Prions are proteins that have become bent out of shape. Their chain of amino acids folds up in an abnormal ways, and they can transmit this rogue alignment to their normal counterparts. As their numbers increase, they gather in large clumps that can kill neurons and damage brains. They most famously cause BSE in cows, CJD in humans and scrapie in sheep. But other mammals suffer from prion diseases too – the deer equivalent is called chronic wasting disease or CWD and it is shedding light on how prions are transmitted in the wild.
Gultekin Tamguney from the University of California, San Francisco, has found that even infected deer are contagious even when they are apparently healthy and show no outward symptoms. Their faeces are bursting with prions, and through these infectious dollops, deer effectively seed their environment with sources of contagion.
CWD can be very common in both wild and captive deer, and its quick spread suggests that individuals must be able to pass on infections from one to another, probably via their environment. Scientists have suggested that faeces could act as a vehicle for prions, but so far, no one had ever shown that.
To do that, Tamguney spent one to two years collecting samples of faeces from five mule deer that had been infected with CWD prions. They liquefied these samples and exposed them to brief doses of radiation that would kill off any bacteria and viruses, but leave prions unharmed. They then injected the liquid into the brains of mice (the prion protein differs by just one amino acid between mice and deer, and proteins from one species can successfully infect the other).
Samples that were taken from uninfected deer didn’t cause the mice any harm. But those that were taken 4-5 months after the deer became infected successfully transmitted the disease to 29% of the mice. By directly searching for the prion proteins, and looking at the infected brains under the microscope, Tamguney confirmed that these prions collected from the droppings of deer could infect fresh brains.
At 4-5 months, the deer aren’t showing any visible signs of the prions building up in their brains. They won’t start showing symptoms for another year or so, but during this time, they are continuously shedding infectious proteins into their environment, every time they relieve themselves. Over the course of its remaining lifetime, Tamguney estimates that a deer could shed as many prions in its faeces as accumulate in its brain by the time of its death.
The amount of prions in any one pile of droppings would be fairly low, but it’s such a regular source of contagion that deer could potentially come into contact with several “doses”. Transmission is even easier when you consider that mule deer eat small amounts of soil every day, for the important minerals it contains. But some mouthfuls contain not just minerals, but prions too. In fact, coming into contact with soil particles actually makes it easier for these rogue proteins to infect animals that accidentally lick contaminated ground.
Reference: Nature doi:10.1038/nature08289
Image: Mule deer by Daniel Mayer
More on prions:
September 9th, 2009 at 2:09 pm
How do the prions get from their brains to their feces? It seems several steps must be involved, first to get out of the brain cells, then across the blood-brain barrier, and finally to the gut. Do they circulate in the blood? Get picked up by the liver and injected along with bile? Or go directly from innervation of the gut?
Do the precursor proteins exist in all nerve cells, or only in the brain?
Finally, are humans at risk from exposure to these prions?
September 9th, 2009 at 3:10 pm
Mechanism for how ingested prions probably get in host cells here
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0003314
Lipid raft micropinocytosis. These were neuroblastoma cell cultures, but their evidence seems solid. Excretion? Who knows, but it is not just neurons that take these prions up so it could be a product of cell lysis or undescribed secretion pathway.
And if I was a deer/elk hunter, I’d be concerned. But that’s me (Teach undergrad bio/zoo/micro)
September 9th, 2009 at 3:22 pm
Thank you, Spirula.
I don’t recall hearing about prions afflicting mountain lions. I wonder if they have some mechanism to keep prey prions away from their own precursor proteins, or if their proteins can no longer be persuaded to fold that way, or to stay folded.
September 9th, 2009 at 3:53 pm
No problem Nathan. Gave a lecture last week which included prions and CWD.
I don’t know about the predator situation, nor if it has even been considered let alone studied. The only major predator deer have now comes with either head-lights or a gun. And the even though the deer population as exploded, I doubt anyone has even looked at potential cost to predators. It seems to be just focused on potential human impacts.
(As I recall, the first cases showed up on those loathsome game ranches out west.)
And the existence of sites like these are why I would be concerned.
http://www.mndeerhunters.com/index.php?option=com_content&task=view&id=16&Itemid=28d.
There are suspicious cases, just no confirmed ones. Considering it has a lengthy incubation, and that prions are not known to be species specific, I’m not the type of person play hunky-dory with venison until someone says “Oops, looks like we DO have a confirmed case.”
Again, that’s me.
September 9th, 2009 at 4:18 pm
Uptake in sheep and elk is considered to be most likely occuring through lymphoid tissue along the alimentary track, particularly the Peyer’s patches and tonsils, with subsequent migration up to the CNS. Even at early stages there is detectable levels of PrP-Sc in mucosal neurons. It seems reasonable that the shedding is ocurring from those initial accumulation sites along the alimentary track.
Reference:
Antemortem detection of PrPCWD in preclinical, ranch-raised Rocky Mountain elk (Cervus elaphus nelsoni) by biopsy of the rectal mucosa. J Vet Diagn Invest 21:15–24 (2009)
http://jvdi.org/cgi/content/abstract/21/1/15
September 10th, 2009 at 12:20 am
I wonder how much that 29% would go down if a study was done where mice were fed deer poop. It seems that injecting them straight into the mouse brains doesn’t really prove anything on the infection end, just that there are prions in deer poop.
September 10th, 2009 at 9:38 am
Yong! You are an amazing writer.. The topics you cover are simply uncovered by anyone.
September 17th, 2009 at 6:46 pm
There are many many deer in my community by the Chesapeake Bay, which has woods, open fields and nature parks, but is ringed tightly by suburbs, city and water, so the deer can’t really leave. In my small community alone, there have been 4 cases of Lou Gehrig’s in recent years, all have died. Have you heard that ALS could be a prion disease, or perhaps somehow deer could be involved? I’m afraid to eat the produce from my own garden, since the deer eat the tops of everything grown there. By the way, everyone in the family but one has had confirmed Lyme disease.
Thanks,
-V
October 11th, 2009 at 9:20 pm
This is very interesting! I have encountered speculation that Alzheimer’s is prion-related (tau). Considering that beef is quite hospitable to this prion (BSE), especially when due to poor industrial practices (cannibalism, forced carnivorism, etc), I wonder how many Alzheimer’s cases are related to BSE. If, indeed, many Alzheimer victims are suffering a rogue prion, they still eat, and thus excrete too. The product aka “sludge” is quite interesting in this case, in that it is made directly from treated/processed human sewage, and most often spread over pastures, where cattle sometimes feed. Prions are known for their durability, and are quite unlikely destroyed through the processes involved in producing “sludge”. I have read that 40percent of people over 85 years of age, are victims of Alzheimer’s. If so, that’s a lot o’ poo. As far as I know, Japan has a ban on American beef, for the very reason of BSE. I have read that many Koreans are interested in following suit. I think this is a somewhat-forbidden subject considering the general void of discourse and awareness of it. It also potentially threatens large industry, as well as opening a very large can of…worms in healthcare and other areas.