Articles, Blog

How parasites change their host’s behavior – Jaap de Roode

August 23, 2019

Which of these entities
has evolved the ability to manipulate an animal
many times its size? The answer is all of them. These are all parasites, organisms that live on
or inside another host organism, which they harm and sometimes even kill. Parasite survival depends on transmitting
from one host to the next, sometimes through
an intermediate species. Our parasites elegantly achieve this
by manipulating their host’s behavior, sometimes through direct brain hijacking. For example, this is the Gordian worm. One of its hosts, this cricket. The Gordian worm needs water to mate,
but the cricket prefers dry land. So once it’s big enough to reproduce, the worm produces proteins that garble
the cricket’s navigational system. The confused cricket
jumps around erratically, moves closer to water, and eventually leaps in,
often drowning in the process. The worm then wriggles out to mate and its eggs get eaten
by little water insects that mature, colonize land, and are, in turn, eaten by new crickets. And thus, the Gordian worm lives on. And here’s the rabies virus,
another mind-altering parasite. This virus infects mammals, often dogs, and travels up the animal’s
nerves to its brain where it causes inflammation
that eventually kills the host. But before it does, it often increases
its host’s aggressiveness and ramps up the production
of rabies-transmitting saliva, while making it hard to swallow. These factors make the host
more likely to bite another animal and more likely to pass
the virus on when it does. And now, meet Ophiocordyceps,
also known as the zombie fungus. Its host of choice is tropical ants
that normally live in treetops. After Ophiocordyceps spores
pierce the ant’s exoskeleton, they set off convulsions
that make the ant fall from the tree. The fungus changes the ant’s behavior,
compelling it to wander mindlessly until it stumbles onto a plant leaf with
the perfect fungal breeding conditions, which it latches onto. The ant then dies, and the fungus parasitizes its body
to build a tall, thin stalk from its neck. Within several weeks,
the stalk shoots off spores, which turn more ants
into six-legged leaf-seeking zombies. One of humanity’s most deadly assailants
is a behavior-altering parasite, though if it’s any consolation, it’s not our brains
that are being hijacked. I’m talking about Plasmodium,
which causes malaria. This parasite needs mosquitoes
to shuttle it between hosts, so it makes them bite more frequently
and for longer. There’s also evidence
that humans infected with malaria are more attractive to mosquitoes, which will bite them
and transfer the parasite further. This multi-species system is so effective, that there are hundreds of millions
of malaria cases every year. And finally, there are cats. Don’t worry, there probably
aren’t any cats living in your body and controlling your thoughts. I mean, probably. But there is a microorganism
called Toxoplasma that needs both cats and rodents
to complete its life cycle. When a rat gets infected
by eating cat feces, the parasite changes chemical levels
in the rat’s brain, making it less cautious
around the hungry felines, maybe even attracted to them. This makes them easy prey, so these infected rodents
get eaten and pass the parasite on. Mind control successful. There’s even evidence that the parasite
affects human behavior. In most cases,
we don’t completely understand how these parasites manage
their feats of behavior modification. But from what we do know, we can tell that they have
a pretty diverse toolbox. Gordian worms seem to affect
crickets’ brains directly. The malaria parasite, on the other hand, blocks an enzyme
that helps the mosquitoes feed, forcing them to bite over
and over and over again. The rabies virus may cause
that snarling, slobbering behavior by putting the immune system
into overdrive. But whatever the method, when you think about how effectively these parasites control
the behavior of their hosts, you may wonder how much of human behavior
is actually parasites doing the talking. Since more than half of the species
on Earth are parasites, it could be more than we think.


  • Reply Victor Nilsson July 24, 2019 at 10:25 pm

    The last of us fan boi here.

  • Reply circle July 25, 2019 at 3:43 pm

    You forgot one more. My toxic friend.

  • Reply N Williams July 26, 2019 at 12:13 am

    Real question.. since rabies increase the immune system in to a hyperactive state, can rabies be used and manipulated in a way to kill cancer cells?

  • Reply It's Candy Time Boys and Girls July 26, 2019 at 3:42 pm

    My parents tell me I'm a parasite.

    I dunno why, I'm just planning to live inside the basement until I die.

  • Reply Andrei Luca July 26, 2019 at 6:42 pm

    I really like the animation . Keep going!!

  • Reply Ricardo Cabrera July 27, 2019 at 2:01 am

    Can you make a video about MRSA I'm survivor

  • Reply D V. July 27, 2019 at 3:50 am

    Me: mom dad I am staying inside for today! … Ok?

