The World’s Deadliest Animal

Dr. Biology:

This is Ask A Biologist a program about the living world and I'm Dr. Biology. Let's start off with a question based on the title of this episode. What animal causes the most human deaths each year? Let's think about that question for a moment. Now, do you have an answer? Many people think the most deadly animals are snakes, but that's not correct. Snakes are number three on the list. So, what is number two on the list? Humans come in second on the deadliest animal list. Which brings us to the number one deadliest animal for humans. And to be more specific, a blood-sucking insect [sound of an insect flying and then swatted] The animal is the mosquito. Now, just to clear up something that people misunderstand. Insects are animals. They are part of the kingdom Animalia. So, this was not a trick question. 

Now that we see the mosquito as more than a pest. It can carry some of the world’s deadliest diseases. It’s time to learn more about this pesky and deadly insect. And our guest who will help us do that is Silvie Huijben an evolutionary biologist and faculty member in the School of Life Sciences, and she's also a researcher in the Center of Evolution and Medicine. As a bonus, Huijben is also developing a cool game app that could help all of us reduce the number of these potentially dangerous insects that live in our neighborhoods and backyards. Welcome to Ask A Biologist, Silvie. 

Silvie:

Thank you very much. It's nice to be here.

Dr. Biology:

All right. Let's talk about the world's deadliest animal, or perhaps I should say animals, since there are about 3500 species of mosquitoes. So, let's begin there. And I'll even be more specific. Are some species more deadly than others?

Silvie:

Yes, for sure. So, we have, as you were saying, about 3500 different mosquito species. It's only about 200 of those that we should really be worrying about. Those are the ones that can transmit diseases to humans. So, a lot of mosquitoes are really quite harmless and some of them don't even bite at all. They don't take any blood meals.

Dr. Biology:

So, we have 3500 mosquitoes. We've got a subset that we need to really worry about. You talked about these diseases. What are the top ones that are the most challenging for humans, in this case, the most deadly? 

Silvie:

Right. So very good question. And the very first one is malaria. That's on top. Malaria kills about 600,000 people every year. Most of those are kids under the age of five. So, this is a very dramatic disease, and it mostly occurs in Africa a little bit also South America, and Asia. But Africa gets the big share. About 90% of all mortality is happening in Africa. So, that's by far the most worrisome one. 

Then we also have dengue virus. So, dengue is also a very big vector-borne disease. It infects a lot of people. It doesn't have as high of a mortality than malaria does, but it is a very debilitating disease. 

And then we also have Zika that, you know, a lot of you will probably have heard of from 2016 when there was a big outbreak and a lot of worry about that disease. And there's some others. We have chikungunya, which is also a bad disease. 

West Nile virus is a problem in the United States, particularly in Arizona, where we are here is having a big problem with West Nile virus. And there are several more. So, really, there is a lot of vector-borne diseases and mosquito-borne diseases.

Dr. Biology:

All right. So, let's talk a little bit about a term we're using here, vector-borne.

Silvie:

Right?

Dr. Biology:

When we talk about vector, it's actually like you can imagine a starting point. Right. There is a basic area where something is originating from. So, the vector in our case is?

Silvie:

A mosquito.

Dr. Biology:

Right. Or a group of mosquitoes.

Silvie:

Or a group of mosquitoes. Right. Different species.

Dr. Biology:

Right. So, they're bad?

Silvie:

Yep.

Dr. Biology:

So, if they're so bad. Is there any benefit to having mosquitoes around?

Silvie:

Well, that's the million-dollar question. And a lot of people ask me this question, why do we have mosquitoes? Can't we just get rid of all mosquitoes and be done with it? And the answer is, we don't know. Mosquitoes might have a role in the food chain. We know that there are fish that eat the larvae of mosquitoes. If we get rid of all those mosquitoes, we get rid of their food sources and those little fish might be in trouble. And then the big fish that eat the little fish might be in trouble. And the same for the adults. Another thing is we don't quite know how important mosquitoes are as a pollinator. So, mosquitoes, believe it or not, eat nectar from the plants and from the flowers. And therefore, there is some evidence that mosquitoes do play a small role in pollination of certain plants. So, getting rid of mosquitoes might not be our best approach.

Dr. Biology:

Right. And I've also done some reading and it seems like they're an important source of food for, you know, birds. Bats. 

Silvie:

Correct.

Dr. Biology:

Dragonflies.

Silvie:

Yes.

Dr. Biology:

And I think some frogs.

Silvie:

Frogs? Yes, exactly. So, there is quite a lot of animals that depend on mosquitoes as a food source. What we don't really know is how important they are. And, of course, if we want to get rid of mosquitoes, we don't want to get rid of all mosquitoes. We just need to get rid of those mosquitoes that cause diseases. And then we need to know how important is this specific species in the food chain and in the ecosystem?

Dr. Biology:

So tell me why there are certain species of mosquitoes that carry a particular disease. That's interesting to me because there can be multiple species of mosquitoes in one neighborhood or in your backyard. Why is one carrying the disease while the other is not?

