Toxoplasmosis — you may have heard about it, but how much do you really know about it? Toxoplasma gondii is a single-celled parasite known for being found in cat droppings, with a reputation of getting into human beings and making them slightly... off. It's true, and more complicated, than you may expect. This particular parasite has also found its way into way more people than you may expect — maybe even you.
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00:00Are cats taking over your mind? No, but for 60 million people around the world,
00:05they are changing the way you think.
00:06What are you all afraid of? Cats, cats, cats, cats.
00:10You've probably heard cats are one of the major carriers of Toxoplasma gondii.
00:15The parasites live in cat droppings, but that's not the only place you can find them.
00:19You can also pick up these parasites by eating contaminated meat. According to the CDC,
00:24the problematic parasites usually reside in pork, venison, and lamb — although
00:28any undercooked meat could potentially carry them. You can also come into contact with these
00:33parasites in soil, on unwashed fruits and vegetables, or on food prepared in a kitchen
00:38in which cross-contamination occurs. It's also terrifyingly common. The CDC says around 60
00:44million U.S. residents are living with this little hitchhiker, and researchers estimate
00:48about a third of the world's population is infected.
00:51Okay, a little more than expected.
00:54If you want to find out your own status with these parasites,
00:58there are tests that can detect them. Otherwise, you probably wouldn't know you caught them.
01:03When you're first infected, you might come down with what feels like a bit of a cold.
01:06That passes, after which your immune system keeps the parasites in line.
01:10They only start creating real havoc if your immune system is seriously compromised.
01:15If that happens, it'll attack your brain and eyes. At that point, you're in trouble.
01:20The parasite is estimated to kill as many people as malaria does.
01:24Yeah, it's worse than I thought.
01:26The story of how he discovered the parasite's powers is brilliant,
01:29and it's proof science happens in weird ways. In the 1990s, a Czech scientist named Dr. Jaroslav
01:35Fleger became convinced that there was a parasite living in his brain, influencing his personality
01:40and guiding his decisions. Fleger said his work began when he read a book on the flatworm,
01:45which invades the nervous systems of ants and paralyzes them to make them easy prey for sheep,
01:49a worm's final destination. Studying the impacts of these worms made him take a look at his own
01:54behavior, and he realized he was developing a tendency to do some nutty stuff. He reportedly
01:59noticed himself doing such things as walking into traffic, crossing busy streets without looking,
02:04and sharing political opinions that would have been better kept to himself.
02:08Gunfire didn't bother him either, and he realized that's not normal.
02:12In 1990, he went to Charles University in Prague and joined an in-depth study into
02:16Toxoplasma gondii. There, he was able to examine infected individuals and the parasite's life
02:22cycle. He realizes that, essentially, the only place the parasite can reproduce is inside a cat.
02:28Once it leaves the cat, it needs to get back in or die trying. Since humans aren't usually
02:33prey for cats, most of us are a dead-end host for these parasites. Or are we?
02:39We don't have parasites!
02:41By the time Fleger got on board with studying Toxoplasma gondii,
02:45researchers had already discovered something weird about infected rats.
02:48They were more active, less cautious, and more likely to wander into open spaces.
02:53Things got even weirder when an Imperial College London parasitologist introduced the scent of a
02:59cat to the research rats. She put infected rats into enclosures that had different areas
03:04treated with different scents — cat urine, rabbit urine, and the rats' own smell.
03:09The rats became so obsessed with the cat odor that researchers dubbed their condition
03:13fatal feline attraction. Next up for researchers were animals closer in the evolutionary chain
03:18than rats. In 2016, researchers in Gabon presented both infected and non-infected chimps with the
03:24smell of their natural predator, the leopard. Infected chimps were all about investigating
03:29the smell, while non-infected counterparts weren't so keen. But when scientists exposed
03:33infected subjects to the scents of lions and tigers, which are not their natural predators,
03:38there was no difference in their behavior. The conclusion was clear.
03:42The parasites modified the behavior of their hosts to make it more likely that they would get
03:46eaten by a cat. Another piece of the puzzle was provided by research from the National
03:50Institutes of Health in 2011. That's when researchers found that the presence of toxoplasma
03:55in rats activated parts of the brain linked to sex. Instead of causing fear,
04:00the smell of cats turned the rats on.
04:02"'Cats love this, trust me."
04:05The parasite's influence on human behavior wasn't documented until 2002,
04:09when Dr. Fleger found evidence that the parasite was modifying human behavior in similar ways to
04:14rats and chimpanzees. One behavioral difference he noticed in himself was his tendency to become
04:20oblivious to car horns. He also started crossing the street without looking. It's not surprising,
04:26then, that he found his first bit of evidence by analyzing traffic patterns. Infected drivers,
04:31he discovered, were around 2.6 times more likely to find themselves in a car accident.
04:36It was the first indication the parasite we thought was harmless and completely under our
04:40control was, in fact, affecting us, causing an increase in risky behavior.
