Before we get started, here's a message for the adults who are listening.
Support for But Why comes from Progressive Insurance.
Do you ever think about switching insurance companies to see if you could save some cash?
Progressive makes it easy to see if you could save when you bundle your home and auto policies.
Try it at Progressive .com.
Progressive Casualty Insurance Company and Affiliates.
Potential savings will vary, not available in all states.
This is But Why, a podcast for curious kids from Vermont Public.
I'm Jane Lindholm. On this show, we take questions from curious kids just like you, and we find answers.
In making today's episode, I learned a lot, and I had my mind kind of changed.
So I want to know at the end of the episode if you experienced the same thing.
Let me know. Have you ever heard the word parasite?
A parasite is, by definition, bad.
At least, it's bad for its host.
A parasite is a living organism that uses a different organism.
It either lives in or on this other organism called the host and the parasite uses its host to gain food or be able to reproduce in a way that's bad for the host.
Basically, the parasite thrives at the expense of the host.
Sounds bad, right? And it can be really bad.
Sometimes parasites make their hosts too weak to eat.
Sometimes they even kill their hosts.
And sometimes humans can be the hosts to parasites.
There's a lot of medicine designed to help us fight off and get rid of parasites.
Of course, parasites aren't always super dangerous.
Sometimes they're just really annoying and itchy, like head lice.
But still, I would say that those head lice are still thriving at our expense, right?
But our special guest today loves parasites.
She says they're fascinating and often very beautiful.
And she says they're really important, even good for our planet.
She says without parasites, our ecosystems would fall apart.
So without further ado, let's meet this scientist and see if she can convince you to at least appreciate, if not love, the world of parasites.
I'm Chelsea Wood. I'm an associate professor in the School of Aquatic and Fishery Sciences at the University of Washington.
But I prefer to be known as a parasite ecologist because parasites are my favorite things.
What does that mean exactly?
Well, an ecologist is a person who studies living things, organisms, where they occur in the world and why.
And a parasite ecologist is someone who studies that question, but for parasites.
Chelsea is also the author of a book for kids called Power to the Parasites.
I read it and I learned a lot.
We already went over what a parasite is, but let's hear it again just so we're clear.
Parasites are organisms that live in or on other organisms and cause harm to those hosts that they're living in or on.
And so my research extends from worms and other creepy crawlies that you'd find in your gut to ticks and lice crawling through your hair.
But mostly I study the parasites of wildlife, not of people.
Parasites are alive, but they aren't all animals.
They can also be plants or other living organisms.
And lots of things behave in parasitic ways.
Our definition of parasite encompasses parasites that are animals like worms, ticks, lice, but also things that aren't animals like bacteria, fungi, protozoa, viruses.
So the term parasite applies to a whole broad swath of the tree of life.
Why do they need a host?
What do we mean when we say a parasite needs a host?
It's just another way of making a living in the world.
You know, a lot of us are familiar with the term predator.
A predator is an organism that eats other organisms.
And a parasite is a similar kind of category.
Parasitism is a way of living in the world and a way of finding nutrition in the world.
For parasites, they are living in or on their host and eating little bits of that host.
For a predator, they don't live in or on their host, but they eat the whole thing when they come across it.
So it's just a different way of eating and living in the world.
How long do parasites live?
Oh man, it really depends.
Anywhere from a couple of minutes to decades.
Wow. There are many human parasites where we know that they can live inside of their host for 30 years, sometimes more.
Sometimes we are not sure how long their lifetime could extend because, of course, they depend on the host.
So if the host dies, the parasite dies with it.
It really depends on the parasite.
they are so different from one another, and they encompass the full range of lifetimes from very short to very long.
But there are some that only live for minutes, and that's their whole life cycle?
Yeah, that's their whole life stage.
There are lots of parasite species that use transmissive stages, basically like little larval stages that are out in the environment for a period of time in search of a host.
And those larval stages can be very short -lived.
They can have only a matter of minutes to hours to find the right host, and if they don't, they die.
Now, you might think we're just doing this episode to tell you all about parasites and that none of you has ever sent us a question about parasites.
