Australian researchers studying the risk factors linked to sporadic CJD have found that the greatest is surgery for conditions like hysterectomies and heart surgery.

BBC Science Correspondent James Wilkinson: Sporadic cases of CJD have until now had no known cause

They think their findings could have implications for "new variant CJD" caused by eating beef infected with bovine spongiform encephalopathy (BSE).

CJD - a degenerative, fatal brain disorder that leads to progressive dementia and other conditions - comes in several forms. The most common is sporadic CJD that mainly affects elderly people and accounts for between 85 and 95% of cases worldwide.

Risk factors

Scientists do not know what causes it, but researchers from the Australian National Creutzfeldt-Jakob Disease Registry sought to isolate some risk factors.

CJD is diagnosed by an examination of brain tissue on death

They studied 241 definite and probable cases of CJD and compared them with 784 healthy volunteers.

The information on the CJD cases came from the registry's records of cases between 1970 and 1993.

The greatest risk factor was for people who had had three or more operations, including hysterectomies, heart surgery, haemorrhoid removal, cataracts, varicose veins and carpel tunnel syndrome.

But people who had organ transplants, blood transfusions and major dental work were at no increased risk.

Other risk factors included living or working on a farm or in a market garden for more than 10 years.

However, people who worked in butchers shops or abattoirs were not at any greater risk.

Hospital infection

The researchers say their findings suggest that CJD could be linked to infections transmitted during operations.

Writing in the Lancet, they say they shoud "reinforce the heightened vigilance about infection control at all levels of care in hospital settings".

Last year, there were some 39 cases of sporadic CJD in the UK and 12 cases of new variant CJD.

Recent UK research has suggested a link between new variant CJD and surgical instruments.

Some doctors have called for the introduction of disposable instruments to prevent infection being spread.