It's been awhile since I've discussed prions on here. (Indeed, so long that the last time was on my old blog, but I imported a few of them that can be found here, here, and some background on prions here). Allow me to copy a bit of that to re-introduce the topic:
Prions are, of course, the transmissible agents that cause diseases such as kuru and Creutzfeldt-Jakob in humans, and related disease such as "mad cow" disease, scrapie, and chronic wasting disease in animals. Though there was initially much controversy about these agents in the early years (most notably, because they did not contain any nucleic acids), Bottaro notes that this is a "heresy" that the science community has embraced.
I mention all this because a reader passed along a recent article published in PLoS Pathogens showing that prions remain infectious in soil.
Prion disease results from the transmission of a misfolded form of an ordinary protein, PrP. Transmission can occur whenever one comes in contact with the abnormal protein, which are incredibly stable. While one way such contact can occur is via contaminated meat (such as tainted beef, giving rise to "mad cow" disease in humans), it's less clear how transmission occurs between animals. Prions have been found in both milk and urine of animals, but it's not known exactly what significance these have in the spread of the infectious protein.
Some environmental reservoir of contamination, however, has been noted in several studies. For example, it's known that prions can enter the environment when carcasses of infected animals decompose or are buried, or when they urinate as mentioned above. Additionally, a 2004 paper published in Emerging Infectious Diseases showed that, when deer were placed into paddocks that contained 1) live, prion-infected deer; 2) a decomposed deer carcass; or 3) deer excreta, they could become infected with the prion under all three circumstances, suggesting the prion persists in the environment and can be spread in ways that don't involve direct contact with other live, infected animals.
The new manuscript takes this a step farther, and examines the interaction of the prion with soil minerals. They found that it does bind to both whole soil as well as a variety of soil minerals, particularly montmorillonite (abbreviated Mte, which they describe as "a expandable layered silicate clay mineral") in a stable attachment. They tested whether this Mte-bound prion remained infectious, and it was when they inoculated the Mte-prion complex into hamsters' brains. They didn't, however, test if it was infectious via ingestion, which would be the most obvious route of transmission to animals--they note in the introduction that cattle, deer, sheep and other animals can consume hundreds of grams of soil every day.
So, that loose end still remains to be tied in order to really make the case that animals can acquire prion disease from contaminated soil, but the current manuscript certainly strengthens the case that this is so. This has serious implications for eradication of the prion once it enters an environment, since it's so stable (prions are resistant to radiation, UV light, heat, freezing, many chemicals, etc.). This mode of transmission also increases the likelihood of transmission of the agent between species, since by this model, they wouldn't need to have close direct contact to spread the infection: just to happen upon the same chunk of land (quite literally).
References
Johnson CJ, et al. (2006) Prions Adhere to Soil Minerals and Remain Infectious. PLoS Pathog 2(4): e32. http://pathogens.plosjournals.org/perlserv/?request=get-document&doi=10…
Miller, MW et al. (2004) Environmental sources of prion transmission in mule deer. 10:1003-6. http://www.cdc.gov/ncidod/EID/vol10no6/04-0010.htm
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One of the properties of montmorillonite (one of a family of clay minerals that also includes talc) is an ability to soak up and hold water like a sponge, swelling in the process. For this reason it is often used by farmers as a soil additive, especially in sandy areas, to help retain water in times of draught. It's also used in earthern dams and levees, and it's used to fill old drill-holes to absorb possibly contaminated well-water before it reaches the higher level aquifers used for drinking.
This is truly creepy. It sounds like a very disturbing variation of Cat's Cradle ice-nine.
Thanks for the info, Dave!
Mark,
There's a pretty good book on prions, Deadly Feasts, that makes exactly that comparison.
That is extremely scary. I had no idea. I thought that the threat was largely addressed by avoiding consumption of these proteins - simply elimating rendering and the like. I did not think these proteins were nearly as stable or transmissive as they are. I suppose it's not difficult to imagine that deer would ingest just about anything in close proximity like that.
Next we'll find out that ticks and other blood feeders are reservoir/vectors for prions.
Why are there so many prion diseases? I mean, why did people get Kuru and not vCJ (and vice versa)?
Why did people apparently not get either disease from eating sheep that had scrapie?
Most prions are thought to be species-specific (Mad Cow Disease is an exception) and molecule-specific. So, the Kuru prion will only replicate its shape on the Kuru protein; same for CJ and vCJ. While this isn't a hard and fast rule, it does seem to be a trend. Prions are an area of hot research in biophysical chemistry. They are really neat, until you realize how dangerous they are.
Peter, people do get scrapie from consuming infected meat, delivering lambs, or possibly from consuming goat's milk. It is impossible to trace where a person may have contracted scrapie, as infection could have occurred as much as 15 years before symptoms appear.