Like all parasites that can live inside people, tapeworms probably fill you with disgust. But they’re also fascinating. Specially adapted and uniquely armored, they inhabit a menagerie of animals, including us. Here are some facts about our ancient companions.
1. TAPEWORMS ARE MORE THAN JUST WORMS.
People love to call pretty much any long, skinny animal a “worm.” There’s a legless lizard, for example, that we’ve named the slow worm. Inchworms are really moth caterpillars—they’ve even got legs. And tapeworms get their name because they’re long, thin, and flat like tape.
But if you take a closer look at a tapeworm (eww), you’ll find that they’re unique parasitic specialists, only distantly related to the familiar earthworms in your garden. For example …
2. THEIR HEADS HAVE SUCTION CUPS AND GRAPPLING HOOKS.
Adult tapeworms live in animal intestines—and that’s not an easy place to call home. For one thing, it’s regularly rocked with muscular contractions that push along digestive material and waste. Tapeworms need to hang on for dear life.
To do this, they’ve evolved specialized “heads” that carry an arsenal of attachment devices. Some have spines or retractable hooks. Some have circular suckers or grooves that work like suction cups. Many have a combination of these. No matter what weapons the tapeworm has at its disposal, the purpose is the same: to keep it anchored in the intestinal wall so that its body can dangle free and soak up nutrients. Yum!
3. THEY WEAR THEIR STOMACH ON THE OUTSIDE.
Tapeworms don’t have a gut of their own. Instead, they use their specialized outer surface to absorb nutrients and excrete waste. And they do it pretty efficiently.
Your small intestine has finger-like bumps called villi that basically add more intestinal surface, creating extra space for absorbing food. Tapeworms are also covered in little villus-like bumps that help them soak up more of whatever their host is eating.
4. MUCH OF THEIR BODY IS FOR MAKING BABY TAPEWORMS.
Beyond the “head” and a sort of “neck,” a tapeworm is just a series of segments, each with its own male and female sexual parts. As the tapeworm adds newer segments near the head, older segments move down the body. Eventually, they mature and make eggs. The parasite’s host then poops out either eggs or egg-filled segments … and that’s when things get interesting.
5. TAPEWORMS USUALLY NEED MORE THAN ONE HOST.
Those new baby tapeworms are about to take an amazing cross-species journey. Most tapeworms hang out in two or three types of animal hosts across their lifespan. The first host gets infected by eating eggs from the environment, and the subsequent hosts get infected by… eating the previous hosts. Tapeworms only grow to their full adult form—and reproductive ability—in their final animal home.
6. YOUR FAVORITE ANIMAL GETS TAPEWORMS.
Humans aren’t the only animals that get tapeworms—not by a long shot. These parasites are found across much of the animal kingdom. Dogs and cats can get them. Birds, too. Fish get tapeworms, as do hyenas, antelopes, moose, wolves, insects such as beetles and ants … and the list just goes on.
7. TAPEWORMS CAN BE HUGE.
Larger animals have larger intestines—and bigger tapeworms. Whales get tapeworms, and these onboard companions can grow to 100 feet or more. That’s over twice the length of the longest giant squid ever discovered.
8. THEY’RE ANCIENT.
Not only are tapeworms everywhere, but they’ve also been around for a loooong time. In 2013, scientists from Brazil announced the discovery of tapeworm eggs in fossilized poop from a 270-million-year-old shark. For context, that’s tens of millions of years before the first dinosaurs walked the Earth.
9. THEY'VE BEEN ALL OVER THE PLACE.
As Carl Zimmer writes in his book Parasite Rex, there’s a rare tapeworm group that inhabits marsupials in both Bolivia and … Australia. How did these parasites get to two such faraway spots without hopping on a jet plane?
The answer is continental drift. Long ago, the continents of South America and Australia were smushed together, and an ancestral marsupial—with an ancestral tapeworm—roamed their forests. When the continents split up, so did marsupial populations. And the tapeworms came along for the ride.
10. THEY CAN MANIPULATE THEIR HOSTS.
Tapeworms rely on their first host getting eaten. And some of them don’t just sit around and wait for their lucky day (and their host’s unluckiest one).
For example, one tapeworm species needs three hosts across its lifespan: first a crustacean, then a stickleback fish, and, finally, a bird. While it’s inside the fish, the tapeworm encourages its host to seek out the warmer waters that help tapeworms grow. An infected stickleback becomes larger and more lethargic, which makes it an easier target for predatory birds.
Another tapeworm species exerts similar mental pressure on ants, making them less likely to flee when a hungry bird drops by for a snack. It also turns the ants yellow for reasons that aren’t clear—but probably aren’t purely aesthetic.
11. IF YOU GOT A TAPEWORM, YOU PROBABLY WOULDN’T FEEL SICK.
People usually get these parasites by eating raw or undercooked meat, such as beef, pork, or fish. Sometimes people with tapeworm infections will feel weak or nauseated, and they may experience anemia or a vitamin B-12 deficiency. But most of the time, an infected human won’t feel any different. Often, the only sign is those yucky but harmless tapeworm segments in poop.
As a side note: anyone can get these and other tapeworm infections, but they are sadly much more common in developing nations—and in underserved communities with poorer sanitary infrastructure.
12. BUT SOME INFECTIONS CAN CAUSE PROBLEMS.
Here’s where it gets (really) gross. Improper sanitation may lead someone to consume tapeworm eggs that came out of another person. The larval tapeworms then hatch and start roaming the body, looking for a safe place to hide out. They can latch onto muscle tissue, brain tissue (where they cause seizures), and other body parts, causing an infection called Cysticercosis. It’s relatively rare in the United States, though the CDC has labeled it one of five neglected parasitic infections—ailments that need more attention.
13. ONE TIME, A TAPEWORM GAVE SOMEONE CANCER.
In 2013, a 41-year-old Colombian man showed up at the doctor’s, and tests showed that he had many cancerous tumors. But a closer look revealed something peculiar: the cancer cells were too small to be human. That man also had a tapeworm infection—and those cells were from his tapeworm. Yes, a tapeworm gave him cancer.
There’s no need to worry that something similar will happen to you. This event was astonishing, and astonishingly rare.
14. NO, YOU CAN’T DRAW OUT A TAPEWORM WITH MILK.
There’s a persistent myth that placing bowl of warm milk in front of an infected person’s mouth will cause their tapeworm to crawl out. While this may attract some nearby stray kittens, it won’t work on a tapeworm.
Here’s the thing: adult tapeworms hang out in your intestines. To reach the mouth, they’d have to crawl through a veritable gauntlet of digestive tubing, squeeze through a sphincter up into the stomach and scale the mighty esophagus. Oh, and they’d have to smell the milk through all of those guts. Tapeworms are very happy staying put, thank you.
If you suspect you have a tapeworm infection, visit a doctor for the proper prescription and let medical science remove your unwanted passenger.
This article has been corrected from the original version.