Horsehair worms can control a host's brain and steer it to water, where it drowns.
Scientists have studied the worms' mind-controlling power for years. But what's really going on?
New research shows these clever creatures figured out how to steal DNA in order to manipulate hosts.
Horsehair worms look like simple creatures on the surface, but they're actually some of nature's most sophisticated bodysnatchers.
These parasitic worms, which resemble dark and stringy horse hair, take control of their host's brain and drive it to suicide.
Scientists have been studying hairworms and their mind-control for years, but how these clever highjackers achieve such a feat, at molecular scales, was a scientific mystery until recently.
Hairworms' magic copy-cat trick
As larvae, a hairworm's first mission in life is to get eaten, usually by a tadpole or young mosquito. But it's not until that animal gets eaten by a larger land insect, such as a cricket, millipede, grasshopper, or mantis, that the worm takes over.
After the insect's digestive juices dissolve the first host's body, the horsehair worm breaks out Trojan-horse style. It saps its new host's nutrients, for about three months, before driving the host to drown itself.
Hairworms both breed and lay eggs in the water, so after their host kills itself, the worm can swim over to the nearest ball of orgy-ing hairworms to start the cycle anew.
The researchers found that worms mimic chemicals in their host's body that act just like the host's neurotransmitters — chemical messengers that help the brain communicate with the rest of the body.
In the study, researchers examined the genetic code of the horsehair worm species Chordodes fukuii and its favorite host, mantises, aka mantids.
They found that when Chordodes manipulate a mantis toward suicide, the worm's genome undergoes a lot of changes. Nearly 3,200 of its genes become more active and start producing more proteins, while around 1,700 genes become less active.
Meanwhile, researchers observed minimal change in the genes of the mantis.
This gave the scientists a key clue to discover how the worms take control: If the only changes happened within the Chordodes' genome, this meant the Chordodes controls the mantis by creating its own chemicals instead of using its host's.
The worms were likely able to produce these mind-controlling chemicals by stealing copies of mantis DNA via a process called horizontal gene transfer, said study author Tappei Mishina. This idea was further supported by the fact that the researchers found 1,420 Chordodes genes closely matched mantis genes.
Usually, genetic code transfers vertically from parent to child, but in horizontal gene transfer, the Chordodes nabs the mantis DNA directly.
Hairworms aren't the only organisms that can do this — viruses do it a lot. But hairworms are definitely one of the larger, more complex organisms to demonstrate this ability.
Rather than committing one giant DNA heist, Mishina said the Chordodes likely stole bits of genetic material that accumulated over generations. This DNA gave them the tools they needed to brainwash their hosts.
3 ways hairworms may manipulate a host
The study authors went one step further to try and figure out what exactly these chemicals are doing to the mantis' brain to control it. They linked the hairworm's genetic changes to three potential mechanisms.
The first mechanism is manipulating neurotransmitters like dopamine.
"We found extremely high dopamine levels from some manipulated mantids compared with un-manipulated and un-infected mantids," Mishina said, adding that more research is needed to know for sure. But previous studies have shown that dopamine drives movement and motivation in insects, much like it does in humans.
Another possible way, the authors report, is that a worm also activates genes that encourage the mantis to move toward light, ideally light with horizontal polarization, like what's reflected off bodies of water.
Lastly, the study authors found genetic changes linked to the mantis' circadian rhythm. Horsehair worms make their hosts more active in the middle of the day, when it's easier to find light reflecting off water. The more the host moves around, the more likely they find a suitable place to drown.
Should humans worry?
Fortunately, there are no reports of horsehair worms brainwashing humans, according to the University of California's Integrated Pest Management Program.
If you somehow ate an insect that was infested with one, the worst you would probably get is some mild intestinal upset.
Hairworms only mimic chemicals from their favorite species of host. So the Chordodes worms can only control mantises and can't puppeteer other insects or mammals, according to the study.
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