The brain-wasting diseases BSE, known as mad cow disease, and human CJD are caused by different forms of mutant proteins called prions. A number of people, mainly in England, have also suffered from what is called variant CJD, a brain disease believed to be acquired by eating meat from infected cows. No Americans have been reported with variant CJD.
Now, the team of Italian researchers reports a study of eight cows with mad cow disease found that two of them had brain damage resembling the human victims of CJD. They said the cows were infected with prions that resembled those involved in the standard form of the human disease, called sporadic CJD, not the variant caused by eating infected meat.
Salvatore Monaco, lead author of the new study, said the findings may indicate that cattle can also develop a sporadic form of the disease, but it might also be a new food-borne form of the illness.
Dr. Paul Brown of the National Institutes of Health said the finding does not indicate an increased threat to humans.
If a new form of the disease were affecting humans there should be an increase in the incidence of CJD, said Brown, who was not part of the research team.
However, scientists in Europe have studied all cases of sporadic CJD for the last decade and the incidence has not changed, said Brown, an expert in the disease, who works at the National Institute of Neurological Disease and Stroke.
Both the human and cattle diseases cause holes to form in the brain. The Italian researchers found that, in addition to the holes, two cows had an accumulation of amyloid plaque in their brains. Amyloid plaques are an indication of Alzheimer's disease in humans. They have also been found in people with sporadic CJD but had not been found in cattle, the researchers said.
Mad cow disease is formally known as BSE — bovine spongiform encephalopathy — and the Italians named the new form with amyloid plaques BASE.
"Although observed in only two cattle, the BASE phenotype could be more common than expected," they reported.
Monaco said in an interview via e-mail that he believes the incidence could be as high as 5 percent among cattle with mad cow symptoms.
But while human CJD and BASE share several characteristics, the Italian researchers cautioned against assuming a link between the two.
The findings of the team led by Monaco, of the Department of Neurological and Visual Science, Policlinico G.B. Rossi, in Verona, Italy, are reported in this week's online edition of Proceedings of the National Academy of Science.
Brown said there have also been some unpublished reports from Japan of cows with a different form of mad cow disease.
Cattle are believed to develop BSE from eating infected tissues of other animals. Such feed has now been banned in many countries, including the United States.
The first case of mad cow disease in the United States was reported in December in Washington state, involving a cow imported from Canada.
An investigation seeking other cattle from the same herd ended last week with Agriculture officials saying they had located all but 11 suspect animals and concluding the rest could not be found.
By Randolph E. Schmid