November 21, 2024 • 32 mins
For thousands of years, humans have shaped mosquito evolution while mosquitoes have shaped human history. Today on the show, Noah Rose, an ecologist at UC San Diego, tells us how mosquitoes came to love human blood. Then, Georgetown historian John McNeill makes the case for how mosquitoes – and the viruses they carry – changed the course of history in the Americas.
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Speaker 1 (00:02):
If you're a virus, you have this constant problem. There.
You are successfully reproducing in some rabbit or some chicken,
or let's say some person, and then one day the
person's immune system gets wise starts attacking you, destroying you.
You need to find a new host, and you need
to do it fast. Viruses have evolved lots of strategies
(00:23):
for solving this problem. Some of them fly out of
people's mouths and noses and sneezes and coughs. Others ooze
out in bodily fluids like blood and saliva, and some viruses,
in a truly amazing twist of evolution, have enlisted mosquitoes.
They're like these missiles that fly through the air, sucking
(00:44):
virus out of one host and injecting it into another.
Diseases like Zeka, West Nile, Japanese encephalitis, and yellow fever
have killed millions of people over the course of human history,
and they couldn't have done it without mosquitos. I'm Jacob Goldstein,
and this is Incubation, a show about viruses. Today on
(01:04):
the show, how mosquitoes learned to love people and how
those mosquitoes and the viruses they carried changed human history.
In the second half of today's show, we'll hear the
(01:26):
story of how mosquitoes carrying yellow fever were enlisted as
a kind of biological weapon and how they played a
key role in shaping the history of the Western hemisphere. First, though,
we're going to talk about how those particular mosquitoes, a
species called eighties a gypty, came to love humans in
the first place, and we'll also get a sense of
(01:46):
what climate change and urbanization mean for the future of humans, mosquitoes,
and mosquito born viruses. My guest for this part of
the show is Noah Rose. He's an assistant professor in
the Department of Ecology, Behavior and Evolution at the University
of California, San Diego, and he is really really into mosquitoes.
(02:09):
What's the most mosquito bites you ever had at one time?
Speaker 2 (02:13):
You Know what's kind of funny is in my old job,
I would feed my own mosquitoes.
Speaker 1 (02:20):
When you say I would feed my own mosquitoes, what
does that mean?
Speaker 2 (02:24):
Like I would stick my arm in a cage and
they would bite me, and I always dress.
Speaker 1 (02:29):
You would fed them just like giving food to a dog,
but instead of putting kimble on a plate. You'd put
your arm in a box and your arm was not protected,
they would just come and bite you so that they
could live.
Speaker 2 (02:39):
That's exactly right. I was like, I'm gonna feed all
my mosquitoes, and I'm just gonna like, I'm gonna like
bite the bullet, and I'm gonna like know that I
can just do this. The buck stops here. I can
keep my mosquitos alive. And so I like went all in.
I got I don't know, like tens of thousands of
bites over a short period of time. And then I
got to I got to my new job, and they're like, yeah,
we don't really like let people do that here. We
(03:00):
think it's gross and maybe bad.
Speaker 1 (03:04):
Fair enough, I guess fair enough. So, okay, if we
go to the time when you're starting out studying mosquitoes
as a postdoc, like, what are the big questions you're
trying to figure out?
Speaker 2 (03:15):
Yeah, so, really, what we were trying to understand is
what makes mosquitoes particularly good at spreading disease? Because there's
lots of mosquitoes in the world, and not all of
them are equally good at spreading disease. Actually, you know,
of the over three thousand species that are out there,
the vast majority are not really causing so many problems
for people, but a few of them are really really
good at spreading disease. Maybe the best example of that
(03:37):
is eighties a gypty. Eighties agypty is like an amazing
vector of viral diseases, in particular diseases like yellow fever,
dangay zeka, chicken gunya. It's really good at spreading those
diseases for a lot of reasons. But kind of high
up on that list of reasons is it loves to
live around and bite people. And if you want to
spread disease, you know, from person to person, living around
(03:59):
people biting people super super helpful.
Speaker 1 (04:02):
So just a sort of threshold question if you're looking
at any of the thousands of species of mosquitoes, so
obvious I wouldn't even think of it. Does it bite
people a lot? Because if it doesn't, then for the
most part, we're not going to be so worried about it.
Speaker 2 (04:15):
That is exactly right. Most of the species of mosquitoes,
their life cycle, it just doesn't really involve people. It's
not about us. It's you know, they're living in the forest,
they're biting animals. They're breeding in some sort of natural
standing water. They're just not spreading a ton of disease
from person to person because they don't survive that well
in human habitats. But a few mosquitoes are really, really
good at that, and eighties Egypty is one of the best.
And so the next question is how did it come
(04:37):
to be that way? Because this mosquito wasn't always that way.
It evolved from something, right, It evolved from an ancestral population,
probably with a life cycle that did not involve living
in a plastic bucket in near backyard and biting a human.
Speaker 1 (04:50):
And so to answer this question, you traveled to sub
Saharan Africa right to study eighties agypty. What did you find?
Speaker 2 (04:59):
Yeah, so, basically, most populations of eighties of Gypty in
its native range of sub Saharan Africa, they don't particularly
love human hosts. They're willing to bite humans, but they
actually they would like to bite a wide variety of
vertebrate hosts, and humans are not their favorite of those.
Humans are like, yeah, I'll take it if I have to,
But they don't love how we smell.
Speaker 1 (05:19):
We're an acquired taste.
Speaker 2 (05:21):
We are a very acquired taste exactly. But a few
populations are really really strongly specialized on human hosts and
human habitats. They love how we smell. And those are
populations living in this region called the Sahel. It's just
south of the Sahara Desert, where for most of the
year it's really hot, really dry, a terrible place to
(05:41):
be a mosquito. You know, mosquitoes thrive on standing water,
but for a couple months it's mosquito paradise. They have heavy,
heavy rains, pools of water kind of everywhere, but that
dry season is so hot and so long that it
seems like the ancestral generalist form of the mosquito it
just doesn't survive that well. There. Instead, we find these
populations of eighties of gipty that are really tightly interconnected
(06:03):
with settled human populations, right, So they're not breeding in
their normal treeholes or rock pools. They're breeding in things
like heavy clay vessels used to store water. It's an
amazing habitat for a human specialist mosquito. And so we
think that this is actually the original context that sort
of drove eighties Egypty to specialize on humans.
Speaker 1 (06:22):
You observe this, and you're also doing essentially genomic analysis, right,
You're trying to figure out kind of the history of
how this came to be by studying the genetics of
these different populations, and in particular the subpopulation that is
like kind of a weird o mosquito, right, this weird
oh mosquito that happens to love human dwellings, that happens
(06:44):
to love these clay vessels where people store their water.
