Italian scientists have found a second form of "mad cow disease" that more closely resembles the human Creutzfeldt-Jakob disease.
Italian researchers report a study of eight cows with mad cow disease found that two of them had brain damage resembling that in 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.
Dr Paul Brown of the National Institutes of Health in the United States 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 Dr Brown, who was not part of the research team.
But scientists in Europe have studied all cases of sporadic CJD for the past decade and the incidence has not changed, said Dr Brown.