Council Prion Unit in London.

Genetic risk

Prions are infectious agents that cause fatal brain diseases such as vCJD in humans and bovine spongiform encephalopathy (BSE) and scrapie in animals.

Following infection, there is a very long incubation period before symptoms of the disease occur.

According to the latest figures from the Department of Health, 99 cases of vCJD have been recorded to date.

But it remains unclear how many other people have been exposed to BSE and what proportion of these will eventually develop the human form of the disease.

One clue is our genes. As with conditions like cancer and heart disease, genetic factors are thought to be involved in determining an individual's risk of developing CJD, after exposure to the infective agent.

All patients identified so far have a particular variation in their genetic make-up (MM), shared by about 40% of white Britons. Two other subtypes (VV and MV) are seen.

The current estimates assume that only people with the MM genetic make-up will contract the disease if they come into contact with the infective agent, for example by eating contaminated meat.

'Complacent'

However, Professor Collinge, who led the team that carried out the new research, warns that such predictions may be "overly optimistic".

"This study reminds us that we cannot be complacent about the potential risks to public health posed by BSE," says Professor Collinge.

"We cannot rule out an epidemic that evolves over decades."

The new work confirms that in mice at least, a number of genes are involved in susceptibility to prion diseases.

And although it may take longer for symptoms to appear in some animals because of their genetic make-up, that does not mean they will not eventually succumb to the disease.

As the mouse and human genomes are so similar, corresponding genes are almost certain to be found in humans.

-- Anonymous, May 14, 2001