The long-term risk of developing vCJD may be higher than thought, according to research among a Papua New Guinea tribe.

It has been a widely accepted theory that only a few people with a particular genetic makeup will be susceptible to the infectious agent, or prion, which is believed to be the cause of the illness.

And, indeed, it has been shown that all 88 victims of vCJD in the UK shared certain characteristics which could conceivably have placed them at a higher risk of developing the human form of mad cow disease.

Some experts have even taken the view that those without this genetic makeup might be immune to vCJD.

However, research into a closely related disease, presented at last week's Millennium Festival of Medicine, and reported in New Scientist magazine, suggests that this protection may be illusory.

Scientists travelled to Papua New Guinea in the South Pacific where a form of human CJD has killed more than 2,500 members of the Fore tribe in the past century.

The reason behind the swift spread of the disease was the custom of eating the brains of dead relatives as a sign of respect.

When this practice was banned more than four decades ago, the number of deaths from the variant, called Kuru, fell away.

Long incubation period

But more recently, there has been a rise in the number of elderly Kuru victims.

Interestingly, tests on 11 victims revealed they all had the genetic makeup thought to offer protection against CJD infection or development.

This suggests that those with the "protective" genes may still get ill - although not as quickly as those with the susceptible makeup. The delay in the onset of symptoms could be as much as 30.

This would mean that new cases of vCJD in the UK might crop up well into the second half of this century.

Professor Peter Smith, who chairs the UK Government's Spongiform Encephalopathy Advisory Committee, said: "It doesn't rule out the possibility that the incubation period is longer in people of that genotype."

The unknown incubation period of any possible vCJD infection is the main reason why scientists are unable to place a reliable figure on how many people are likely to fall ill with the disease.

Any number from 150 to 100,000 people in the UK might fall ill and die, according to some estimates.

If the suggestion of the Papua New Guinea researchers is correct, the number of likely victims will increase.