British researchers have found a second person who became infected with the human version of mad cow disease as the result of a contaminated blood transfusion.

Reporting in the journal Lancet, the researchers said they had found the malformed proteins, or prions, that cause the disease in an unidentified person who died this year of unrelated causes. The discovery of a second transfusion-associated prion infection, experts said, suggests that the risk of mad cow disease to the population is higher than they had realized, because it appears to confirm that eating infected beef is not the only way of spreading the disease.

The prions were found through an intensive autopsy, which was conducted because the victim was one of 17 people known to have received blood from donors who later developed the incurable disease.

Both the United Kingdom and the United States took steps several years ago to protect their blood supplies from mad cow contamination because of what was then a theoretical concern that it could be transmitted through blood transfusions. In 2001, the American Red Cross began to turn away donors who had spent three months in Britain, or six months anywhere in Europe, since 1980.

The first confirmed transmission of mad cow disease through a blood transfusion was reported in Britain late last year. The second case was in someone who had received the transfusion five years ago. The person showed none of the neurological symptoms typical of the disease.

Mad cow disease, or, in humans, variant Creutzfeldt-Jakob disease, causes death by attacking the brain and central nervous system, but in the second case the infection had not spread to those areas. The prions were found in the spleen and in a cervical lymph node.

The second transfusion-infected victim also had a different genetic makeup than the approximately 150 people who had contracted the disease. That discovery led principal researcher James Ironside, of the British CJD Surveillance Unit, to conclude that more people might be susceptible to the disease. Researchers had believed that people with the second victim's genetic makeup, or genotype, were in some way protected from mad cow disease.

In an accompanying Lancet commentary, blood policy specialists Kumanan Wilson of Toronto General Hospital and Maura N. Ricketts of Health Canada argue that aggressive and sometimes costly steps taken to protect blood supplies from mad cow disease have been proven to be well-founded and necessary.

"The key lesson from this policy-making experience is that lack of definitive evidence should not preclude action for serious potential exposures," they wrote in support of applying the "precautionary principle" in medicine.

They also wrote that "there now appears to be sufficient evidence that individuals without clinical signs of [mad cow disease] harbor, and therefore potentially transmit, the infection."

In another Lancet article, researchers at the University of Maryland reported that removing white blood cells from donated blood, called leucoreduction, reduces the risk of mad cow disease transmission, but only by about 40 percent. French scientists reported they had identified a new technique for disinfecting prion-contaminated medical devices.