Speaker 2 (06:47):
That's exactly right. And so with the genetics, basically the
question we're asking is like what is the genetic signature
of this human specialist mosquito? And in particular, what we
find is that there's a shared genetic basis in the
Sahelian populations and the invasive populations that came to spread
all over the whole world. Right, So we think that
these populations in the Sahel are likely the origin of
(07:11):
this human specialist form. But they didn't stay there. They
spread everywhere across the global tropics, at least in urban areas,
and they became this like total menace.
Speaker 1 (07:20):
Okay, so you are going off to sub Saharan Africa,
so I get that you want to understand how eighties
of gypsy evolved to love people and therefore spread disease.
But what's the sort of smaller set of things you
have to do to actually figure that out.
Speaker 2 (07:34):
Yeah, so there's a bunch of things, but the two
main things are studying and quantifying their behavioral preference for humans.
How much do the mosquitoes want to bite humans? And
sequencing their genomes. That's the big thing. And the way
we do that is using a device called a live
host two port old factometer, which is a lot of
very fancy words to say, a big plastic box with
(07:56):
a lot of mosquitos on the inside and has two
holes in the side.
Speaker 1 (07:58):
Old factometer, like measuring their smell or something. Is that
what that is?
Speaker 2 (08:03):
Yeah, exactly, it's like measuring their smell. And basically they
can choose to go in one of two traps. One
leads to a human arm. One leads to non human animal,
in many cases a literal guinea pig, because they have
a nice temperament and they smell good to mosquitos.
Speaker 1 (08:19):
And what's the human arm? You got some undergrad sitting
there all day for minimum wage or what.
Speaker 2 (08:25):
Usually it's me.
Speaker 1 (08:27):
It's your own arm sitting there to see what the mosquitoes.
Speaker 2 (08:30):
Like, yeah, it doesn't have to be me. And in fact,
you know, we made sure that we got similar results
with different people, because it would be awfully embarrassing to
do a whole study of like what a mosquito's like
and then have it be like, oh, no, it's just
like it's just that they like just you.
Speaker 1 (08:43):
Yeah, So, how much time did you spend with your
arm in the mosquito box?
Speaker 2 (08:50):
I mean, cumulative months of my life? It was it
was a lot of sitting with my arm in the box. Yeah,
it was you learn to type with.
Speaker 1 (08:57):
One hand or something. So would you listen to podcast?
That's what you do?
Speaker 2 (09:01):
I mostly did audiobooks and just kind of zoning out.
You know, it's it's I'm not gonna say it's meditative,
but you know, it's it's there's something not so terrible
about being like, I'm working while you're clearly just sitting
in a chair. There's worse things in life.
Speaker 1 (09:15):
And are you getting bitten by the mosquitoes?
Speaker 2 (09:17):
Now there's like a piece of screen in the way,
so they fly towards the arm or the guinea pig
and they sort of accumulate up against the screen where
they're like furiously you can see them. It's kind of
it's kind of horrifying to see. They're like straining against
the screen, pushing their body up against it and pushing
their little prebosses through it.
Speaker 1 (09:33):
They want so badly to suck your blood.
Speaker 2 (09:36):
They really really, they're quite motivated, but they can't get
to it.
Speaker 1 (09:41):
So so you've done this work this, you know, collecting
the data, studying the sense of smell with your arm
in the box, doing the genetic analysis, and in the
end you publish a couple of papers that really kind
of tell this thousands of years long story of how
mosquitoes came to you know, bite people and more importantly,
(10:02):
came to be this really important disease vector. So like,
tell me that story. Start a long time ago.
Speaker 2 (10:10):
Go back thousands of years ago. There's eighties agypty mosquitos
living in the forest, breeding in tree holes and rock
pools and sort of opportunistically biting the first vertebrate hosts
that they come across. So like, yep, over there, I
see that little animal, I'm going to bite it great,
and then they lay their eggs and you know, life
cycle complete. It's like any one of those other three
thousand mosquitos that were not that worried about. The Sahara
(10:32):
Desert between about fifteen thousand to five thousand years ago,
more or less, was not the desert we know it
as today. It was sort of a grassland, the green
Sahara people call it. And during that time, there were
lots of sort of distributed human societies, like living as
hunter gatherers. You can find rock carvings in the middle
of the Sahara Desert in places where you know it's
(10:53):
totally inhospitable, and they like depict these sort of idyllic
scenes of like hunting big game on a rolling grassland.
It's totally and a little bit strange. But what happened
is that about five thousand years ago the Sahara Desert
just dried up. That whole area dried up, and that
changed a lot of things. But one of the most
important things it did for humans was it drove people
(11:15):
to kind of settle down and shift to these different
ways of living. And in this hell you have the
emergence of settled human societies on the edge of the
Sahara Desert, storing water, farming things like millet. And when
all that happens, that modern niche for human specialist mosquitos
kind of emerged where you have this place that's otherwise
(11:35):
inhospitable to mosquitoes, but there's an amazing ecological niche living
with humans. And so it's the drying of the Sahara
desert and the emergence of that human specialist niche that
seems to have driven the evolution of the mosquito.
Speaker 1 (11:48):
So you have the humans presumably who were living their
best life when it was a grassland hunting big game,
and then it dries up, and the humans are adapting
to this new environment conditions, you know, storing water, maybe
farming in a way they didn't before, and kind of
alongside them, you have the mosquito also adapting to survive.
Speaker 2 (12:09):
Yeah, yeah, exactly. It's a really interesting thing right where
it's like the way that time works for humans and
for mosquitoes is very different, right, Like mosquitos get like
ten to fifteen generations a year a.
Speaker 1 (12:20):
Year, so like a month a mosquito, mosquito lives for
like a month.
Speaker 2 (12:24):
They live for like a month exactly. And you know,
the average generation time of a human is like twenty
nine thirty years roughly, and so you know, for every
human generation, you get about four hundred mosquito generations.
Speaker 1 (12:35):
H huh. So that's just that much more evolution, that
much more genetic iteration exactly.
Speaker 2 (12:40):
So it's like the last ten thousand years of human
history and cultural change is like, you know, equivalent for
the mosquitoes to like the last four million years of
human evolution. And when you think about like four million
years of human evolution, it's like all the fun stuff, right,
it's like everything that we care about. And so for
these mosquitoes, all of their most interesting, like really interesting
(13:02):
evolutionary changes they're taking place like over the time scale
of human history, right, And so it's this really cool
thing where we can simultaneously study how humans are sort
of shaping the planet, how humans are changing the planet,
and how that's changing mosquitoes.
Speaker 1 (13:17):
So we've been talking about the past, right, and your
insights into how eighties egypty came to be this very
important disease vector in the world. What does your work
tell us about the future.
Speaker 2 (13:30):
Yeah, well, I mean this is a topic I'm really
interested in, in particular because the story of eighties egypty
it's not over. It's still playing out, right, we still
have populations of eighties agypty that are not the amazing vector.
In most of the native range, eighties e gypty is
the generalist form, but that seems to be changing a
lot of the fastest growing cities in the world. They're
(13:51):
in sub Saharan Africa. There's places like Waga Doogu and Brikina, Faso,
Kumasi and Ghana. And in those cities the mosquitos seem
to be evolving a greater specialization on human hosts and habitats.
