Mosquitoes, Viruses and the History of the World

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 for solving this problem. Some of them fly out of

You Know what's kind of funny is in my old job, I would feed my own mosquitoes.

When you say I would feed my own mosquitoes, what does that mean?

Like I would stick my arm in a cage and they would bite me, and I always dress.

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.

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 think it's gross and maybe bad.

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?

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

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.

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 to answer this question, you traveled to sub Saharan Africa right to study eighties agypty. What did you find?

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.

We're an acquired taste.

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 be a mosquito. You know, mosquitoes thrive on standing water,

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 to love these clay vessels where people store their water.

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

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.

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 a lot of mosquitos on the inside and has two holes in the side.

Old factometer, like measuring their smell or something. Is that what that is?

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.

And what's the human arm? You got some undergrad sitting there all day for minimum wage or what.

Usually it's me.

It's your own arm sitting there to see what the mosquitoes.

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.

Yeah, So, how much time did you spend with your arm in the mosquito box?

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.

One hand or something. So would you listen to podcast? That's what you do?

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.

And are you getting bitten by the mosquitoes?

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.

They want so badly to suck your blood.

They really really, they're quite motivated, but they can't get to it.

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, came to be this really important disease vector. So like, tell me that story. Start a long time ago.

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

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.

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.

Year, so like a month a mosquito, mosquito lives for like a month.

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.

H huh. So that's just that much more evolution, that much more genetic iteration exactly.

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,

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.

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 in sub Saharan Africa. There's places like Waga Doogu and Brikina, Faso,

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.

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

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.

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 urbanization or precipitation variation, but maybe not mean temperature per se.

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.

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.

From like the mosquito pov urbanization domini, Yeah, exact.

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.

Yeah.

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.

Now, how do you think about the world differently.

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

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.

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.

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 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

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 for the first time.

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.

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 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.

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 lot of North America. And then this battle happens when the British launched this huge attack against the Spanish in Cartagena.

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 to take Spanish America away from the Spanish, what they most wanted was silver.

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.

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.

So the British are bringing twenty nine thousand men How many men do the Spanish have to defend Cartagena?

Approximately four thousand, seven hundred, so they're out manned what six six to one approximately?

Yes? What are Spain's defenses like in Cartagena?

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, permit an assault, and that would take weeks.

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.

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 experience of yellow fever, so they are maximally susceptible and

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.

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.

It's almost like a kind of early form of biological warfare.

Yes, in effect, it was biological warfare.

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?

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.

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?

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. Being bitten by mosquitoes did not automatically seem to anyone at the time to carry any particular risk.

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?

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 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.

Yeah, it's like the virus with the real conqueror. For Spain, the.

Virus killed probably twenty men for every one killed in combat in this particular encounter.

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.

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 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

How should this story about yellow fever and colonial warfare in the Caribbean, How should that change the way I think about history?

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 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.

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.

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 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.

But by the.

Twentieth century that constraint was relaxed, and mass armies could last long enough to kill one another. That's a horrible irony.

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?

What does it teach us about the way the world works today?

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 population from the ravages of infectious disease, and that is a comparatively new phenomenon and it is not to be taken for granted.

Thank you so much for your time. It was great to talk with you, Jacob.

Thank you very much. Thanks for inviting me on the show.

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 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.