Oh, no, no, no. We've gotten lots of questions about parasites.
One type that a lot of you have asked us about is called a tapeworm.
There are several thousand species of tapeworm, and they generally spend their adult stage inside the intestines of another animal.
There are a few that sometimes live inside us humans, but it's really rare for a human to get a tapeworm.
For humans that do get infected, it's often because they've eaten raw or undercooked beef or pork.
My name is Trinity, and I'm nine years old, and I live in canada and my question is are tapeworms sticky oh great question no no you'd think from looking at a picture of them that they might be sticky because they look like a spool of tape right they look like what tape looks like after you've unspooled
it from its roll and they kind of glisten a little bit so i can imagine you might think that they are sticky but they're actually slippery and slimy.
I'm Oliver, and I'm five years old.
I live in Bloomfield, New Jersey, and my question is, how do tapeworms need to be so long?
Why do tapeworms need to be so long, wonders Oliver.
Tapeworms are really interesting because while they look like an unspooled strip of tape, they're actually made up of tens, sometimes hundreds, of individual segments all strung together along that length of tape.
And each one of those segments is a separate reproductive unit with its own male and female parts.
And each one of those little units, which the technical term for them is proglottid, each one of those proglottids can break off from the larger tapeworm tape and go out of the gut that it's living in, in the poop of the host, and out into the world.
So the reason that tapeworms are long is that the longer they are, the more capacity they have to reproduce, the more babies they can make.
And so there's a lot of incentive for a tapeworm to be long because every tapeworm wants to make lots of babies.
What kind of hosts do tapeworms live in?
Humans can be hosts for some tapeworms, right?
Absolutely. We can be hosts for a couple different species of tapeworms.
Tapeworms usually mature in the gut of a vertebrate, so an animal with a backbone.
Those tend to be the hosts for the adult form of the tapeworm, which is the form that we're talking about, the one that takes that long shape of an unspooled strand of tape.
But tapeworms actually evolved alongside sharks.
When they first evolved, sharks were their only hosts.
And since then, they've evolved to use many different species of vertebrates.
But because they originally evolved alongside sharks, still the vast majority of tapeworm species use sharks as their final hosts.
Hello, my name is Bobby.
I am 11 years old and I live in Nigeria.
I want to know why worms are in our stomach.
That's an example of species just trying to make their way in the world.
Wherever there is a resource that can be used, you will find a species trying to use that resource.
A term that we use in biology a lot is nature abhors a vacuum, which means as soon as there is a resource that's not being used, some species is going to figure out a way to use that resource.
Can I stop you to just say, I love the word abhors, and it's a really great vocabulary word.
It's spelled A -B -H -O -R -S, and it means hates, like really hates.
So if you say, I abhor broccoli, you're saying, I really hate broccoli.
So when nature abhors a vacuum, it doesn't like when there's a vacuum.
Exactly. It hates it deeply.
So in this sense, vacuum means an empty space.
Nature will figure out a way to use that empty space.
And your stomach could be considered an empty space, even if it's full of food, at least as far as parasites are concerned.
Your stomach is full of food.
Food for the taking.
And of course a parasite is going to evolve to use that food because nature abhors a vacuum.
But we just did an episode about how food turns into poop.
And one of the things that we talked about was that you have a lot of bacteria in your gut that's really good.
So those bacteria aren't parasites.
That's right. Yeah, that's why our definition of parasitism has to involve the effect it has on a host.
A parasite has a negative effect on a host 100 % of the time.
If it doesn't have a negative effect on a host, it's not a parasite.
And so those bacteria that have a beneficial effect, we would actually call them mutualists because they're living in or on the host, but they're having a beneficial effect for that host.
There can also be bacteria living in your gut that really don't benefit or hurt you, and we call those commensals.
So from parasites to commensals to mutualists, we go from organisms that have a negative effect to no effect to a positive effect.
Grossed out yet? Coming up, we're going to talk about other parasites that might creep you out even more, lice and ticks.
But we're also going to see if Chelsea can convince you that parasites are cool.
