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Wikimedia Commons // Public domain

14 Surprising Truths About Tapeworms

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Wikimedia Commons // Public domain

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.


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 …


Head of a Taenia solium, or pork tapeworm. Image credit: Rjgalindo via Wikipedia // CC BY-SA 3.0

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!


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. 


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. 


Tapeworm in a human appendix. Wikimedia Commons // Public Domain

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. 


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. 


AdultTaenia saginata tapeworm. Wikimedia Commons // Public Domain

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.


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. 


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. 


A kingfisher holding a stickleback fish. Image credit: Getty Images

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. 


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. 


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.


Adult Hymenolepis nana tapeworms. Georgia Division of Public Health via Wikimedia  // Public Domain 

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. 



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.

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Big Questions
Should You Keep Your Pets Indoors During the Solar Eclipse?
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By now, you probably know what you’ll be doing on August 21, when a total solar eclipse makes its way across the continental United States. You’ve had your safety glasses ready since January (and have confirmed that they’ll actually protect your retinas), you’ve picked out the perfect vantage point in your area for the best view, and you’ve memorized Nikon’s tips for how to take pictures of this rare celestial phenomenon. Still, it feels like you’re forgetting something … and it’s probably the thing that's been right under your nose, and sitting on your lap, the whole time: your pets.

Even if you’ve never witnessed a solar eclipse, you undoubtedly know that you’re never supposed to look directly at the sun during one. But what about your four-legged family members? Shouldn’t Fido be fitted with a pair of eclipse glasses before he heads out for his daily walk? Could Princess Kitty be in danger of having her peepers singed if she’s lounging on her favorite windowsill? While, like humans, looking directly at the sun during a solar eclipse does pose the potential of doing harm to a pet’s eyes, it’s unlikely that the thought would even occur to the little ball of fluff.

“It’s no different than any other day,” Angela Speck, co-chair of the AAS National Solar Eclipse Task Force, explained during a NASA briefing in June. “On a normal day, your pets don’t try to look at the sun and therefore don’t damage their eyes, so on this day they’re not going to do it either. It is not a concern, letting them outside. All that’s happened is we’ve blocked out the sun, it’s not more dangerous. So I think that people who have pets want to think about that. I’m not going to worry about my cat.”

Dr. Jessica Vogelsang, a veterinarian, author, and founder of pawcurious, echoed Speck’s statement, but allowed that there’s no such thing as being too cautious. “It’s hard for me to criticize such a well-meaning warning, because there’s really no harm in following the advice to keep pets inside during the eclipse,” Vogelsang told Snopes. “It’s better to be too cautious than not cautious enough. But in the interest of offering a realistic risk assessment, the likelihood of a pet ruining their eyes the same way a human would during an eclipse is much lower—not because the damage would be any less were they to stare at the sun, but because, from a behavior standpoint, dogs and cats just don’t have any interest in doing so. We tend to extrapolate a lot of things from people to pets that just doesn’t bear out, and this is one of them.

“I’ve seen lots of warnings from the astronomy community and the human medical community about the theoretical dangers of pets and eclipses, but I’m not sure if any of them really know animal behavior all that well," Vogelsang continued. "It’s not like there’s a big outcry from the wildlife community to go chase down coyotes and hawks and bears and give them goggles either. While we in the veterinary community absolutely appreciate people being concerned about their pets’ wellbeing, this is a non-issue for us.”

The bigger issue, according to several experts, would be with pets who are already sensitive to Mother Nature. "If you have the sort of pet that's normally sensitive to shifts in the weather, they might be disturbed by just the whole vibe because the temperature will drop and the sky will get dark,” Melanie Monteiro, a pet safety expert and author of The Safe-Dog Handbook: A Complete Guide to Protecting Your Pooch, Indoors and Out, told TODAY.

“If [your pets] have learned some association with it getting darker, they will show that behavior or at a minimum they get confused because the timeframe does not correspond,” Dr. Carlo Siracusa of Penn Vet Hospital told CBS Philly. “You might put the blinds down, but not exactly when the dark is coming but when it is still light.” 

While Monteiro again reasserts that, "Dogs and cats don't normally look up into the sun, so you don't need to get any special eye protection for your pets,” she says that it’s never a bad idea to take some extra precautions. So if you’re headed out to an eclipse viewing party, why not do your pets a favor and leave them at home. They won’t even know what they’re missing.

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Big Questions
Why Can't Dogs Eat Chocolate?
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Even if you don’t have a dog, you probably know that they can’t eat chocolate; it’s one of the most well-known toxic substances for canines (and felines, for that matter). But just what is it about chocolate that is so toxic to dogs? Why can't dogs eat chocolate when we eat it all the time without incident?

It comes down to theobromine, a chemical in chocolate that humans can metabolize easily, but dogs cannot. “They just can’t break it down as fast as humans and so therefore, when they consume it, it can cause illness,” Mike Topper, president of the American Veterinary Medical Association, tells Mental Floss.

The toxic effects of this slow metabolization can range from a mild upset stomach to seizures, heart failure, and even death. If your dog does eat chocolate, they may get thirsty, have diarrhea, and become hyperactive and shaky. If things get really bad, that hyperactivity could turn into seizures, and they could develop an arrhythmia and have a heart attack.

While cats are even more sensitive to theobromine, they’re less likely to eat chocolate in the first place. They’re much more picky eaters, and some research has found that they can’t taste sweetness. Dogs, on the other hand, are much more likely to sit at your feet with those big, mournful eyes begging for a taste of whatever you're eating, including chocolate. (They've also been known to just swipe it off the counter when you’re not looking.)

If your dog gets a hold of your favorite candy bar, it’s best to get them to the vet within two hours. The theobromine is metabolized slowly, “therefore, if we can get it out of the stomach there will be less there to metabolize,” Topper says. Your vet might be able to induce vomiting and give your dog activated charcoal to block the absorption of the theobromine. Intravenous fluids can also help flush it out of your dog’s system before it becomes lethal.

The toxicity varies based on what kind of chocolate it is (milk chocolate has a lower dose of theobromine than dark chocolate, and baking chocolate has an especially concentrated dose), the size of your dog, and whether or not the dog has preexisting health problems, like kidney or heart issues. While any dog is going to get sick, a small, old, or unhealthy dog won't be able to handle the toxic effects as well as a large, young, healthy dog could. “A Great Dane who eats two Hershey’s kisses may not have the same [reaction] that a miniature Chihuahua that eats four Hershey’s kisses has,” Topper explains. The former might only get diarrhea, while the latter probably needs veterinary attention.

Even if you have a big dog, you shouldn’t just play it by ear, though. PetMD has a handy calculator to see just what risk levels your dog faces if he or she eats chocolate, based on the dog’s size and the amount eaten. But if your dog has already ingested chocolate, petMD shouldn’t be your go-to source. Call your vet's office, where they are already familiar with your dog’s size, age, and condition. They can give you the best advice on how toxic the dose might be and how urgent the situation is.

So if your dog eats chocolate, you’re better off paying a few hundred dollars at the vet to make your dog puke than waiting until it’s too late.

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