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  • Is there a disease that makes us love cats,

  • and do you have it?

  • Maybe,

  • and it's more likely than you'd think.

  • We're talking about toxoplasmosis,

  • a disease caused by toxoplasma gondii.

  • Like all parasites, toxoplasma lives at the expense of its host,

  • and needs its host to produce offspring.

  • To do that, toxo orchestrates a brain manipulation scheme

  • involving cats,

  • their rodent prey,

  • and virtually all other birds and mammals,

  • including humans.

  • Documented human infections go as far back as ancient Egypt.

  • We found samples in mummies.

  • Today, about a third of the world's population is infected,

  • and most of them never even know it.

  • In healthy people, symptoms often don't show up at all.

  • When they do, they're mild and flu-like.

  • But those are just the physical symptoms.

  • Toxoplasma also nestles into our brains

  • and meddles with our behavior behind the scenes.

  • To understand why, let's take a look at the parasite's life cycle.

  • While the parasite can multiply in practically any host,

  • it can only reproduce sexually in the intestines of cats.

  • The offspring, called oocysts, are shed in the cat's feces.

  • A single cat can shed up to a hundred million oocysts.

  • If another animal, like a mouse, accidentally ingests them,

  • they'll invade the mouse's tissues and mature to form tissue cysts.

  • If the mouse gets eaten by a cat,

  • the tissue cysts become active and release offspring

  • that mate to form new oocysts,

  • completing the cycle.

  • But there's a problem.

  • A mouse's natural desire to avoid a cat makes it tough to close this loop.

  • Toxoplasma has a solution for that.

  • The parasites invade white blood cells to hitch a ride to the brain

  • where they seem to override the innate fear of predators.

  • Infected rodents are more reckless and have slower reaction times.

  • Strangest of all, they're actually attracted to feline urine,

  • which probably makes them more likely to cross paths with a cat

  • and help the parasite complete its life cycle.

  • How does the parasite pull this off?

  • Although the exact mechanism isn't known, toxo appears to increase dopamine,

  • a brain neurotransmitter that is involved in novelty-seeking behavior.

  • Thus, one idea is that toxo tinkers with neurotransmitters,

  • the chemical signals that modulate emotions.

  • The result?

  • Fatal attraction.

  • But mice aren't the only animals that end up with these parasites,

  • and that's where humans, and all of toxo's other hosts, come in.

  • We can accidentally ingest oocysts in contaminated water,

  • or unwashed produce,

  • or from playing in sandboxes,

  • or cleaning out litter boxes.

  • This is behind the common recommendation that pregnant women not change cat litter.

  • Toxo can cause serious birth defects.

  • We can also get toxo from eating undercooked meat

  • from other animals that picked up some oocysts.

  • And it turns out that toxo can mess with our brains, too.

  • Studies have found connections between toxo and schizophrenia,

  • bipolar disorder,

  • obsessive compulsive disorder,

  • and aggression.

  • It also slows reactions and decreases concentration,

  • which may be why one study found that people involved in traffic accidents

  • were almost three times more likely to have toxoplasma.

  • So is toxo manipulating our brains as an evolutionary strategy

  • to get predatory cats to eat us?

  • Or are our brains just similar enough to a rodent's

  • that the same neurological tricks that lure them in catch us in the net, too?

  • And is toxo the reason so many people love cats and keep them as pets?

  • Well, the jury's still out on that one.

  • Some recent studies even contradict the idea.

  • Regardless, toxoplasma has definitely benefitted from humans

  • to become one of the world's most successful parasites.

  • It's not just our willingness to let cats on our dining room tables

  • or in our beds.

  • Raising livestock and building cities which attract rodents

  • has provided billions of new hosts,

  • and you and your cat may be two of them.

Is there a disease that makes us love cats,

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B2 US TED-Ed parasite mouse offspring rodent litter

【TED-Ed】Is there a disease that makes us love cats? - Jaap de Roode

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    VoiceTube posted on 2016/07/22
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