This is But Why, a podcast for curious kids.
I'm Jane Lindholm. We're learning about parasites with parasite ecologist Chelsea Wood today.
Before we get back to your questions, are you curious how someone like Chelsea even becomes a parasite scientist?
When she was your age, she wanted to study whales and dolphins.
I got involved in a project early on in my career because I wanted to do marine biology and it happened to be a parasite project.
And I went into this project thinking parasites were gross.
And I was honestly pretty disappointed that the marine project that I had drawn was one that involved parasites because I thought that it was going to be boring and I was going to have to look at a bunch of disgusting worms and I just was not into it.
And as I got deeper and deeper into this project, I realized how amazing these things were.
And one of the things that really drove that home for me was that when I was a kid, I used to collect snails into buckets.
And I lived on the East coast of the United States where the most common snail was the common periwinkle.
And I probably put hundreds of these into my bucket over the course of my childhood.
And then as I was working on this project, my job was to crack these snails open, same species, and to look at the parasites that were inside of them.
It was the first time that I'd ever done that, and I realized that they were full of worms.
I'd been looking at these snails for decades and missing the most interesting part.
And it made me realize that, you know, when you look across a beautiful landscape, you see trees and deer and rabbits and all that stuff is obvious to the naked eye, but what you don't see is that each one of those things is basically a package of parasites walking around in the world.
And you just have to look only a tiny bit below the surface to see this whole other world of parasites that exists just beneath what's familiar to all of us.
Okay, so that's how Chelsea got hooked.
Here's her pitch to maybe hook you.
One of the things that I think people don't appreciate about parasites is that they are really, really beautiful.
I know it sounds strange when I say that because when you look at a worm in your hand, it's slimy and it writhes.
It doesn't look like anything beautiful doesn't enter into the equation at all.
But when you look at those worms under the microscope, when you look at them at their own scale, at the scale where they exist, you see these spectacular adaptations to the parasitic life that have evolved over millennia.
And it's just mind -boggling to see how perfect they are for the things that they need to do.
And a great example is one of the shark tapeworms.
This is a group of tapeworms in the order Trepenarenca.
Informally, we call them the Trepenarenc tapeworms.
And they're one of those very, very old orders of tapeworms that stuck around with the sharks instead of evolving to use other groups of organisms.
And they are perfectly adapted to living in the shark gut, which as you can imagine, just like picture yourself inside a shark gut.
It's dark, it's slippery, and there's this constant tide of poop running in one direction that if you didn't resist it would take you straight out and into the ocean.
And that would be death for a tapeworm.
It can only survive in the gut of a shark.
I'm starting to regret imagining myself inside of a shark.
It's a hard place to live for a lot of reasons, but the Trepenarynx have evolved these beautiful structures that allow them to resist the tide of poop that's constantly threatening to blast them out into this hostile ocean.
What they have are these four tentacles on their head end that they are able to kind of like pull inside of their head, but they can also like shoot them out really fast if they need to.
They're coiled up and each tentacle is armed with thousands and thousands of backward facing spikes.
So what this tapeworm can do is, you know, as it wants to move around in the shark intestine, it can burst those tentacles right out of its head end and lay them down on the gut wall of the shark.
And then it can use its hooks to like form a really close attachment to that gut wall and then pull the tentacles back into its head, securely anchoring itself to the gut wall of the shark.
And in that way, it can kind of crawl up if it wants to against the tide of poop.
It can move around and it can always make sure that it maintains a close connection with that gut wall, ensuring that it doesn't get burst out of that shark into the hostile ocean.
So not only is it really good at doing this, but if you look at these tentacles under the microscope.
They are just so beautiful.
People are always really surprised.
These animals are just spectacularly beautiful.
And I think it's a real shame that more people don't appreciate them.
That is disgustingly amazing to think about a river of fast -flowing poop that these parasites are able to resist.
All right, what do you think?
