Scientists have developed a device that filters from blood the mutant proteins that cause the human form of mad cow disease. This new tool could boost the safety of donated blood.

Infectious proteins called prions cause mad cow disease, scrapie in sheep, and variant Creutzfeldt-Jakob disease

(vCJD) in people. Since the early 1980s, doctors have diagnosed more than 200 cases of the fatal human disease worldwide, most of which seem to have resulted from eating beef tainted with prions. However, there's evidence that at least three people contracted the disease from blood transfusions that carried prions.

Scientists are working to reduce the risk of obtaining prions from beef. Researchers in the United States and Japan reported online Dec. 31, 2006 in Nature Biotechnology that they have engineered cattle that are free of the proteins that mutate to cause mad cow disease.

However, Robert Rohwer of the Veterans Affairs Medical Center in Baltimore, who studies vCJD and other prion-related diseases, notes that if these cattle enter the food supply, disease risk won't drop right away. Prions can linger in a person's blood from beef that they ate years ago.

"This is a disease with a very long incubation period during which people infected with vCJD appear completely normal," Rohwer says.

To develop a way to extract prions from blood, Rohwer and his colleagues searched a library of millions of chemicals for ones that stick tightly to prions. The team coated tiny beads with each chemical and incubated the beads with prions isolated from people, hamsters, and other animals. After excluding those chemicals that were too expensive or too toxic or that stuck too readily to other blood components, the researchers narrowed their focus to a compound that they call L13.

For testing how well L13 traps prions in blood, Rohwer and his colleagues used hamster blood spiked with prions that cause scrapie. The scientists first passed the blood through a filter that removes white blood cells because previous research had shown that about half of blood-borne prions stay in or around those cells.

The team then ran half the blood through another filter embedded with beads coated with L13.

When the researchers injected blood that hadn't gone through the second filter into 99 hamsters, 15 contracted scrapie. However, of 96 hamsters injected with the L13-filtered blood, none became ill with the disease.

Rohwer's team reports its results in the Dec. 23/30, 2006 Lancet.

Marc Turner, who studies prions in blood at Edinburgh Blood Transfusion Center in Scotland, calls the new report "very promising work." Filtering disease-causing prions from blood could keep the blood supply safer if people infected with vCJD donate blood, he says.