'Mad Cow' and Other Prion Diseases Hide Out in Spleen

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http://www.scientificamerican.com/article.cfm?id=mad-cow-prion-diseases-hide-spleen

Prion diseases such as bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (vCJD) are able to jump species much more easily than previously thought. A study published in Science today shows that in mice, prions introduced from other species can replicate in the spleen without necessarily affecting the brain.

The study reinforces the concern that thousands of people in the United Kingdom might be silent carriers of prion infection, potentially able to pass a lethal form of the disease to others through surgery or blood transfusions.

Prions are infectious pathogens, primarily composed of the misfolded form of a protein called PrP. Normal PrP molecules that are converted into the misshapen type then aggregate in the brain to form hard, insoluble clumps -- with fatal consequences.

Previous studies have judged the ease of cross-species transmission by looking for clinical symptoms as well as the presence of prions in infected animals' brains. Results from these studies suggested that in many cases there is an effective 'species barrier', with most inoculated animals seemingly free of prions at the end of their lives.