Speaker 1 (14:03):
Is this a future where disease transmission mosquito by mosquitos
is going to get worse just because there's more density,
more urbanization. Is that the story you're telling.
Speaker 2 (14:13):
I think there's good reasonably that that might be playing out,
not just in the future, but even in the present day.
For example, Wagga Doogo, Historically speaking, it was not thought
to be a majorly problematic area for things like dangay fever,
and in the last couple of years there have been
these massive outbreaks of dang gay fever and waggadogo. And
at the same time we can see in the genomes
(14:35):
of the mosquitoes there that they're becoming more specialized on humans,
they have more ancestry from that human specialist kind of
gene pool. Right, the mosquitoes are changing like on contemporary
time scales, Like the mosquitoes twenty years ago are different
from the mosquitoes of today are different from the mosquitoes
of twenty years from now. So it's not just like
a hypothetical future thing. I think it's like a present
(14:57):
day dynamic.
Speaker 1 (14:59):
So the initial wave of specialization thousands of years ago
was triggered by environmental change, right by the drying out
of the Sahara. In the modern context, is climate change
relevant to this discussion.
Speaker 2 (15:13):
Yeah, that's a great question. It's one that I get
all the time, and it's one that I'm very interested
in understanding. It's hard to say for sure, because climate
has really complex effects on mosquito born disease transmission. When
it comes to some of the topics I've studied, Like
so much of what we know about what seems to
drive mosquitoes to specialize on humans, it's things like either
(15:33):
urbanization or precipitation variation, but maybe not mean temperature per se.
But there's indirect ways that I still think climate change
might contribute to this. For example, climate change can very
directly contribute to things like people moving very quickly into
sort of informal settlements in rapidly growing urban areas, because
(15:54):
it's just like the old way that people were living
isn't working anymore.
Speaker 1 (15:58):
Right, So it seems like from what you're saying, urbanization
is a much clearer driver than climate change of basically
more mosquitoes and more mosquito spread disease.
Speaker 2 (16:10):
That's exactly right. Urbanization is so fast and so extreme
and represents such a major change to the sort of
ecosystem that these mosquitoes are living in.
Speaker 1 (16:19):
From like the mosquito pov urbanization domini, Yeah, exact.
Speaker 2 (16:24):
Yeah, if you're a mosquito sitting in a forest and
the forest gets chopped down and replaced with a giant metropolis,
that's a bigger change than an environmental change playing out
more subtly over longer time scales.
Speaker 3 (16:35):
Yeah.
Speaker 4 (16:36):
So if you zoom out and think about what you knew,
what your worldview was when you started studying mosquitoes, and
what you know, and kind of the way you think
about the world.
Speaker 1 (16:47):
Now, how do you think about the world differently.
Speaker 2 (16:51):
I mean, it's an interesting question because you know, the
thing that got me interested in mosquitos was trying to
understand how human are affecting our planet, right, like trying
to understand how we're affecting the environment, how we're reshaping ecosystems.
And you know, in those days, my main motivation was
wanting to protect the biodiversity, wanting to protect ecosystems, which
(17:15):
I still care a lot about. One thing that changes
just you know, like I sort of realized not everything
is like about us either like helping or heard things like,
you know, mosquitos are evolving really quickly to exploit us,
to make us sick. It's kind of amazing to me
that mosquitos could do something like evolved to be better
(17:36):
at taking advantage of us on the same kind of
timescale that we're talking about, like oh, we're degrading ecosystems,
or we're removing habitat for charismatic megafauna that we care
about or things like that, like nature. Also nature is responding.
Speaker 1 (17:51):
So it's like it's like before it was like, oh,
nature is a victim, and now it's like, yo, mosquitoes
are kind of kicking ass.
Speaker 2 (17:58):
Yeah, they're really really good at what they do, and like,
you know, we change the whole way that the place
they live looks, and they're like, I can do this too.
I can totally live here, and I can totally bite
the organisms that live here. They make it work.
Speaker 1 (18:14):
Thanks to my guest Noah Rose, now as an assistant
professor in the Department of Ecology, Behavior and Evolution at
the University of California at San Diego. After the break,
mosquitoes become a biological weapon. Eighties of gypty first came
(18:41):
to the New World on European slave ships, and along
with the mosquito came a new disease, a disease that
until that point had not existed in the Americas, yellow fever.
John McNeil is a professor of history at Georgetown and
the author of a book called Mosquito Empires. In the book,
John makes the case that yellow fever and the mosquitoes
(19:03):
that transmitted it had a profound effect on the fight
among European powers to control the Western Hemisphere. Tell me
about yellow fever historically, going back a couple hundred years,
so in the seventeen hundreds. What happens in the absence
of modern medicine if you get yellow fever.
Speaker 5 (19:21):
That all depends on where you were born and raised,
and what age you are. Children often show no symptoms,
don't know that they're sick, survive it, and in the
process acquire lifelong immunity. The worst thing to be is
in the prime of your life and encountering yellow fever
(19:43):
for the first time.
Speaker 1 (19:44):
This is going to be very important for our story today.
What happens if you were in the prime of your
life and you get yellow fever and you have never
had it before.
Speaker 5 (19:54):
Odds are you get a high fever and jaundice, that
is a sure white person in your skin begins to
turn yellow, and after several days, either you get better
you're among the lucky ones and immune for life, or
after a brief respite, you get sicker and you begin
(20:18):
to vomit up partially coagulated blood with the consistency and
coloring of coffee grounds. And when that happens, not only
are your day's numbered, your hours are numbered.
Speaker 1 (20:34):
So there's this one particular moment that you talk about
in the book where yellow fever plays a central role
in world history, and that moment is the Battle of Cartagena,
which took place in modern day Columbia. It happened in
seventeen forty one. And at this point, right, the Spanish
control a lot of South America, the British control a
(20:57):
lot of North America. And then this battle happens when
the British launched this huge attack against the Spanish in Cartagena.
So tell me about the battle. Why was this battle
so important?
Speaker 5 (21:10):
Cartagena was one of the two or three lynchpins of
Spanish colonial defense. It was also central to the trading
system of Spanish America, and that included the annual exports
of silver from mines in the Andes. Everybody who wanted
(21:33):
to take Spanish America away from the Spanish, what they
most wanted was silver.
Speaker 1 (21:39):
So was the notion that if a Britain conquered Cartagena,
they could, like you know, to some degree, conquer the
Spanish in South America, make Spanish colonies British colonies.
Speaker 5 (21:52):
Yes, that would be the first step. So back in
London they sent huge reinforcements. They had a twenty nine
thousand men between soldiers and sailors, and that was probably
the largest amphibious military expedition in world history up to
that time.
Speaker 1 (22:13):
So the British are bringing twenty nine thousand men How
many men do the Spanish have to defend Cartagena?
Speaker 5 (22:19):
Approximately four thousand, seven hundred, so they're out manned what
six six to one approximately?