  • Reply ved pandhare July 27, 2019 at 12:08 pm


  • Reply Praise the sun July 27, 2019 at 8:05 pm

    Parasite causes me to feel dead inside
    That would have no effect on me
    Because I’m already dead inside

  • Reply Javier Rodriguez July 28, 2019 at 5:02 pm


  • Reply xX_Apophis_Xx July 29, 2019 at 9:17 pm

    2:10 me when my mom tells me to do my homework

  • Reply amarjeet singh July 30, 2019 at 10:47 am

    Venom has entered the chat.

  • Reply Shalin Chopra July 30, 2019 at 7:15 pm

    Actually none of these parasites "evolved" because no living creature has ever evolved unless you call micro variations within a species and non beneficial mutations evolving. Visit kent hovind
    For the truth about evolution

  • Reply Scarly Dahl August 4, 2019 at 5:05 am

    Nature: Go in water to reproduce
    Gordian worm: I’m about make this harder than this has to be.

  • Reply why lol August 4, 2019 at 5:24 am

    Is this commensalisn?

  • Reply 0bAm3 August 4, 2019 at 8:16 am

    the years have been hard on patrick i see

  • Reply jill August 4, 2019 at 1:16 pm

    lol the zombie one used an ant head on the table as cash

  • Reply 3APNHA August 5, 2019 at 4:51 am

    This is insane, I love TedEd

  • Reply jacqueline schael August 6, 2019 at 3:37 am


  • Reply Kristopher Salunga August 7, 2019 at 2:29 am

    They can cause malaria

  • Reply Ben Berkebile August 7, 2019 at 11:28 pm

    Wait is that mouse sentient than umm that mouse is a murderer

  • Reply Xuaeb Izec August 9, 2019 at 2:03 pm

    saw this at school

  • Reply Ibrahim Alomari August 10, 2019 at 10:02 am

    Anyone notice how smart they are ?

  • Reply Cheeseburger Monkey August 13, 2019 at 9:50 pm

    The nicest parasite
    -makes food for you
    -makes water for you
    -builds shelter for you
    -helps with your money
    receive an injection of parafeeder for only $0.99

  • Reply Marie Smith August 15, 2019 at 2:09 am

    Well, that was educational, but depressing,

  • Reply Smoke-This August 15, 2019 at 10:15 am

    Haha i remember when i was little and i understood dinosaurs were gone and I'd never see one, it kinda broke my heart..

  • Reply Glen Joseph August 15, 2019 at 11:57 am

    Every TED-Ed video I watch, I always hear this informative style voice which sounds very good for explaining stuff.

  • Reply Khaleesi Romaerys August 17, 2019 at 12:12 am

    I love coffee and foods. Maybe there's a parasite within me that loves coffee and foods.

  • Reply Anna Mathiang August 17, 2019 at 2:48 am

    I thought the tile said meet the plastics (mean girls)

  • Reply ana the cat gamer aj August 17, 2019 at 10:00 pm

    wait why did you guys did anthro animals ARE YOU GUYS FURRIES?! (luv the videos the animals are really cute)

  • Reply gamingFuria August 18, 2019 at 10:06 am

    Your animations are so interesting and fun to watch

  • Reply Janelle Ryan August 19, 2019 at 11:07 pm

    Why talk about parasites that effect animals.Cause human parasites that effect humans arent a issues right,No issues here. Lol

  • Reply claire dose that August 20, 2019 at 7:11 pm

    It's not a disease video without metioning plague Inc in the coments section.

  • Reply May Phan August 20, 2019 at 9:16 pm

    The three house-vise

  • Reply Derelict Rv August 21, 2019 at 3:57 am

    toxoplasma 100% affects humans, it messes with natural fear response, making a host 3 times more likely to get into a car accident.

  • Reply This Mc Mustard August 22, 2019 at 6:39 am

    Feeling itchy yet you should

  • Reply Sofia's Glitter Time TV August 22, 2019 at 12:19 pm

    flashes back to watching venom

  • Reply Sofia's Glitter Time TV August 22, 2019 at 12:20 pm

    flashes back to watching venom

  • Reply Sofia's Glitter Time TV August 22, 2019 at 12:20 pm

    flashes back to watching venom

  • Reply NO NAME August 23, 2019 at 12:49 pm

    i have plasmodium

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