Scientists have just challenged the notion that airborne prions are innocuous. It is known that prions can be transmitted through contaminated surgical instruments and, more rarely, through blood transfusions. However, prions are not generally considered to be airborne – in contrast to many viruses such as influenza and chicken pox. 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 not 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.
The prion is the infectious agent that caused the epidemic of “mad cow” disease, also termed bovine spongiform encephalopathy (BSE). BSE claimed the life of more than 280,000 cows in the past decades. Transmission of BSE to humans, e.g. by ingestion of food derived from BSE-infected cows, causes variant Creutzfeldt-Jakob disease which is characterized by a progressive and invariably lethal breakdown of brain cells. Consumption of food made from BSE-infected cows has caused the deaths of almost 300 people. The precautionary measures against prion infections in scientific laboratories, abattoirs, and animal feed factories have not typically included stringent protection against aerosols. These new findings suggest that it may be advisable to consider the possibility of airborne prion transmission, and to create regulations aimed at minimizing the prion infection risks to humans and animals.
Aerosols Transmit Prions to Immunocompetent and Immunodeficient Mice. (2011) PLoS Pathog 7(1): e1001257. doi:10.1371/journal.ppat.1001257
Prions, the agents causing transmissible spongiform encephalopathies, colonize the brain of hosts after oral, parenteral, intralingual, or even transdermal uptake. However, prions are not generally considered to be airborne. Here we report that inbred and crossbred wild-type mice, as well as tga20 transgenic mice overexpressing PrPC, efficiently develop scrapie upon exposure to aerosolized prions. NSE-PrP transgenic mice, which express PrPC selectively in neurons, were also susceptible to airborne prions. Aerogenic infection occurred also in mice lacking B- and T-lymphocytes, NK-cells, follicular dendritic cells or complement components. Brains of diseased mice contained PrPSc and transmitted scrapie when inoculated into further mice. We conclude that aerogenic exposure to prions is very efficacious and can lead to direct invasion of neural pathways without an obligatory replicative phase in lymphoid organs. This previously unappreciated risk for airborne prion transmission may warrant re-thinking on prion biosafety guidelines in research and diagnostic laboratories.