Silvie:

Right. So, the true answer to that question I don't have. As with any kind of disease-causing virus or parasite is they can be very host-specific. And what I mean with that is that some diseases only affect humans. Other diseases only affect birds. Others only affect a snake. And so, the same is for the mosquitoes. So, some of these viruses and some of these parasites can only complete their lifecycle in a certain species of a mosquito. The exact mechanism of that, I do not know, and I'm not sure if we as a scientific community really have the answer to that question.

Dr. Biology:

So, future scientists, there's another question out there. Boy, I'm telling you, there are millions of them—lots to do.

Silvie:

Yes.

Dr. Biology:

All right. When we talk about mosquitoes and we talk about deadly mosquitoes, we're actually talking about only the females.

Silvie:

Yes. 

Dr. Biology:

So, let's talk a little bit about what is the process of a female mosquito and how does that play in the role of transmitting diseases?

Silvie:

All right. To answer that question, I think we should go back to first. What is the life cycle of a mosquito? And a mosquito is very similar to a butterfly. We start with an egg and a female mosquito lays an egg in the water or on the surface of the water. The egg hatches and from that, we have a little larva. The larva goes through four different stages, so it molds every time and becomes bigger and bigger, and then it becomes a pupa. Now the pupa of a mosquito is actually we call it a tumbler. It's not stationary. Actually, it tumbles through the water, hence the name Tumbler. 

The tumbler doesn't eat at all. That takes about one or two, three days, and then an adult mosquito emerges. Now we have male and female mosquitoes. And they need to mate to be able to get the next offspring right to make the babies. And so the female makes the eggs and the female needs blood to be able to incubate those eggs. She needs the proteins and the iron that she takes from the human blood or from any kind of blood. It doesn't have to be human blood to produce her eggs. 

Now, in the process, when she bites a person and that person has a certain disease like malaria, for instance, let's take malaria as an example. When she's taking up that blood meal, she's also taking up some of those parasites that are living in the blood of that person that's infected with malaria. And now that parasite is in the mosquito. Now, that mosquito can't immediately transmit it to the next person. 

The parasite needs to complete the whole life cycle through the mosquito and that takes about 7 to 14 days for the whole parasite to go through to the mosquito, which means the parasite goes through the midgut of the mosquito forms a big sac of parasites on the outside of the guts, and then bursts out and then they migrate to the salivary glands of mosquitoes.

So, what mosquitoes need to do, this is very interesting, is when they bite you, they don't want to be smacked. Right, for very obvious reasons. So, what a mosquito does is they inject a little bit of a numbing agent so that you don't feel her when she's piercing your skin and also a little bit of an anticoagulant. So basically, that your blood flows very freely so she can take it up. So, what's happening is those parasites are inner salivary glands and she spits those parasites out with those substances and now she's injecting those parasites into a new person. And now that person is infected with malaria. In this case.

Dr. Biology:

Wow. Quite a process.

Silvie:

It is quite a process. And what boggles me as an evolutionary biologist or a biologist, in general, is this process takes up to two weeks for a parasite to complete their lifecycle. But an adult mosquito really only lives about two weeks as well. So, it's almost mind-blowing that we have so much malaria because there shouldn't be as much. Many mosquitoes are dying before they are able to transmit malaria.

Dr. Biology:

I hadn't thought about that. And when you mention that it's not just humans that we'd have to worry about here. So, are other animals impacted by malaria or other diseases that the mosquitoes transmit?

Silvie:

Yes. So, malaria is a big problem all throughout the animal kingdom. There are lots of different species of malaria. Now, most of those species can't infect us as humans, but we have a lot of different species of avian malaria. Malaria can infect reptiles. It can infect birds. It can infect apes. Basically, it can infect almost any animal on the planet. 

And how I got into malaria research actually was through a project where penguins in a zoo in the Netherlands were infected with malaria every single summer. And this was a big problem because a lot of the penguins would actually die because of this disease. And so, the zoo wanted to know what's causing this particular malaria. They knew it was malaria. But what is a parasite and what is the species of mosquitoes that is transmitting this? That's how I got into malaria. But then the penguins left, but the malaria stuck.

Dr. Biology:

That’s interesting. I wouldn't have even thought about the penguins. Although I was doing to me reading and I think there was, what, one continent that didn’t have mosquitos and one other place, think. Let’s see the continent was Antarctica.

Silvie:

Right. Only in very, very cold places. You wouldn't find any mosquitoes. But there's not many places on the earth where you can't find mosquitoes. To be honest, I don't know the distribution of mosquito species in general. But yes, Antarctica wouldn't be the place.

Dr. Biology:

Right. And I think there was. Oh, I know the other one. Iceland.

Silvie:

Oh, Iceland doesn't have mosquitoes?

Dr. Biology:

Yeah. According to my latest research. So, you mentioned that you got into this through penguins, which fascinates me. I just wouldn't have thought of them as being susceptible to malaria.

