There will no doubt be a lot of news generated on this PLoS article, published today, demonstrating the aerosol transmission of prions – the etiologic agents of Bovine Sponigform Encephalopathy (“Mad Cow Disease”) and Creutzfeldt-Jakob Disease (CJD). My guess is that much of the press it generates will be panic-inducing and will misinterpret the results, so I’m going to take a swipe at it (familiarize yourself with prions at the bottom of this post).
First, the authors used nebulizers to aerosolize “brain homogenates” – this is an unrealistic method of transmission. I was unable to locate research demonstrating whether or not prions “shed” as viruses and other infectious agents are (perhaps in a manner opposite that which was used to inoculate the mice in the study, i.e. neurons in the cribiform plate or BALT). If they are, it’s clear that they aren’t shed in numbers sufficient for infection, or we would have had a true global pandemic on our hands by now (there are familial & sporadic forms of CJD, and these people don’t walk around infecting others).
Second, as implied above, the concentrations are higher than anything one would be exposed to by interacting with a person or animal with a prion disease. In the lowest concentration (0.1% weight/volume percentage), there was a 0% attack (infection) rate, as shown below. While only a 2.5% concentration was necessary for an apparent 100% attack rate, it doesn’t matter – you will never be exposed to that much “aerosolized brain” through casual interaction with a prion carrier!
So while I’ve already read articles that attest to the 100% lethality of “airborne” prions, I would like to make this humble attempt to quell any irrational fear that’s been triggered in those who happen to pass through my humble portion of the blogosphere: the bottom line is that aerosolizing infectious tissue that never normally leaves the host and using it to directly inoculate mice does NOT demonstrate airborne transmission of prions. And remember, because prions have not been shown to carry nucleic acids, they cannot mutate to acquire new characteristics (e.g. airborne transmission).
In fact, the most worrisome thoughts these findings brought to mind were not those of contracting prion diseases in the hospital, but the implications these results may carry for biological warfare and bioterrorism. Not wanting to be one of the fear mongers myself, I’d like to remind you that such weapons would require sophisticated dispersal methods to be a real threat.
I’d encourage you to share this article with anyone who’s avoiding walking down the meat aisle in the supermarket for fear of contracting a prion disease (and those who post the fear-mongering articles!).
Edit (1/14/11): One final addition – when infected mice were housed with uninfected mice (and thus shared the same air), none of the uninfected mice developed the infection).
Edit (1/18/11): Because this post continues to receive hits, I decided to include part of an email conversation on the subject I had with one of the neuroscientists at my institution (she raises an interesting point about the dentist’s office):
“No, I don’t know that anyone has ever shown the prion protein is “shed” like a viral particle. Given the low probability of infection, if prion protein were deposited extracellularly, it would probably be very sticky and stay close to the cell it came from, not be released and easily form an aerosol.In any case, I agree with you – if it does mean “airborne” prion transmission is possible, this is more likely to happen in the lab or the slaughterhouse. But if it’s true, an aerosol could potentially be generated under other more common circumstances such as in the dentist’s office when drilling through bone/tissue.”
Learn stuff below:
What Are Prions?
Prions are similar to viruses in that they are small, filterable, and “self-replicating” (see below) infectious agents. They are dissimilar in that they contain no genetic information, because they’re proteins (if nucleic acids are present, they’re scarce and have yet to be identified). Even more unique, and troublesome, is that prions are very resistant to sterilization and may not cause an inflammatory response (PrP is an inherently-coded protein).
Endogenous PrP-C is a mostly alpha-helical structure, but can be induced to fold itself in a malicious manner that results in a deadly encephalopathy. This misfolded protein, PrP-SC (for scrapie, another transmissible spongiform encephalopathy), has extensive beta-pleated sheets, rather than alpha-helices. Further, once converted, PrP-SC can induce PrP-C to convert, resulting in a cascade effect – accumulation of PrP-SC in the central nervous system is responsible for the symptoms and physical findings of the disease.
How Are Prions Transmitted?
In the mid-1900s, an epidemic of kuru was identified among the Fore people of Papua New Guinea. Known to practice ritualistic cannibalism during funeral ceremonies, it’s been hypothesized that the epidemic began after the ingestion of an individual with a sporadic case of CJD. (Cases of kuru dwindled after the cessation of cannibalistic practices.)
The “mad cow disease” epidemic in the UK was before my time, but the principle is the same: cattle were essentially force-fed the remains of other cattle, which were worked into the livestock feed. Since then, strict control of cattle feed practices, surveillance, and slaughter of infected animals has brought the epidemic under control.
Variant Creutzfeldt-Jakob Disease (vCJD) is the prion encephalopathy associated with ingestion of contaminated food products from infected cattle (and some other mammals). However, there have been cases reported of transmissions linked to human corneal & dura mater transplants, cadaveric pituitary hormones, and improperly sterilized neurosurgical equipment.
Thus, while transmission between humans and across species has been established, it should be noted that the above methods are all unnatural modes of transmission, and that aside from cannibalism, there is no evidence for direct human-human transmission (or animal-human transmission, for that matter).