Speaker 1 (22:27):
Yes? What are Spain's defenses like in Cartagena?
Speaker 5 (22:31):
The most important thing they had was massive stone fortifications, okay,
and the significance of that, which the Spaniards well understood
by seventeen forty, was that the massive fortifications would require
any attacking force to lay siege to the city, to
try to bring artillery within range, bash down the walls,
(22:56):
permit an assault, and that would take weeks.
Speaker 1 (23:01):
So the fortifications wouldn't necessarily prevent an attack, but they
would slow it down. Why is that important? Why is
it important that it would take weeks for the British
to get through the fortifications.
Speaker 5 (23:13):
It's of central importance. The reason is because in the
course of those weeks, the attacking force would be bitten
by the local mosquitoes, would acquire the yellow fever virus,
and the men in question young men in the prime
of their lives. Born and raised in places such as
the British Isles. These were people who had no prior
(23:36):
experience of yellow fever, so they are maximally susceptible and
within six weeks. The Spanish tended to think the climate,
which really means yellow fever, would wreak havoc upon the
attackers and force them to call off their siege.
Speaker 1 (23:57):
So the Spanish are using this, this virus yellow fever
as like a core part of their military strategy, because
they know that their own forces are made up of
people who have immunity. Right, they've grown up locally, they've
had the disease, whereas that is not true for the attackers.
For the British.
Speaker 5 (24:17):
Correct they didn't understand yellow fever's mode of transmission, but
they knew it was extremely deadly to people are fresh
off the boat from Europe.
Speaker 1 (24:32):
It's almost like a kind of early form of biological warfare.
Speaker 5 (24:37):
Yes, in effect, it was biological warfare.
Speaker 1 (24:41):
Okay, so this is the Spaniard's idea. The British arrive
with their dozens of ships and their tens of thousands
of men. What happens?
Speaker 5 (24:54):
So the British prosecuted a siege, tried to move man
and artillery across the difficult and damp terrain close by
to Cartagena. Progress was slow. People started to get sick
within days of disembarking from their ships.
Speaker 1 (25:14):
You mentioned that the British soldiers who had landed at
Cartagena and were laying siege to the city didn't have tents,
so they were just lying on the ground, getting bit
by mosquitoes and getting yellow fever. Yes, why what happened?
Why didn't they have tents? Like tents had been invented?
Speaker 5 (25:33):
Tents had been invented. There are two things to bear
in mind here. One is the difficulty of outfitting a gigantic,
amphibious military expedition on schedule in the eighteenth century is
not all that easy in the twenty first century. Second
one is that nobody knew how yellow fever was transmitted.
(25:56):
Being bitten by mosquitoes did not automatically seem to anyone
at the time to carry any particular risk.
Speaker 1 (26:06):
A right, like they just thought it was like my
asthma or something. So what ends up happening in the battle?
How does it end?
Speaker 5 (26:13):
The admiral got frustrated with the army commander with slow progress,
and before long they were losing hundreds of men daily
to yellow fever, and after thirty three days and a
premature attempt to storm the fortifications, the British withdrew and
(26:36):
the Spanish defenders were triumphant. They became great heroes in
Spanish military history. But what they did was not lose
before yellow fever forced their enemies to withdraw.
Speaker 1 (26:53):
Yeah, it's like the virus with the real conqueror. For Spain, the.
Speaker 5 (26:58):
Virus killed probably twenty men for every one killed in
combat in this particular encounter.
Speaker 1 (27:07):
Wow. And then you say that the same dynamic plays
out once the fighting in the Caribbean and South America
shifts from being one colonial power against another two being
wars of independence.
Speaker 5 (27:24):
Yes, and the large scale wars for independence in the
Caribbean really begin with the Haitian Revolution in seventeen ninety one,
and there British forces and then French forces tried to
prevent a slave uprising, and those British and French forces
(27:47):
suffered extreme disease mortality, and they both went after the
other gave up and Haiti won its independence. I think
it is in some ways analogous to the way that
Spanish colonial authorities used their understanding of the disease climate
(28:09):
to military advantage in places like Cartagena. And the reason
for that is their own armies were composed of men
who had either grown up in the Caribbean amid yellow fear, malaria, denge,
and other infections, or had grown up in Africa and
similarly had encountered these diseases and were either immune or resistant,
(28:32):
Whereas the forces sent out to quell the Haitian Revolution
were recruited from the British Isles, North America, France, in short,
people who had virtually no immunity or resistance against yellow
fever and malaria.
Speaker 1 (28:50):
How should this story about yellow fever and colonial warfare
in the Caribbean, How should that change the way I
think about history?
Speaker 5 (29:04):
We should understand that until the twentieth century, the great
majority of death and dying in military campaigns was not
a result of combat. It was a result of various diseases.
And there were certain times in places, including the Caribbean
(29:24):
and the seventeenth, eighteenth, and nineteenth century, in which those
diseases were you could say partisan, that is, they systematically
favored one side over another and powerfully influenced the results.
Speaker 1 (29:38):
So it wasn't until the twentieth century, after thousands of
years of war, that most people who died in war
died from injuries inflicted by the enemy rather than died
from infectious disease.
Speaker 5 (29:52):
Yes, and that entails a gigantic and brutal irony. The
mass slaughter for which the First World War is justifiably
famous was possible only because military medicine had become sufficiently
effective that large armies of millions of men could be
(30:14):
kept alive long enough to butcher one another. Prior to that,
it was impossible to maintain gigantic armies for any length
of time because they would get too sick.
Speaker 2 (30:28):
But by the.
Speaker 5 (30:28):
Twentieth century that constraint was relaxed, and mass armies could
last long enough to kill one another. That's a horrible irony.
Speaker 3 (30:40):
So if we think about this history we've been talking about,
and then we think about the modern world, where obviously
we have lots more treatments, medicine is just better now
than it was then, what lessons does this story hold
for the dynamics of the modern nace?
Speaker 1 (31:00):
What does it teach us about the way the world
works today?
Speaker 5 (31:03):
It should help us recognize the fragility of what we
might consider the Golden Age of health, an era ushered
in by public health systems, by urban sanitation, by vaccination
regimes which protect a significant proportion of the entire human
(31:26):
population from the ravages of infectious disease, and that is
a comparatively new phenomenon and it is not to be
taken for granted.
Speaker 1 (31:38):
Thank you so much for your time. It was great
to talk with you, Jacob.
Speaker 5 (31:41):
Thank you very much. Thanks for inviting me on the show.
Speaker 1 (31:49):
John McNeil's a professor of history at Georgetown and author
of the book Mosquito Empires. Thanks to both my guests today,
Noah Rose and John McNeil. Next week on the show,
Ken Measle's Cure Cancer, A very weird good thing was
going on. Incubation is a co production of Pushkin Industries
(32:10):
and Ruby Studio at iHeartMedia. It's produced by Kate Ferby
and Brittany Cronin. The show is edited by Lacey Roberts.
