Mad cow disease is one of those  things that’s so terrifying you just have to pretend it doesn’t exist.

Or maybe that’s just me. It’s a fatal disease with no  cure that’s spread by proteins and you can catch it just  by eating a cheeseburger.

Since this disease popped up so suddenly  and is so absolutely horrifying, researchers have spent a LOT of time studying it. And one thing that jumped out at them was that  a lot of the people who got sick were young. Like, couldn’t even run for U.

S. president young. So researchers really wanted  to figure out why that was. And after decades of connecting the  dots, we may finally have an answer. [♪ INTRO] Mad cow disease, more formally known  as variant Creutzfeldt-Jakob disease, or vCJD, is a prion disease.

They’re super rare, which is good  news, because they’re also terrifying. And to explain why, we have  to start with what a prion is. Prion proteins exist all through our bodies, but the majority are concentrated in our  central nervous systems, mainly in our brains.

But sometimes these proteins get misfolded,  and that’s where the problems start. See, a protein is just long strings of amino  acids that are folded up into a 3D shape, and that shape is critical to letting  that protein do its little protein job. And that’s true of all of them, prions included.

And the cells that make these proteins  are also in charge of making sure they get folded correctly before they get sent  off into the wide world of your body. Normally when a protein is  misfolded, it gets destroyed, and the cell that made it tries  to get it right the next time. But sometimes misfolded proteins get out, and  that’s where they can start to cause problems.

So just to recap our vocab:  When we say prion protein, that means the normal and correctly  folded guys that we all have. But when we say the word prion on its own, that means the disease-causing, bad guy protein. Which leads us to the reason that  prion diseases are next level scary.

Regardless of how it’s  folded, a protein isn’t alive. And since it isn’t alive, the  immune system can’t kill it. Like, our immune systems work by  identifying a bacteria or a virus or any other pathogen as  being foreign and killing it.

Once it’s dead, that bad thing  usually can’t make you sick anymore. But because prions aren’t alive, they can’t die. And the other problem with CJD  is that a prion is like an ant.

There’s never just one. That’s because one misfolded prion can bind  to other, correctly-folded prion proteins, and bend them out of shape too. So even though there was just one badly  folded protein to start, that prion will bend others, which causes affected brain  cells to die and release even more prions.

Eventually enough brain cells are killed that  clusters of these prions will form in the brain. All those misfolded proteins in your neurons  can cause major neurological problems, including trouble moving, insomnia,  personality changes, dementia, coma, and death. So yeah, CJD is super scary.

It’s always fatal, often within a  few months or years of diagnosis. There are a few types of CJD,  all of which are classified based on where the patient’s  first messed up protein came from. There’s Sporadic, meaning a  prion protein native to your body just got folded wrong one  day and we have no idea why.

Hereditary, where there was a mutation  on the gene that codes for the protein, resulting in instructions to misfold it. Or acquired, which means that, well,  you acquire the misfolded protein. This happens via exposure to blood, brain, or  spinal tissue from somewhere in the environment.

Most of the time, this is iatrogenic, which  is a fancy term meaning it happened during medical treatment, like if equipment  that was used in your brain surgery was contaminated with misfolded prions,  or if you got a corneal transplant or graft of the tissue that covers your  brain from someone already infected. There are even some super rare cases  of transmission via blood transfusion. But acquired CJD doesn't have to be iatrogenic.

It can also happen when someone  eats beef that has been infected with the cow version of a prion disease. And CJD that is acquired from cow  proteins is called variant CJD. So to figure out how variant CJD got into humans, we need to figure out how  prion diseases got into cows.

Which takes us back to the 1980s. Back when parachute pants and leg warmers  were all the rage, some cattle farmers in the UK noticed their cows started  acting strange, and then just up and dying. They determined that these cows had a new disease, which researchers called Bovine Spongiform  Encephalopathy, or BSE for short, but everyone else just called it mad cow disease.

This wasn’t the first prion disease  we’d found, either in animals or people. There’s this one that sheep get called scrapie, which causes them to rub their sides  against walls and stuff, hence the name. And at this time, we already  knew about classic CJD in humans, so the concept of us having a  prion disease was already out there.

But what we didn’t know was that  prion diseases could cross species. Which might be why these UK  farmers treated this new disease like a nuisance and not a public health concern. At least, not right away.

And because nobody thought this thing  was contagious, they didn’t exactly tread cautiously when it came to  disposing of cows that had died of BSE. So… let’s talk about what  they did with those dead cows. Feeding livestock takes a lot of food.

So to cut costs, farmers would take the  waste products from slaughtering their cows, grind them up, and turn it into a high-protein  animal feed, called meat and bone meal. Which is as gross as it sounds. They’d also do this with the bodies of any  of the cows that died ahead of slaughter, since it was thought that  the way they were processed would kill just about any pathogens  that made it into the grinder.