Disregard the part about the river of poop and just concentrate on how impressive it is that these parasites are able to survive and thrive in such a harsh environment let's get back to some of your other questions hi but why my name is leeway i'm nine years old i live in canvra australia and my question is what is headlice and how
does it spread my name is ramona i'm six years old i'm from providence rhode island my question is where did the first lice come from Hi, I'm Hazel from Austin, Texas, and I'm 10 years old.
My question is, how did the first person get head lice, and how have they survived all these years?
Thank you. My name is Luda.
I'm from order Minnesota, and I'm five years old.
My question is, why do lice eggs itch?
That's a great question.
Head lice are one species of many, many species of lice in the world.
And this one has evolved to exploit humans only.
Head lice can survive nowhere except on a human head.
But they're certainly not the only species of louse in the world.
Lots of birds have their own host -specific species of lice, meaning their lice can only survive on that one species of bird and nowhere else.
And there are many, many more examples among mammals and other vertebrates.
So our head lice are a very special louse species that can only use us, and they live among the hair shafts of our head, and they eat little bits of dead skin that fall off of your head.
They eat living skin, and they also sometimes eat our blood.
And that's part of what causes the itching associated with having a head full of head lice.
These little bugs eating the skin off of your scalp.
Now luckily, they're pretty easy to get rid of, but that doesn't mean they're not annoying while you have them.
Human head lice and body lice have evolved alongside humans, so we've had lice for about as long as we've been human.
Let's talk about some other things that like to make us itch.
fleas, ticks, and mosquitoes.
Hi, my name is Marlo.
I am five years old.
I have an awesome tick list.
And my question is, are fleas related to mosquitoes?
Distantly, but yes.
Yeah, fleas are insects, just like mosquitoes are.
But there's many, many thousands of years of evolution between the fleas and the mosquitoes.
And their habits of drinking blood are things that have evolved independently in those two groups.
So it's not a trait that they share way back in their evolutionary history.
It is something that each group evolved independently.
Hi, my name is Lily.
I live in Maryland and I am six years old.
And my question is, why do fleas bite cats and dogs and not people?
Unfortunately, fleas do also bite people.
If your dog or cat has fleas, you also can be bitten by their fleas.
Now, the fleas prefer dogs and cats, so they're less likely to bite you than they are to bite your dog or your cat.
But that doesn't mean that they won't bite you.
In fact, my brother's dog had fleas a couple of months ago, and we all got bitten by her fleas, unfortunately.
That is not fun. Hi, my name is Ginny.
I'm seven years old.
I live in Front Hill, Ontario, and my question is, why is there such a thing as ticks and mosquitoes?
Why is there such a thing as ticks and mosquitoes?
We talked about fleas and mosquitoes.
What about ticks and mosquitoes?
Any connection? Yeah, they are also related to one another, although distantly.
And the reason that they exist is the same reason why worms in our stomachs exist.
Nature abhors a vacuum.
Nature hates a vacuum.
We're walking around big packages of blood, and blood is so nutrient rich.
It can sustain many, many flea and tick and mosquito lives.
And so why wouldn't a species evolve to be able to use that food, to be able to use that resource?
If they can manage to get at it, then they will absolutely use it.
My name is Theo, and my age is three.
And I live in South I'm Karen, Canada, Saskatchewan.
And my question is, why do ticks bite you and also suck your blood?
Hi, my name is Lars.
I'm five years old.
I live in Minneapolis.
And my question is, why do ticks suck blood?
My name is Jasper. I live in Georgia.
And my question is, why do ticks need blood to survive?
Ticks actually have a totally fascinating life cycle.
usually involving three different stages.
They're born out of eggs, and the eggs are laid underneath leaves, usually on the forest floor.
Those eggs hatch out these teeny tiny larvae, and they're so small that they're hard even to see on your skin with the naked eye.
Typically, they're too small to ever be picked up by a person because they're just like down underneath the leaves.
So they'll usually feed on something small that's skittering around underneath that leaf litter like a mouse.
Once the larvae take that blood meal from a mouse, they drop off and they do this thing called molting, which is where they shed their exoskeleton, they shed the outer layer of their body and grow into the next stage, which is called a nymph.