It's mastered by Sarah Bruguire, fact checking by Joseph friedman Or.
Executive producers are Lacy Roberts and Matt Romono. I'm Jacob Goldstein.
Thanks for listening.
If you're a virus, you have this constant problem. There.
You are successfully reproducing in some rabbit or some chicken,
or let's say some person, and then one day the
person's immune system gets wise starts attacking you, destroying you.
You need to find a new host, and you need
to do it fast. Viruses have evolved lots of strategies
(00:23):
for solving this problem. Some of them fly out of
people's mouths and noses and sneezes and coughs. Others ooze
out in bodily fluids like blood and saliva, and some viruses,
in a truly amazing twist of evolution, have enlisted mosquitoes.
They're like these missiles that fly through the air, sucking
(00:44):
virus out of one host and injecting it into another.
Diseases like Zeka, West Nile, Japanese encephalitis, and yellow fever
have killed millions of people over the course of human history,
and they couldn't have done it without mosquitos. I'm Jacob Goldstein,
and this is Incubation, a show about viruses. Today on
(01:04):
the show, how mosquitoes learned to love people and how
those mosquitoes and the viruses they carried changed human history.
In the second half of today's show, we'll hear the
(01:26):
story of how mosquitoes carrying yellow fever were enlisted as
a kind of biological weapon and how they played a
key role in shaping the history of the Western hemisphere. First, though,
we're going to talk about how those particular mosquitoes, a
species called eighties a gypty, came to love humans in
the first place, and we'll also get a sense of
(01:46):
what climate change and urbanization mean for the future of humans, mosquitoes,
and mosquito born viruses. My guest for this part of
the show is Noah Rose. He's an assistant professor in
the Department of Ecology, Behavior and Evolution at the University
of California, San Diego, and he is really really into mosquitoes.
(02:09):
What's the most mosquito bites you ever had at one time?
Speaker 2 (02:13):
You Know what's kind of funny is in my old job,
I would feed my own mosquitoes.
Speaker 1 (02:20):
When you say I would feed my own mosquitoes, what
does that mean?
Speaker 2 (02:24):
Like I would stick my arm in a cage and
they would bite me, and I always dress.
Speaker 1 (02:29):
You would fed them just like giving food to a dog,
but instead of putting kimble on a plate. You'd put
your arm in a box and your arm was not protected,
they would just come and bite you so that they
could live.
Speaker 2 (02:39):
That's exactly right. I was like, I'm gonna feed all
my mosquitoes, and I'm just gonna like, I'm gonna like
bite the bullet, and I'm gonna like know that I
can just do this. The buck stops here. I can
keep my mosquitos alive. And so I like went all in.
I got I don't know, like tens of thousands of
bites over a short period of time. And then I
got to I got to my new job, and they're like, yeah,
we don't really like let people do that here. We
(03:00):
think it's gross and maybe bad.
Speaker 1 (03:04):
Fair enough, I guess fair enough. So, okay, if we
go to the time when you're starting out studying mosquitoes
as a postdoc, like, what are the big questions you're
trying to figure out?
Speaker 2 (03:15):
Yeah, so, really, what we were trying to understand is
what makes mosquitoes particularly good at spreading disease? Because there's
lots of mosquitoes in the world, and not all of
them are equally good at spreading disease. Actually, you know,
of the over three thousand species that are out there,
the vast majority are not really causing so many problems
for people, but a few of them are really really
good at spreading disease. Maybe the best example of that
(03:37):
is eighties a gypty. Eighties agypty is like an amazing
vector of viral diseases, in particular diseases like yellow fever,
dangay zeka, chicken gunya. It's really good at spreading those
diseases for a lot of reasons. But kind of high
up on that list of reasons is it loves to
live around and bite people. And if you want to
spread disease, you know, from person to person, living around
(03:59):
people biting people super super helpful.
Speaker 1 (04:02):
So just a sort of threshold question if you're looking
at any of the thousands of species of mosquitoes, so
obvious I wouldn't even think of it. Does it bite
people a lot? Because if it doesn't, then for the
most part, we're not going to be so worried about it.
Speaker 2 (04:15):
That is exactly right. Most of the species of mosquitoes,
their life cycle, it just doesn't really involve people. It's
not about us. It's you know, they're living in the forest,
they're biting animals. They're breeding in some sort of natural
standing water. They're just not spreading a ton of disease
from person to person because they don't survive that well
in human habitats. But a few mosquitoes are really, really
good at that, and eighties Egypty is one of the best.
And so the next question is how did it come
(04:37):
to be that way? Because this mosquito wasn't always that way.
It evolved from something, right, It evolved from an ancestral population,
probably with a life cycle that did not involve living
in a plastic bucket in near backyard and biting a human.
Speaker 1 (04:50):
And so to answer this question, you traveled to sub
Saharan Africa right to study eighties agypty. What did you find?
Speaker 2 (04:59):
Yeah, so, basically, most populations of eighties of Gypty in
its native range of sub Saharan Africa, they don't particularly
love human hosts. They're willing to bite humans, but they
actually they would like to bite a wide variety of
vertebrate hosts, and humans are not their favorite of those.
Humans are like, yeah, I'll take it if I have to,
But they don't love how we smell.
Speaker 1 (05:19):
We're an acquired taste.
Speaker 2 (05:21):
We are a very acquired taste exactly. But a few
populations are really really strongly specialized on human hosts and
human habitats. They love how we smell. And those are
populations living in this region called the Sahel. It's just
south of the Sahara Desert, where for most of the
year it's really hot, really dry, a terrible place to
(05:41):
be a mosquito. You know, mosquitoes thrive on standing water,
but for a couple months it's mosquito paradise. They have heavy,
heavy rains, pools of water kind of everywhere, but that
dry season is so hot and so long that it
seems like the ancestral generalist form of the mosquito it
just doesn't survive that well. There. Instead, we find these
populations of eighties of gipty that are really tightly interconnected
(06:03):
with settled human populations, right, So they're not breeding in
their normal treeholes or rock pools. They're breeding in things
like heavy clay vessels used to store water. It's an
amazing habitat for a human specialist mosquito. And so we
think that this is actually the original context that sort
of drove eighties Egypty to specialize on humans.
Speaker 1 (06:22):
You observe this, and you're also doing essentially genomic analysis, right,
You're trying to figure out kind of the history of
how this came to be by studying the genetics of
these different populations, and in particular the subpopulation that is
like kind of a weird o mosquito, right, this weird
oh mosquito that happens to love human dwellings, that happens
(06:44):
to love these clay vessels where people store their water.
Speaker 2 (06:47):
That's exactly right. And so with the genetics, basically the
question we're asking is like what is the genetic signature
of this human specialist mosquito? And in particular, what we
find is that there's a shared genetic basis in the
Sahelian populations and the invasive populations that came to spread
all over the whole world. Right, So we think that
these populations in the Sahel are likely the origin of
(07:11):
this human specialist form. But they didn't stay there. They
spread everywhere across the global tropics, at least in urban areas,
and they became this like total menace.
