Infectious prions are misshapen protein particles that coax normal proteins to misfold as well, forming hard-to-dissolve clumps that build up, rupture cells and lead to the characteristic "spongy" appearance of affected brains. Salvatore Monaco of the University of Verona and his colleagues analyzed the brains of eight Italian cattle that had tested positive for BSE. In two of the oldest animals, the researchers found that the disease-causing prions differed from those typically associated with BSE. In addition, they detected widespread protein plaques in parts of the brain usually left unaffected by mad cow disease and found that the brain stems of the two animals were less altered by the disease than those of the other six animals were.
The results support the hypothesis that, like viruses and the sheep prion disease scrapie (which takes as many as 20 different forms), BSE may encompass a number of strains. The authors have dubbed the novel form BASE and note that "although observed in only two cattle, the BASE phenotype could be more common than expected."