The nymph is a little bit bigger and it can feed on humans, but lots of other mammals and birds.
It takes another blood meal, it drops off, it then goes through that period of molting again, shedding its skin, and then it becomes an adult, which is a much bigger animal.
Now the adults, because they're bigger, have to take a bigger blood meal, which means that they need a bigger host.
And so they'll typically feed on larger mammals like deer, and then they'll drop off.
And once they've had that third and final blood meal, as an adult, they're able to mate and produce eggs, and the life cycle begins again.
So at each stage of the tick's life cycle, it has to take a blood meal.
And that blood meal is what fuels its ability to either molt or produce a batch of eggs.
Hi, my name's Holden, and I'm from Ilgin, Illinois, and I'm five.
And my question is, how do ticks get Lyme disease, and how do they spread it to other people?
Many kids know that some ticks can make people sick.
One of the diseases you can get from a tick is called Lyme disease, and there are others that ticks can carry and that humans can get.
So we have several kids who want to know how do ticks get Lyme disease or other diseases and how do they spread it to other people?
How do they make us sick?
Yeah, ticks are full of all kinds of different bacterial and viral parasites that they can pass on to people and other mammals.
And that's part of the reason why it's really good to check yourself for ticks after you've been in a place where you know that they're hanging around.
You want to pull them off before they're able to feed on your blood and transmit any of those viruses or bacteria to you.
Now for Lyme disease, that's typically transmitted to people in the bite of the black -legged tick in the eastern part of North America and then different species of tick here on the West Coast.
But all those ticks go through the kind of life cycle that I just described where they're feeding on rodents like mice and rats in their first larval stage and then feeding on people only once they've molted and become nymphs.
And that's where Lyme disease comes in because mice are really, really good hosts for the Lyme disease bacterium.
They can replicate tons and tons of that bacterium in their bodies.
And so when they're fed on by a larval tick, the larval tick It takes that blood into its body, and it is now infected with the Lyme disease bacterium.
And after it molts and becomes a nymph, it can now pass that bacterium on to the next host that it feeds on.
Now, the nymphs are the most dangerous for people because they're still pretty small.
And that means that most people who contract Lyme disease are getting it from a nymphal tick that's fed on a rodent and then on them.
And they're not able to notice it before it's too late because that nymph is too small and too difficult to notice.
Now, Lyme isn't the only thing that can be transmitted to people in the bite of ticks, but that's generally how it works, where the ticks are picking up that virus or bacterium in their larval stage and then passing it along when they're nymphs.
Hi, my name is Karen and I'm five years old.
I live in Lewis, Delaware.
And my question is, why do tick bites hurt or stain?
That's a good question because it seems like the tick would want to be more stealthy and not let you know that it's there by making—same with a mosquito.
Why do they let you know that you've been bitten?
They don't want to.
Ticks and mosquitoes and, you know, all biting arthropods have some strategies that they use to reduce the pain or at least the initial pain of the bite so that they can feed undetected.
You know, mosquitoes have those delicate little feet so that when they land on you they cause as little disturbance as possible.
Ticks, their saliva is full of numbing agents that are intended to make the bite totally painless.
But nothing's perfect, and they can still make mistakes and not have their chemicals perfectly tuned to the host's nervous system, their ability to sense that pain or that itchiness.
And so the initial pain of the bite is often a mistake on the part of the arthropod.
But then once it's done feeding, it doesn't care what you feel, right?
It doesn't care if you're itchy or if the bite hurts because it's already had its blood meal and is off and safe.
And so often the itchiness or pain you feel of a bite after the mosquito or tick is done feeding is your immune system reacting to the fact that a foreign object has been pushed into your body.
It's your immune system trying to protect you against all the stuff that's been introduced in the breach of your skin in that bite.
So maybe you should think of an itchy mosquito bite.
The itching is actually a good thing.
That's your body going to work.
Exactly. A lot of what we've been talking about sounds pretty scary, especially when we talk about human diseases and the idea that for every parasite, they are bad for the host species.
should we be afraid of parasites?