Speaker 1 (07:20):
Okay, so you are going off to sub Saharan Africa,
so I get that you want to understand how eighties
of gypsy evolved to love people and therefore spread disease.
But what's the sort of smaller set of things you
have to do to actually figure that out.
Speaker 2 (07:34):
Yeah, so there's a bunch of things, but the two
main things are studying and quantifying their behavioral preference for humans.
How much do the mosquitoes want to bite humans? And
sequencing their genomes. That's the big thing. And the way
we do that is using a device called a live
host two port old factometer, which is a lot of
very fancy words to say, a big plastic box with
(07:56):
a lot of mosquitos on the inside and has two
holes in the side.
Speaker 1 (07:58):
Old factometer, like measuring their smell or something. Is that
what that is?
Speaker 2 (08:03):
Yeah, exactly, it's like measuring their smell. And basically they
can choose to go in one of two traps. One
leads to a human arm. One leads to non human animal,
in many cases a literal guinea pig, because they have
a nice temperament and they smell good to mosquitos.
Speaker 1 (08:19):
And what's the human arm? You got some undergrad sitting
there all day for minimum wage or what.
Speaker 2 (08:25):
Usually it's me.
Speaker 1 (08:27):
It's your own arm sitting there to see what the mosquitoes.
Speaker 2 (08:30):
Like, yeah, it doesn't have to be me. And in fact,
you know, we made sure that we got similar results
with different people, because it would be awfully embarrassing to
do a whole study of like what a mosquito's like
and then have it be like, oh, no, it's just
like it's just that they like just you.
Speaker 1 (08:43):
Yeah, So, how much time did you spend with your
arm in the mosquito box?
Speaker 2 (08:50):
I mean, cumulative months of my life? It was it
was a lot of sitting with my arm in the box. Yeah,
it was you learn to type with.
Speaker 1 (08:57):
One hand or something. So would you listen to podcast?
That's what you do?
Speaker 2 (09:01):
I mostly did audiobooks and just kind of zoning out.
You know, it's it's I'm not gonna say it's meditative,
but you know, it's it's there's something not so terrible
about being like, I'm working while you're clearly just sitting
in a chair. There's worse things in life.
Speaker 1 (09:15):
And are you getting bitten by the mosquitoes?
Speaker 2 (09:17):
Now there's like a piece of screen in the way,
so they fly towards the arm or the guinea pig
and they sort of accumulate up against the screen where
they're like furiously you can see them. It's kind of
it's kind of horrifying to see. They're like straining against
the screen, pushing their body up against it and pushing
their little prebosses through it.
Speaker 1 (09:33):
They want so badly to suck your blood.
Speaker 2 (09:36):
They really really, they're quite motivated, but they can't get
to it.
Speaker 1 (09:41):
So so you've done this work this, you know, collecting
the data, studying the sense of smell with your arm
in the box, doing the genetic analysis, and in the
end you publish a couple of papers that really kind
of tell this thousands of years long story of how
mosquitoes came to you know, bite people and more importantly,
(10:02):
came to be this really important disease vector. So like,
tell me that story. Start a long time ago.
Speaker 2 (10:10):
Go back thousands of years ago. There's eighties agypty mosquitos
living in the forest, breeding in tree holes and rock
pools and sort of opportunistically biting the first vertebrate hosts
that they come across. So like, yep, over there, I
see that little animal, I'm going to bite it great,
and then they lay their eggs and you know, life
cycle complete. It's like any one of those other three
thousand mosquitos that were not that worried about. The Sahara
(10:32):
Desert between about fifteen thousand to five thousand years ago,
more or less, was not the desert we know it
as today. It was sort of a grassland, the green
Sahara people call it. And during that time, there were
lots of sort of distributed human societies, like living as
hunter gatherers. You can find rock carvings in the middle
of the Sahara Desert in places where you know it's
(10:53):
totally inhospitable, and they like depict these sort of idyllic
scenes of like hunting big game on a rolling grassland.
It's totally and a little bit strange. But what happened
is that about five thousand years ago the Sahara Desert
just dried up. That whole area dried up, and that
changed a lot of things. But one of the most
important things it did for humans was it drove people
(11:15):
to kind of settle down and shift to these different
ways of living. And in this hell you have the
emergence of settled human societies on the edge of the
Sahara Desert, storing water, farming things like millet. And when
all that happens, that modern niche for human specialist mosquitos
kind of emerged where you have this place that's otherwise
(11:35):
inhospitable to mosquitoes, but there's an amazing ecological niche living
with humans. And so it's the drying of the Sahara
desert and the emergence of that human specialist niche that
seems to have driven the evolution of the mosquito.
Speaker 1 (11:48):
So you have the humans presumably who were living their
best life when it was a grassland hunting big game,
and then it dries up, and the humans are adapting
to this new environment conditions, you know, storing water, maybe
farming in a way they didn't before, and kind of
alongside them, you have the mosquito also adapting to survive.
Speaker 2 (12:09):
Yeah, yeah, exactly. It's a really interesting thing right where
it's like the way that time works for humans and
for mosquitoes is very different, right, Like mosquitos get like
ten to fifteen generations a year a.
Speaker 1 (12:20):
Year, so like a month a mosquito, mosquito lives for
like a month.
Speaker 2 (12:24):
They live for like a month exactly. And you know,
the average generation time of a human is like twenty
nine thirty years roughly, and so you know, for every
human generation, you get about four hundred mosquito generations.
Speaker 1 (12:35):
H huh. So that's just that much more evolution, that
much more genetic iteration exactly.
Speaker 2 (12:40):
So it's like the last ten thousand years of human
history and cultural change is like, you know, equivalent for
the mosquitoes to like the last four million years of
human evolution. And when you think about like four million
years of human evolution, it's like all the fun stuff, right,
it's like everything that we care about. And so for
these mosquitoes, all of their most interesting, like really interesting
(13:02):
evolutionary changes they're taking place like over the time scale
of human history, right, And so it's this really cool
thing where we can simultaneously study how humans are sort
of shaping the planet, how humans are changing the planet,
and how that's changing mosquitoes.
Speaker 1 (13:17):
So we've been talking about the past, right, and your
insights into how eighties egypty came to be this very
important disease vector in the world. What does your work
tell us about the future.
Speaker 2 (13:30):
Yeah, well, I mean this is a topic I'm really
interested in, in particular because the story of eighties egypty
it's not over. It's still playing out, right, we still
have populations of eighties agypty that are not the amazing vector.
In most of the native range, eighties e gypty is
the generalist form, but that seems to be changing a
lot of the fastest growing cities in the world. They're
(13:51):
in sub Saharan Africa. There's places like Waga Doogu and Brikina, Faso,
Kumasi and Ghana. And in those cities the mosquitos seem
to be evolving a greater specialization on human hosts and habitats.
Speaker 1 (14:03):
Is this a future where disease transmission mosquito by mosquitos
is going to get worse just because there's more density,
more urbanization. Is that the story you're telling.