So, when the first cows started dying of  BSE, they too got turned into cow food. And eventually, those cows that were fed  this contaminated food ended up as our food. This SciShow video is supported  by Giving What We Can, an organization inspiring more people to  leverage the power of effective philanthropy.

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Like we said, the first cows started  getting sick in the mid 1980s. But it took until the ‘90s for the  first cases to appear in humans. That’s how we learned that mad cow  disease can incubate in your body for over a decade before  you start showing symptoms.

And we’ve since learned that prions  are basically indestructible. They’re heat resistant and survive being boiled, aren’t destroyed with alcohol, and they  stick around in soil for literal years. If that sounds scary, I’m sorry, but  I’m about to make that so much worse.

It’s been estimated that 3 million  cows that had asymptomatic BSE made it onto people’s dinner tables in the UK. That’s horrifying. But here’s the thing.

Even though probably everyone who so  much as looked at a burger in the 1980s probably came in contact with these prions, only a couple hundred people have  ever been diagnosed with variant CJD. So naturally, researchers wanted to know why, which meant looking at the  demographics of exactly who got sick. And right off the bat, they  noticed an unusual pattern.

While classic CJD is most common  in middle-aged or elderly people, the average age of variant  CJD patients was under 30. Given that we know there’s a  decade-long incubation period, that means that the majority of those  patients became infected in their teens. And that’s….

Really weird. Like, it’s not unusual for diseases  to hit really young children harder, because their immune systems  aren’t fully geared up yet. But in this case, it looked like  teenagers were the most susceptible, even more so than little kids.

The easiest explanation was that maybe  teens just got more exposure to the contaminated beef, but a 2004 study looked  into that and found no evidence that teens in the UK ate any more burgers  or meatloaf than any other age groups. So that left researchers wondering  what it might be about teenagers that made them so vulnerable to variant CJD. Well it took some time, but a paper  published in 2016 seems to have the answer.

And it’s in our guts. We all have these small bits of tissue  called Peyer’s patches in our intestines. And on top of these patches are  immune cells called M cells.

M cells are basically the TSA of your gut,  checking over everything that passes through. If the M cells aren’t sure  about something that goes by, they’ll grab it and send it over to your  other immune cells, like T cells and B cells. And it’s the T and B cells’  jobs to give it a pat down and figure out if that thing is a  pathogen or just some Dorito dust.

If that questionable particle  fails the vibe check, those T and B cells raise the  alarm and neutralize the threat. But they can’t seem to do  that to those pesky prions. Like we said, they’re not  alive, so they can’t die.

Plus, T cells only look for intruders based on  their amino acid sequences, not their shape. And these prions have all the right amino  acids, so they can fly under the radar. Remember, the problem is just how they’re folded, but all the amino acids are  the same as a correct protein.

Since properly folded prion proteins already  exist in the body, the T cells would just recognize that sequence of acids as a thing  that’s supposed to be there, and let it through. And at least some B cell  activation relies on T cells too, so if the T cells don’t catch the  intruder, then the B cells won’t either. Even if some B cells tried to destroy the  misfolded proteins that the M cells brought in, prions just so happen to be resistant to the  enzymes they’d usually use to kill pathogens.

So the M cells invite these little protein  disasters right into the body, and then the rest of the immune system doesn’t know what  to do with them, so just lets them go. Then the misfolded prions are free to bump into  healthy prion proteins and start misfolding them. And researchers have some pretty strong  evidence that of all the immune cells, it’s the Ms that are the most at play.

Researchers created lab mice that had either  more M cells than usual, or no M cells at all. Not only did the mice with bonus M cells  get prion diseases faster, but the ones without M cells were immune to getting  the prion diseases through the oral route. Which brings us to the reason that teens  are the most at risk of prion diseases.

See, even though everyone has Peyer’s  patches and M cells, people in their teens and early twenties have way more  Peyer’s patches than other age groups. We don’t have many Peyer’s patches at birth, and as we grow up, we gain  more, right until puberty. And after that, we start to lose them.

More Peyer’s patches means more M cells, which means more chances for  misfolded prions to get into the body. So if you have a whole population  of people eating contaminated beef, the people with the most Peyer’s patches are the ones that are most likely to have  an M cell bring that prion into their body. Then the disease incubates for ten  years, and you get a bunch of people that start showing symptoms in  their late twenties, early thirties.

So it took us some time, but it seems  like there’s a pretty good answer to the weird pattern in who  got sick with mad cow disease. Thankfully mad cow disease is super rare now, as in, there have been fewer than 10  cases globally in the last decade. A lot of that comes from prevention  and monitoring of the cows themselves, but as with all super-scary diseases,  researchers are working hard to find a cure.

Will Peyer’s patches be the answer? It’s tough to say, but knowing more  about how the disease is transmitted is a promising start. So even though this disease sounds  scary, there’s no need to have a cow. [♪ OUTRO]