04:45At first, it doesn't seem to make much sense. How could causing humans to take more risks be
04:50a part of a parasite's journey back into the bodies of cats? According to Fleger,
04:55human beings were once a completely viable vehicle for getting toxoplasma back into cat
05:00hosts. And in some parts of the world, we still are. He suggests that thousands of years ago,
05:05big cats were a much bigger threat to humans in their day-to-day lives,
05:09and the parasite drove us into the mouths of lions or tigers. Fleger says he's found
05:14evidence in personality questionnaires he gives to infected and non-infected people,
05:18and says infected people are much less fearful of things like the forest or darkness.
05:23In other words, they are prime victims for big cats hunting at night.
05:27Fleger has also found toxoplasma gondii doesn't change all human behavior in quite the same way.
05:32He claims that infected women were more likely to become more outgoing, friendly, and moral,
05:37though not more promiscuous. Infected men, on the other hand, became more guarded,
05:41more prone to jealousy, and more likely to break or bend the rules.
05:45Those reactions are pretty much polar opposites, but Fleger thinks he knows what's going on.
05:50He says it's possible the parasite is interacting with the way people react to chronic stress.
05:54Therefore, the opposite personality changes seen in infected people aren't that strange at all.
05:59Since men tend to withdraw during times of stress and women tend to reach out,
06:03the parasite may be putting the host under an extreme amount of stress.
06:07Those around them are just seeing the outward manifestations of that.
06:12The idea that we're not in control of our own emotions or behavior is a terrifying prospect,
06:16but more research suggests it's impossible to tell how it's going to impact us.
06:20Anne Catherine Stock from the University of Dresden puts it like this,
06:24It interferes with brain chemistry, but the parasite itself doesn't intend to harm someone.
06:29It always acts by the same mechanism. It's just that humans are very rarely prey to cats.
06:34That doesn't help its goal.
06:36And that means, at the end of the day, the effect of the parasite on humans is
06:39pretty scattershot. There have been links to an increased likelihood of the development of
06:44mental illness, but Stock's work also shows that there's a potential for some good changes as well.
06:49Her team found infected people had faster response times when they
06:53needed to react to changing stimuli.
06:55Sir, I believe you owe this lady an apology.
06:57Fry, no! He's bulging with what could be muscles.
07:01There's also evidence that toxoplasma increases the amount of dopamine,
07:04the feel-good chemical in the brain. In rats, that's specific to the pathways that attract
07:09them to cats, but in humans, it's more complicated. The entire system gets
07:13flooded and different people can have different reactions.
07:17Researchers have uncovered the process the parasite uses to infect humans,
07:21which the parasites are essentially an interim host. Melbourne scientists found that the parasite
07:26hijacks a healthy human cell to create a sort of bunker that provides it with everything it needs
07:31to survive in a dormant state. Toxoplasma releases proteins into the human cell, which then manipulates
07:37a person's chemistry to stockpile starches. Think of a bear hibernating for the winter.
07:42Fortunately, our immune systems keep on top of this sort of thing, at least in healthy individuals.
07:47Scientists are still working on figuring out some of the precise mechanisms,
07:51but they do know a healthy immune system will form cysts around the toxoplasma pockets.
07:56It's possible that increased levels of dopamine come because of the cysts and an increase in
08:00one particular enzyme that's key in both the development of the cysts and dopamine.
08:05What kind of impact the cyst has on the brain tissue around it isn't entirely clear.
08:10One of the things you may have heard about the Toxoplasma gondii parasite is it makes people
08:14more prone to mental illness, particularly schizophrenia. That's terrifying, but a 2016
08:20study from Duke University looked at more than 1,000 people and found there was no link between
08:25infection and mental illness. Around 28 percent of their sample group, which was made up of people
08:30born in New Zealand in the early 1970s, were infected. Ultimately, it showed no correlation
08:36between infection and IQ, depression, schizophrenia, driving offenses, accident claims, or
08:41criminal convictions. Other studies, including one with a sample pool of 7,440 people, did not
08:48support a link between toxoplasma infection and mood disorders. University College London has
08:53also looked at the claim that when cats are present in the household of a pregnant woman
08:57or a child, the child is more susceptible to mental illness down the road. Good news for
09:02cat lovers. Across the 5,000-person sample pool, there was no link found between cat ownership
09:07and mental illness or impairments.
09:09It's undeniable that cats play a vital role in the life cycle of this weird parasite,
09:14but according to the CDC, there's absolutely no reason to give up your fluffy pet.
09:18They do recommend pregnant women stop cleaning the litter box, though.
09:22I was planning on doing that anyway.
09:24They also say there's next to no chance of picking up the parasite by even petting an
09:29infected cat, as it's not transmitted by fur. All this is a great reason to keep your cats inside,
09:34as the CDC says they become infected by eating small animals that carry the parasite.
09:39You'll be eating a fast food burger and boom! You'll be crawling with us again!
09:45Ever wonder what makes special sauce so special? Yo!
09:50Keep cats inside and the problem is non-existent. And even if your cat's been outside before,
09:55you can still turn things around. Because the cat is part of Toxoplasma gondii's life cycle,
10:00they can only pass along the infection for about two weeks after becoming infected themselves.
10:04And that's it for this video. If you liked it, don't forget to like and subscribe!