Washington ? One of the biggest hurdles in fighting ailments such as mad cow disease and its human version has been the lack of a way to diagnose the illness. A new process may point the way to a useful blood test.

Transfusions can spread the disease among people, but there is no practical test to detect it. That is why blood donors are carefully screened to weed out people who have lived or visited in certain areas where they might have become infected.

Until now, dissecting the brains of victims has offered the only way to detect such brain-wasting diseases in humans.

These diseases are caused by agents called prions. Researchers led by neurology professor Claudio Soto at the University of Texas Medical Branch at Galveston report they have developed a method of multiplying the number prions in a blood sample so a blood test then can detect them.

Such a test could help prevent the spread of the disease through transfusions and could detect the illness in people or animals before it can be spread to others.

The findings, to appear in the September issue of the journal Nature Medicine, were released online Sunday.

The first known prion disease was scrapie, which has infected sheep for many years. In the 1980s, Britain had an outbreak of bovine spongiform encephalopathy, better known as mad cow disease, which spread to Europe and other areas. Two cows have been found with the illness in the United States.

The human form of the illness is called variant Creutzfeld-Jakob disease and is believed to have originated from eating infected beef.

The extent of the vCJD epidemic is not known, but it has killed about 180 people worldwide. Symptoms can take years to develop.

In addition to improving the safety of the blood supply, a practical test could help find infected people and animals before they show symptoms.

The research was supported by the National Institutes of Health and the Intramural John Sealy Endowed Fund for Biomedical Research. Soto has patented the process.