A Mysterious Killer: The Prion

Trigger Warning: There are pictures in this article that may be upsetting to some. Viewer discretion is advised.

‘Ello! This is the start of a new series that deal with mysterious diseases and unusual pathogens. The first of this showcase is the prion, a silent, mysterious and nigh-invulnerable killer with a 100% mortality rate. Let’s start from the beginning.

In the 1950s and 60s, the Fore people of Papua New Guinea started dying of a mysterious disease that first started with the loss of coordination, making movement awkward and jerky, and progressed to the point where the person could no longer walk and in the terminal stages of the disease, the person can’t even sit without support and they’ll die within two years. It was dubbed Kuru, meaning “trembling” in the local language. But what was the cause of this disease?

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A child in the middle stages of kuru

It turns out that the Fore people practice funerary cannibalism, that is, the eating of dead relatives in the belief that the soul can be released from the body in that way. There was also the fact that the ones who were affected by kuru were children and young women. This was because the men of the tribe would take the muscle and the more nutritious organs and leave the less savoury parts for the women and the children, namely the brain. Later, scientists would find out that the main source of infection from cannibalism comes from the consumption of infected brains and spinal tissue.

 

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A leg being eaten

 

As more and more people died, the practice of cannibalism made sure that kuru would spread and infect more and more people. The thing that made the case especially baffling was the incubation period of kuru which could be up to several decades which meant that symptoms of infection would pop up way after the time of exposure. Cannibalism was outlawed afterwards and the number of deaths each year steadily decreased after that. There are still some that are dying from the last of kuru today.

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A graph detailing the number of deaths caused by kuru over five-year periods.

The event that brought greater international scientific attention to the prion’s existence came in the late 1980s and the 1990s in the UK when a massive number of cows suddenly lost coordination and died shortly after. It was dubbed the mad cow disease. At first, the scientists thought of scrapie, a disease that occurred in sheep for the last two centuries. It caused irregular movements in sheep and the name came from how the infected sheep would rub against things like trees until their wool was scraped off, exposing red raw skin. The symptoms were somewhat similar to the cows’ but there were no previous cases of such a disease in cows.

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A sheep with scrapie

So scientists began to look for a source of infection in the cattle population. They found that the carcasses of sheep infected with scrapie were used to make feed for the cattle. The cattle that died from mad cow disease were then also reintroduced into the food supply of cattle as feed. This escalated the range of the disease and it spread quickly through the cattle population of the UK.

The formal name of mad cow disease is Bovine Spongiform Encephalopathy (BSE). Bovine means cow-related. Spongiform Encephalopathy means spongy brain. So what the disease does is poke holes into the nervous tissue of the infected until it resembles a sponge.

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A normal brain vs an infected brain

When the scientists discovered this, they took samples of infected brains and tried to find what was causing the condition. They found that whatever was causing this condition (a substance so small it appeared fuzzy even under an electron microscope) was impervious to radiation that would have destroyed DNA and killed any living being. Short of total incineration, it was also highly resistant to heat so cooking infected tissue wouldn’t work. Scientists have also found that disease-free populations of cattle or sheep that have moved onto pastures previously occupied by a diseased population also came down with the same disease as the previous population. That means that bodily fluids, or at least, excrements, were also responsible for spreading the disease and that whatever was causing the disease could survive and remain pathogenic outside of a living body for a long time.

They have found something that was unlike any other known pathogen. However, its existence have been predicted since the 1960s and was further confirmed with the work of Stanely Prusiner in 1982, who identified a specific protein responsible for most of the cases transmissible spongiform encephalopathies (TSE), which is any disease that is caused by prions. This protein was named the prion or Prion Protein (PrP). They found that the disease-causing version of this protein was a perversion of a naturally-occurring protein in animals. The benign protein and the pathogenic proteins were made up of the same things but the disease-causing protein was folded differently, altering its function (like how they taught you in 9th-grade Biology).

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A normal PrP and a deformed version called PrPSc (which is what causes scrapie)

The spread of prion-caused diseases from sheep to cows raised alarm bells. If it could cross the species barrier, then could it infect humans? They weren’t sure at the time and since kuru was perpetuated by cannibalism and rose from a different source, most people thought that they were safe. That said, many countries stopped the importation of UK beef products to some extent and started testing for BSE periodically in their own cattle populations. The practice of making brain and spinal tissue into animal feed was also banned. But then, a farm-raised mink population started showing signs of TSEs and they were confirmed to have been infected by prions. The cause was also because of contaminated food. They also found that other animals, like deer and cats, could also contract their own forms of spongiform encephalopathy. Suddenly, the immunity of humans no longer seems so certain after all.

The way the prion infects the body is in the way they react with proteins when exposed to nervous tissue. The prion isn’t alive so it isn’t able to reproduce and make extra copies of itself like the usual forms of infection from other pathogens. Instead, it makes it so that the other proteins that come into contact with it become malformed and those converted proteins, in turn, continues to affect other proteins in the same way until enough tissue in the brain has been rendered unfunctional that it affects the organism and eventually kills it. Since it’s not a bacterium, antibiotics don’t work against it. Radiation, like I said before, doesn’t affect it either since it has no genetic information. The only way to treat any diseases caused by prions would be a drug that has something that can bind to the pathogenic prion proteins and make them unable to convert other proteins into itself (something like an antibody). There have been attempts to find such a drug but nothing has been proven to be effective so there remains no way to cure or treat any form of TSEs once you’ve contracted it.

And yes, it turns out that humans can get infected by TSEs and it’s not limited to cases of cannibalism. Consumption of infected brain, spinal or digestive tract tissue can cause you to develop TSEs (don’t worry, there have only been four confirmed cases of BSE in the US and you don’t have to worry about contaminated meat since they don’t sell the potentially contaminated parts in marketplaces anyway). Another possible way to get TSEs, that I mentioned before, is through your environment. If an infected animal defecates somewhere where there is a water source or some other sort of carrying agent nearby, you might come into contact with it if you drink contaminated water or breathe it in. But this method of transmission is unlikely to affect us unless we start eating grass where infected droppings once were so we’re safe from that. There has also been a study that suggests that certain genetic weaknesses will make some people susceptible to developing TSEs on their own without an external source of infection. There’s still a lot to be discovered about the nature of prions and we still don’t know a lot about the basic functioning and structure of prions so the information on this article might change come any new discoveries.

In any case, instances of humans contracting a form of TSE is extremely rare and preventative measures have been taken in most countries to minimise any possible contact to pathogenic prions. So, unless you have extremely bad luck or happen to be in special circumstances, your brain will be safe from this mysterious killer.

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That’s all for today. This is a longer article than usual. As always, like and comment and let me know what you think and if you have any ideas on what topic you’ll like me to tackle next, leave that below as well. Follow if you’d like to see more articles like this.

I’ll talk to you later.

Here’s the wiki page on prions. It has some good sources in the reference section.
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