A video floating around the internet late last month presented a creepy sight: A tangled mess of what appears to be rope flailing about on its own. Captured in Hsinchu, Taiwan by Huang Meilan, the video inspired much speculation. Some identified it as an "alien life form" or a mutant hybrid creature, but a more likely explanation is less otherworldly but equally creepy—a parasite.
"In that video you are definitely seeing one horsehair worm," Ben Hanelt, a biologist at the University of New Mexico, says after reviewing the video sent by Smithsonian.com.
Horsehair worms, part of the taxonomic phylum Nematomorpha, are parasitic worms that resemble long thin strands of hair (hence their nickname). The worms have largely featureless bodies because they're essentially a single "gonad," as Hanelt puts it. They do not eat; their only function is to breed.
The creatures start out as eggs laid in freshwater, where most species of horsehair worm primarily live. Those eggs hatch into tiny larvae, which then make their way into the bodies of invertebrates, often insects like crickets. From there, the larvae slowly grow into adult worms, leaching nutrients from their hosts but otherwise leaving them largely unharmed—until they're ready to break free.
As it approaches its final stages of life, the parasite seizes control of their host's body, Hanelt explained to Matt Simon of Wired in 2014. The worms control the minds of the insects, causing the hapless host to drown itself by jumping into water. Then the parasite will burrow out into the water and start looking for mates.
The worm depicted in the video, which Hanelt says appears to be female, is in this final stage of its life. But somehow it got stuck like a fish out of water.
"It's probably just thrashing around and trying to get back to some water before it dies," Hanelt says. Though the worm looks like a giant tangle of branches, the parasite itself is probably only the thick wiggly strand moving through the middle of the knot. The rest of this "alien" form seems to be something man-made, Hanelt says, speculating it might be the magnetic tape used in video or audio cassettes.
For horsehair worms knots are not unheard of. In the water, they often join into giant clumps that resemble the mythic "Gordian knot." Because of this, some viewers of the video suggested online that the wriggly mass could be several of the parasites tangled together. But Hanelt says these knots of worms usually separate out once on land as the worms struggle to return to the water.
Horsehair worms average about a foot in length in their adult form, but they can get much larger. According to Hanelt, one of his colleagues found horsehair worms nearly seven feet long in the wild.
While they sound like horrific versions of tapeworms, humans have little to fear from horsehair worms. The parasites have evolved to infect invertebrates only, and they have no capacity or desire to acquire a human host. There have been cases of that happening, however, perhaps from people ingesting the worms whole for some reason, Hanelt says. But in those cases, the worms are excreted in feces largely intact—if not vomited up before reaching that stage.
As Hanelt explains, the worms are made like "tanks," able to survive the harsh environment of your digestive tract. The only reported symptom for human ingestion of the parasite is some intestinal distress due to the flailing of the worms as shown in the video.
"They're just sort of passing through your system," he says.