Speaker 2 (14:13):
I think there's good reasonably that that might be playing out,
not just in the future, but even in the present day.
For example, Wagga Doogo, Historically speaking, it was not thought
to be a majorly problematic area for things like dangay fever,
and in the last couple of years there have been
these massive outbreaks of dang gay fever and waggadogo. And
at the same time we can see in the genomes
(14:35):
of the mosquitoes there that they're becoming more specialized on humans,
they have more ancestry from that human specialist kind of
gene pool. Right, the mosquitoes are changing like on contemporary
time scales, Like the mosquitoes twenty years ago are different
from the mosquitoes of today are different from the mosquitoes
of twenty years from now. So it's not just like
a hypothetical future thing. I think it's like a present
(14:57):
day dynamic.
Speaker 1 (14:59):
So the initial wave of specialization thousands of years ago
was triggered by environmental change, right by the drying out
of the Sahara. In the modern context, is climate change
relevant to this discussion.
Speaker 2 (15:13):
Yeah, that's a great question. It's one that I get
all the time, and it's one that I'm very interested
in understanding. It's hard to say for sure, because climate
has really complex effects on mosquito born disease transmission. When
it comes to some of the topics I've studied, Like
so much of what we know about what seems to
drive mosquitoes to specialize on humans, it's things like either
(15:33):
urbanization or precipitation variation, but maybe not mean temperature per se.
But there's indirect ways that I still think climate change
might contribute to this. For example, climate change can very
directly contribute to things like people moving very quickly into
sort of informal settlements in rapidly growing urban areas, because
(15:54):
it's just like the old way that people were living
isn't working anymore.
Speaker 1 (15:58):
Right, So it seems like from what you're saying, urbanization
is a much clearer driver than climate change of basically
more mosquitoes and more mosquito spread disease.
Speaker 2 (16:10):
That's exactly right. Urbanization is so fast and so extreme
and represents such a major change to the sort of
ecosystem that these mosquitoes are living in.
Speaker 1 (16:19):
From like the mosquito pov urbanization domini, Yeah, exact.
Speaker 2 (16:24):
Yeah, if you're a mosquito sitting in a forest and
the forest gets chopped down and replaced with a giant metropolis,
that's a bigger change than an environmental change playing out
more subtly over longer time scales.
Speaker 3 (16:35):
Yeah.
Speaker 4 (16:36):
So if you zoom out and think about what you knew,
what your worldview was when you started studying mosquitoes, and
what you know, and kind of the way you think
about the world.
Speaker 1 (16:47):
Now, how do you think about the world differently.
Speaker 2 (16:51):
I mean, it's an interesting question because you know, the
thing that got me interested in mosquitos was trying to
understand how human are affecting our planet, right, like trying
to understand how we're affecting the environment, how we're reshaping ecosystems.
And you know, in those days, my main motivation was
wanting to protect the biodiversity, wanting to protect ecosystems, which
(17:15):
I still care a lot about. One thing that changes
just you know, like I sort of realized not everything
is like about us either like helping or heard things like,
you know, mosquitos are evolving really quickly to exploit us,
to make us sick. It's kind of amazing to me
that mosquitos could do something like evolved to be better
(17:36):
at taking advantage of us on the same kind of
timescale that we're talking about, like oh, we're degrading ecosystems,
or we're removing habitat for charismatic megafauna that we care
about or things like that, like nature. Also nature is responding.
Speaker 1 (17:51):
So it's like it's like before it was like, oh,
nature is a victim, and now it's like, yo, mosquitoes
are kind of kicking ass.
Speaker 2 (17:58):
Yeah, they're really really good at what they do, and like,
you know, we change the whole way that the place
they live looks, and they're like, I can do this too.
I can totally live here, and I can totally bite
the organisms that live here. They make it work.
Speaker 1 (18:14):
Thanks to my guest Noah Rose, now as an assistant
professor in the Department of Ecology, Behavior and Evolution at
the University of California at San Diego. After the break,
mosquitoes become a biological weapon. Eighties of gypty first came
(18:41):
to the New World on European slave ships, and along
with the mosquito came a new disease, a disease that
until that point had not existed in the Americas, yellow fever.
John McNeil is a professor of history at Georgetown and
the author of a book called Mosquito Empires. In the book,
John makes the case that yellow fever and the mosquitoes
(19:03):
that transmitted it had a profound effect on the fight
among European powers to control the Western Hemisphere. Tell me
about yellow fever historically, going back a couple hundred years,
so in the seventeen hundreds. What happens in the absence
of modern medicine if you get yellow fever.
Speaker 5 (19:21):
That all depends on where you were born and raised,
and what age you are. Children often show no symptoms,
don't know that they're sick, survive it, and in the
process acquire lifelong immunity. The worst thing to be is
in the prime of your life and encountering yellow fever
(19:43):
for the first time.
Speaker 1 (19:44):
This is going to be very important for our story today.
What happens if you were in the prime of your
life and you get yellow fever and you have never
had it before.
Speaker 5 (19:54):
Odds are you get a high fever and jaundice, that
is a sure white person in your skin begins to
turn yellow, and after several days, either you get better
you're among the lucky ones and immune for life, or
after a brief respite, you get sicker and you begin
(20:18):
to vomit up partially coagulated blood with the consistency and
coloring of coffee grounds. And when that happens, not only
are your day's numbered, your hours are numbered.
Speaker 1 (20:34):
So there's this one particular moment that you talk about
in the book where yellow fever plays a central role
in world history, and that moment is the Battle of Cartagena,
which took place in modern day Columbia. It happened in
seventeen forty one. And at this point, right, the Spanish
control a lot of South America, the British control a
(20:57):
lot of North America. And then this battle happens when
the British launched this huge attack against the Spanish in Cartagena.
So tell me about the battle. Why was this battle
so important?
Speaker 5 (21:10):
Cartagena was one of the two or three lynchpins of
Spanish colonial defense. It was also central to the trading
system of Spanish America, and that included the annual exports
of silver from mines in the Andes. Everybody who wanted
(21:33):
to take Spanish America away from the Spanish, what they
most wanted was silver.
Speaker 1 (21:39):
So was the notion that if a Britain conquered Cartagena,
they could, like you know, to some degree, conquer the
Spanish in South America, make Spanish colonies British colonies.
Speaker 5 (21:52):
Yes, that would be the first step. So back in
London they sent huge reinforcements. They had a twenty nine
thousand men between soldiers and sailors, and that was probably
the largest amphibious military expedition in world history up to
that time.
Speaker 1 (22:13):
So the British are bringing twenty nine thousand men How
many men do the Spanish have to defend Cartagena?
Speaker 5 (22:19):
Approximately four thousand, seven hundred, so they're out manned what
six six to one approximately?
Speaker 1 (22:27):
Yes? What are Spain's defenses like in Cartagena?
