Unknown: Theme music.
Jane: 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
experience 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.
Chelsea Wood: 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.
Jane: What does that mean, exactly?
Chelsea Wood: 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.
Jane: 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,
Unknown: 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.
Jane: 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.
Unknown: 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.
Jane: Why do they need a host? What do we mean when we say a
parasite needs a host?
Unknown: It's just another way of making a living in the world.
You know, a lot of us are 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.
Jane: How long do parasites live?
Unknown: Oh, man, it really depends. Anywhere from you know,
a couple of minutes to decades.
Jane: Wow.
Unknown: 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's 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.
Jane: But there are some that only live for minutes, and
that's their whole life cycle?
Unknown: 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
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.
Jane: 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.
Serenity: My name is Serenity, and I'm nine years old, and I
live in Canada, and my question is, are tapeworms sticky?
Unknown: 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.
Oliver: I'm Oliver, and I'm five years old. I live in Bloomfield,
New Jersey, and my question is, how do tapeworms need be so
long?
Jane: Why do tapeworms need to be so long, wonders Oliver.
Chelsea Wood: 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 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.
Jane: What kind of hosts do tapeworms live in? Humans can be
hosts for some tapeworms, right?
Chelsea Wood: 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.
Bobby: 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.
Chelsea Wood: 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.
Jane: 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.
Chelsea Wood: Exactly. It hates it deeply.
Jane: 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.
Chelsea Wood: 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.
Jane: 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.
Chelsea Wood: 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 host 100% of the time. If it
doesn't have a negative effect on 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.
Jane: 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.
BREAK: BREAK
Jane: 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!
Chelsea Wood: 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.
Jane: Okay, so that's how Chelsea got hooked. Here's her
pitch to maybe hook you.
Chelsea Wood: 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
you know, when you look at a worm in your hand, it's slimy
and it writhes. It's just it doesn't, it doesn't look like
anything beautiful. Doesn't enter into the equation at all,
right, but, 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
Trypanorhyncha. Informally, we call them the Trypanorhynch
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.
Jane: I'm starting to regret imagining myself inside of a
shark.
Chelsea Wood: It's a hard place to live for a lot of reasons,
but the Trypanorhynchs 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 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.
Jane: 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.
Levi: Hi, But Why. My name is Levi. I'm nine years old. I live
in Canberra, Australia, and my question is, what is head lice
and how does it spread?
Ramona: 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?
Hazel: 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.
Ludo: My name is Ludo. I'm from Audubon, Minnesota, and I'm five
years old. My question is, why do lice itch?
Chelsea Wood: 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.
Jane: 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.
Marlo: Hi, my name is Marlo. I am five years old. I live in
Austin, Texas, and my question is, are fleas related to mosquitoes?
Chelsea Wood: 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.
Lily: 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,
not people?
Chelsea Wood: 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.
Jane: That is not fun.
Cindy: My name is Cindy. I'm seven years old. I live in
Ontario, and my question is, why is there such a thing as ticks
and mosquitoes?
Jane: Why is there such a thing as ticks and mosquitoes? We
talked about fleas and mosquitoes. What about ticks and
mosquitoes, any connection?
Chelsea Wood: Yeah, they they are also related to one another,
although distantly. And you know, 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.
Theo: My name is Theo, and my age is three, and I live in
Canada, Saskatchewan, and my question is, why do ticks bite
you and also suck your blood?
Lars: Hi, my name is Lars. I'm five years old. I live in
Minneapolis, and my question is, why do ticks suck blood?
Jasper: My name is Jasper. I live in Georgia, and my question
is, why do ticks need blood to survive?
Chelsea Wood: 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 skidding 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 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 ticks 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.
Holden: Hi, my name is Holden, and I'm from Elgin, Illinois and
and I'm five, and my question is, how do ticks get Lyme
disease in, and how do they spread it to other people?
Jane: 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 the ticks cancarry and that humans
can get. We have several kids that 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?
Chelsea Wood: 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 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
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 onto 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 with their nymphs.
Karen: 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
sting?
Jane: 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, like, why do they let
you know that you've been bitten?
Unknown: They don't want to. Ticks and mosquitoes and you
know, all 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 in the breach of your skin,
in that bite.
Jane: 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?
Unknown: 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 stocking 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.
Jane: 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.
Chelsea Wood: 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
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.
Jane: 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@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 Baik is our newest producer. If you follow.
@Butwhy_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 Bodette, Sarah Baik 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!