In the new study, the authors housed immunodeficient and immunocompetent mice in special inhalation chambers and exposed them to prion-containing aerosols, which induced disease. Exposure to aerosols for one minute was sufficient to induce disease in 100% of mice. The longer the exposure, the shorter the incubation time in the recipient mice, after which they developed the clinical signs of a prion disease. These findings indicate that prions are airborne. Prions appeared to transfer from the airways and colonize the brain directly, since various immune system defects – known from previous experiments to prevent the passage of prions from the gut to the brain – did not prevent infection.
Contrary to what some news outlets might conclude from this, it does not mean that prion diseases, such as BSE (“mad cow” disease) and scrapie can be transmitted through casual contact. First, this is a mouse study, using a murine model of prion disease that does not affect humans, cattle, or sheep. Prion diseases do seem to share similar traits, but that doesn’t mean we can automatically extend the results to other species.
Second, aerosols are not the same as breath. Indeed, nobody has yet found any evidence that prions are really airborne contagions in any traditional sense, and the new paper doesn’t change that. The researchers took the brains of prion-infected mice, ground them up into a pulp, fed the pulp into a laboratory-grade atomizer to produce a very fine mist, then sprayed the mist into sealed cages that contained uninfected mice. It’s about as artificial a system as one can imagine.
While it doesn’t frighten me from a public health perspective, the paper should be an eye-opener for people who work with prions in research or diagnostic labs, or who might encounter them in slaughterhouses. If your job entails grinding up animal brains, you should probably do it very carefully, with multiple barriers to prevent you from inhaling the resulting aerosol. It’s still unclear whether animal prions pose a real threat to humans, but there’s no sense taking chances.
Haybaeck J, Heikenwalder M, Klevenz B, Schwarz P, Margalith I, et al. (2011) Aerosols Transmit Prions to Immunocompetent and Immunodeficient Mice. PLoS Pathog 7(1): e1001257. doi:10.1371/journal.ppat.1001257