Although research suggests that prions from one species rarely infect other species, some scientists believe the species barrier was breached when a new version of Creutzfeld-Jakob disease appeared in humans after several recent epidemics of bovine spongiform encephalopathy or "mad cow" disease. Since then, barriers to the transmission of prion diseases between species "have emerged as a major public health issue," according to Eric Jones and Witold Surewicz of Case Western Reserve University.
Prion diseases are caused by misfolded variants of the normal prion protein, which aggregate into fibrous tangles called amyloid fibrils and cause fatal wasting of brain tissue. The abnormally folded protein itself appears to act as an infectious agent, transmitting disease without a DNA or RNA genome such as in a virus. Although disease prions seem to infect normal prions by binding to them and forcing them to take on the abnormal configuration, researchers remain uncertain about the exact molecular details of infection.
Earlier studies identified many "strains" of disease prions across mammalian and yeast species. Researchers thought these strains could be defined by differences in the underlying amino acid sequences of the prions. Under this scenario, disease transmission would be more likely between species with similar prion amino acid sequences.
But a few mysteries stood in the way: Some individuals harbored several different prion strains that caused different disease outcomes, even though all the prions shared the same amino acid sequence. In some cases, a single amino acid change in one species could compl