Speaker 5 (22:31):
The most important thing they had was massive stone fortifications, okay,
and the significance of that, which the Spaniards well understood
by seventeen forty, was that the massive fortifications would require
any attacking force to lay siege to the city, to
try to bring artillery within range, bash down the walls,
(22:56):
permit an assault, and that would take weeks.
Speaker 1 (23:01):
So the fortifications wouldn't necessarily prevent an attack, but they
would slow it down. Why is that important? Why is
it important that it would take weeks for the British
to get through the fortifications.
Speaker 5 (23:13):
It's of central importance. The reason is because in the
course of those weeks, the attacking force would be bitten
by the local mosquitoes, would acquire the yellow fever virus,
and the men in question young men in the prime
of their lives. Born and raised in places such as
the British Isles. These were people who had no prior
(23:36):
experience of yellow fever, so they are maximally susceptible and
within six weeks. The Spanish tended to think the climate,
which really means yellow fever, would wreak havoc upon the
attackers and force them to call off their siege.
Speaker 1 (23:57):
So the Spanish are using this, this virus yellow fever
as like a core part of their military strategy, because
they know that their own forces are made up of
people who have immunity. Right, they've grown up locally, they've
had the disease, whereas that is not true for the attackers.
For the British.
Speaker 5 (24:17):
Correct they didn't understand yellow fever's mode of transmission, but
they knew it was extremely deadly to people are fresh
off the boat from Europe.
Speaker 1 (24:32):
It's almost like a kind of early form of biological warfare.
Speaker 5 (24:37):
Yes, in effect, it was biological warfare.
Speaker 1 (24:41):
Okay, so this is the Spaniard's idea. The British arrive
with their dozens of ships and their tens of thousands
of men. What happens?
Speaker 5 (24:54):
So the British prosecuted a siege, tried to move man
and artillery across the difficult and damp terrain close by
to Cartagena. Progress was slow. People started to get sick
within days of disembarking from their ships.
Speaker 1 (25:14):
You mentioned that the British soldiers who had landed at
Cartagena and were laying siege to the city didn't have tents,
so they were just lying on the ground, getting bit
by mosquitoes and getting yellow fever. Yes, why what happened?
Why didn't they have tents? Like tents had been invented?
Speaker 5 (25:33):
Tents had been invented. There are two things to bear
in mind here. One is the difficulty of outfitting a gigantic,
amphibious military expedition on schedule in the eighteenth century is
not all that easy in the twenty first century. Second
one is that nobody knew how yellow fever was transmitted.
(25:56):
Being bitten by mosquitoes did not automatically seem to anyone
at the time to carry any particular risk.
Speaker 1 (26:06):
A right, like they just thought it was like my
asthma or something. So what ends up happening in the battle?
How does it end?
Speaker 5 (26:13):
The admiral got frustrated with the army commander with slow progress,
and before long they were losing hundreds of men daily
to yellow fever, and after thirty three days and a
premature attempt to storm the fortifications, the British withdrew and
(26:36):
the Spanish defenders were triumphant. They became great heroes in
Spanish military history. But what they did was not lose
before yellow fever forced their enemies to withdraw.
Speaker 1 (26:53):
Yeah, it's like the virus with the real conqueror. For Spain, the.
Speaker 5 (26:58):
Virus killed probably twenty men for every one killed in
combat in this particular encounter.
Speaker 1 (27:07):
Wow. And then you say that the same dynamic plays
out once the fighting in the Caribbean and South America
shifts from being one colonial power against another two being
wars of independence.
Speaker 5 (27:24):
Yes, and the large scale wars for independence in the
Caribbean really begin with the Haitian Revolution in seventeen ninety one,
and there British forces and then French forces tried to
prevent a slave uprising, and those British and French forces
(27:47):
suffered extreme disease mortality, and they both went after the
other gave up and Haiti won its independence. I think
it is in some ways analogous to the way that
Spanish colonial authorities used their understanding of the disease climate
(28:09):
to military advantage in places like Cartagena. And the reason
for that is their own armies were composed of men
who had either grown up in the Caribbean amid yellow fear, malaria, denge,
and other infections, or had grown up in Africa and
similarly had encountered these diseases and were either immune or resistant,
(28:32):
Whereas the forces sent out to quell the Haitian Revolution
were recruited from the British Isles, North America, France, in short,
people who had virtually no immunity or resistance against yellow
fever and malaria.
Speaker 1 (28:50):
How should this story about yellow fever and colonial warfare
in the Caribbean, How should that change the way I
think about history?
Speaker 5 (29:04):
We should understand that until the twentieth century, the great
majority of death and dying in military campaigns was not
a result of combat. It was a result of various diseases.
And there were certain times in places, including the Caribbean
(29:24):
and the seventeenth, eighteenth, and nineteenth century, in which those
diseases were you could say partisan, that is, they systematically
favored one side over another and powerfully influenced the results.
Speaker 1 (29:38):
So it wasn't until the twentieth century, after thousands of
years of war, that most people who died in war
died from injuries inflicted by the enemy rather than died
from infectious disease.
Speaker 5 (29:52):
Yes, and that entails a gigantic and brutal irony. The
mass slaughter for which the First World War is justifiably
famous was possible only because military medicine had become sufficiently
effective that large armies of millions of men could be
(30:14):
kept alive long enough to butcher one another. Prior to that,
it was impossible to maintain gigantic armies for any length
of time because they would get too sick.
Speaker 2 (30:28):
But by the.
Speaker 5 (30:28):
Twentieth century that constraint was relaxed, and mass armies could
last long enough to kill one another. That's a horrible irony.
Speaker 3 (30:40):
So if we think about this history we've been talking about,
and then we think about the modern world, where obviously
we have lots more treatments, medicine is just better now
than it was then, what lessons does this story hold
for the dynamics of the modern nace?
Speaker 1 (31:00):
What does it teach us about the way the world
works today?
Speaker 5 (31:03):
It should help us recognize the fragility of what we
might consider the Golden Age of health, an era ushered
in by public health systems, by urban sanitation, by vaccination
regimes which protect a significant proportion of the entire human
(31:26):
population from the ravages of infectious disease, and that is
a comparatively new phenomenon and it is not to be
taken for granted.
Speaker 1 (31:38):
Thank you so much for your time. It was great
to talk with you, Jacob.
Speaker 5 (31:41):
Thank you very much. Thanks for inviting me on the show.
Speaker 1 (31:49):
John McNeil's a professor of history at Georgetown and author
of the book Mosquito Empires. Thanks to both my guests today,
Noah Rose and John McNeil. Next week on the show,
Ken Measle's Cure Cancer, A very weird good thing was
going on. Incubation is a co production of Pushkin Industries
(32:10):
and Ruby Studio at iHeartMedia. It's produced by Kate Ferby
and Brittany Cronin. The show is edited by Lacey Roberts.
It's mastered by Sarah Bruguire, fact checking by Joseph friedman Or.
Executive producers are Lacy Roberts and Matt Romono. I'm Jacob Goldstein.
Thanks for listening.
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