Absolutely not and I'm not afraid of parasites.
I think parasites are amazing and obviously we should be doing everything that we can to get rid of our own parasites.
If I had a tapeworm in my gut, I would definitely take medicine to get rid of it.
But just because parasites are bad for their hosts doesn't mean that they're bad in general.
Think about a predator, right?
Like a lion stalking a gazelle on the savanna.
that lion is very bad for the gazelle.
If it manages to catch the gazelle, it is absolutely going to kill and eat it.
That's a terrible day for the gazelle.
But we don't think of lions as bad because what they're doing in their ecosystem is keeping a cap on the abundance of gazelle.
They serve a role in their ecosystem.
They're doing something important that keeps the ecosystem in balance.
And in exactly the same way, the parasites are also doing their part to keep ecosystems in balance.
And that includes by making sure no one host species gets too abundant.
The reason that we don't have more cockroaches and rats in the world is that their parasites keep a cap on their abundance.
And we also know that parasites are really important for allowing predators to have enough food to eat.
So parasites, while they might be bad for an individual host, are good for the ecosystem in which they occur.
Yeah. In fact, you wrote a book for kids about parasites that's really cool.
I read it. It's really fun.
And one of the things that you say in your book is that without parasites, ecosystems would fall apart.
And maybe you can give us the example of how a lot of our food, our crops, rely on parasites like aphid -killing wasps.
Yeah, there's this really interesting chapter in the book where I get to delve into parasitoids, which are a special class of parasite that kills their host.
And nowhere is there a better example of this than among the parasitoid wasps.
There are tens of thousands of species of parasitoid wasps.
They usually specialize in just one host species, and many of them tend to go after pests of our farms.
Imagine something like an aphid, which is a kind of insect that sucks the juices out of plants.
Some farmers spend their whole careers trying to get rid of aphids because they can be so destructive for a crop like tomatoes or corn or wheat.
aphids are really, really dangerous.
But the farmer, unbeknownst to him, is actually getting an assist from parasitoids because there are some species of parasitoid wasps who find aphids and then they use this long ovipositor, which is basically like a spike on the butt of the wasp.
And they use that ovipositor to stab the aphid and inject their larvae inside.
And then the larvae grow up inside the aphid, munching on its innards until eventually they kill the aphid.
Now, these parasitoids can actually be a really important control on the abundance of the aphids.
Without the parasitoids, the aphids would grow totally out of control and ruin all of our crops.
And so in that way, farmers really rely on parasitoids and other parasites to make sure that they still have a crop to harvest at the end of the season.
like a lot of things parasites aren't really good or bad I mean they're bad for their hosts but they're just a part of the cycle of life and our ecosystem the same way all the other plants and animals and organisms play a role so do parasites together all the living organisms keep the world in balance
thanks so much to Chelsea L.
Wood she's the author of Power to the Parasites.
It's a book for kids and it's full of interesting facts and stories about parasites.
So if you found this episode interesting, you'll probably like that book.
And I want to know, what do you think about parasites now?
Do you feel differently than you did when I first introduced the episode?
Do you feel better or worse about parasites?
Let me know. And of course, send a question if you've got one.
We take questions on any topic.
So have an adult record you if you're able.
It's easy to do on a smartphone using an app like Voice Memos.
Then have your adult email the file to questions at butwhykids .org.
And of course, if talking or using your voice isn't your thing, just have your adult email us your question.
Oh, and guess what?
We've got a new team member to introduce you to.
Sarah Bake is our newest producer.
If you follow But Why Kids on Instagram, you've seen some videos from her recent trip to Japan.
And you can find her bio where she describes who she is and what her history is on our website, butwhykids .org.
By the way, if you go check out our bios, you can discover the favorite animal of everyone who works on But Why.
So, our show is produced by Melody Beaudet, Sarah Bake and me, Jane Lindholm, at Vermont Public.
Joey Palumbo produces our video series.
Our theme music is by Luke Reynolds, and we're distributed by PRX.
We'll be back in two weeks with an all -new episode.
Until then, stay curious.
From PRX.