Silvie:

Right. And that's the problem because these penguins aren't naturally occurring in the Netherlands as you can imagine. In this case, these were penguins from South Africa. And so, they don't have this particular malaria species in their natural environments. So, they didn't build up any kind of immunity against this avian malaria. So, then we take them out of their normal habitats, place them in a zoo somewhere in Western Europe. And the same problem is in the U.S. as well in the zoos. And now they are infected with these parasites that they don't have any immunity against it. So, that's why they are so affected.

Dr. Biology:

With your research, what are you doing right now? What is your greatest question?

Silvie:

Right. So, I'm an evolutionary biologist. And what I'm really interested in is how do organisms evolve? Resistance against the chemicals that we use to try and control them?

Dr. Biology:

Ah, the arms race!

Silvie:

Yes, exactly the arms race. So, we use insecticides to kill the mosquitoes and we use drugs to kill the parasites. That's our main two mechanisms of action that how we control malaria. But the parasites and the mosquito, they fight back. So, they evolve resistance. The mosquitoes, evolve resistance against the insecticides and the parasites, evolve resistance against a drug. 

So, what I'm interested in is how does that process really work? So how does a resistant parasite? How is it able to spread all over the globe? Basically, because that's the problem that we've been seeing. You start with just one parasite that's resistant and a couple of years later, that one parasite has multiplied and has now replaced all those parasites in Africa that were susceptible to this drug. So, I'm interested in how does that process work? And then if we understand how it works, then we can try and see can we intervene with that? Can we change it so that resistance isn't as much of a problem?

Dr. Biology:

There's also talk about genetically altering the genes, the genome of mosquitoes, to basically make them sterile so that we can get rid of them. No more mosquitoes. The term CRISPR is something that's been in the news for quite a while. We have great information and great videos on it on Ask A Biologist if anyone wants to learn about the process of CRISPR. But the question here is actually I have two questions. One is, can we get rid of all the mosquitoes? And the second one is, should we get rid of all the mosquitoes? And we talk a little bit about the should we a little bit before. But the other question is really, can we get rid of all the mosquitoes?

Silvie:

That's a really good question. [laughter] Can we get rid of all the mosquitoes? I don't think we can. I think there's so many mosquitoes living, particularly in tropical areas, that it's very difficult even to suppress them to very low numbers. We are able to do that to get rid of completely all those even just one species. So far, we haven't been able to do it. We can on small islands. So, if we have an isolated place, then we can get rid of like certain mosquito, species. But on the whole continent of Africa, for instance, I think that's just nearly impossible to do. Never say never, though. 

And with new technologies, things might be possible that we currently think are not possible. I do think that the gene drive technology that you mentioned is really a great step forward. And the reason why I think that is so far our biggest hurdle with targeting mosquitoes and trying to kill mosquitoes is us, humans because it's very difficult to get to all those remote areas where mosquitoes live as a human being. And the idea of gene drives is that the mosquitoes are kind of spreading it themselves and they are able to go to all those places. 

So, taking the human out of the equation and letting the mosquitoes being their own downfall, that might work. But even then, it requires still a lot of releases of mosquitoes. So, we still have that human factor, because I don't think the gene drive is at the moment efficient enough to really have some releases and then let it sweep across the whole continent. So, we're probably going to have to have a lot of independent releases. So, you still have that human factor there.

Dr. Biology:

That brings me to the question of what we can do ourselves. And I'm not talking about opening up the lab in your garage [laughter] and doing your own genetic work or spraying a ton of insecticides around. There are some things we can do to reduce the chances of mosquitoes spreading a parasite or a virus.

Silvie:

Right. We can. And there's quite a bit that we can do. And it depends on where you live across the globe. Now, if you are living in the United States, for instance, then the main mosquitoes that you need to be worried about are Culex mosquitoes. Those are the ones that transmit West Nile virus and Aedes aegypti mosquitoes. Now, this last one, Aedes aegypti mosquito, you can recognize very easily, if you just take a moment to look at the mosquito before you slap it and you look at their legs and they have these black legs with white little stripes on them, it's quite easy to see. So those are Aedes mosquitoes. 

Depending on where you live, you have Aedes aegypti or Aedes albopictus, two different species, and sometimes they're called, a tiger mosquito because of the stripes. Now, this group of mosquitoes loves to be very close to humans. We always find them in places where humans are, too. And that's because we make the perfect breeding grounds for them. So, these types of mosquitoes, these Aedes mosquitoes, they love to lay eggs in very clean water sources. So that can be a cup of water filled with rainwater, for instance, or a birdbath or a flowerpot that has a drainage at the basin where the water collects. So, anything that collects water, it can be as small as the lid of a Coke bottle, for instance. 

If it fills with water, either rainwater or just irrigation water mosquitoes will be able to find it and they're going to lay their eggs. And then you start it with one mosquito, and one mosquito can lay up to 100 eggs. And before you know it, you have a hundred mosquitoes that all can lay eggs again as well. So, during these warm seasons, it can go really fast. 

So, you are asking, what can you do about it? Well, the very obvious answer to that is make sure there are no breeding sites available. So, the best thing to do is check your yard, go outside and just look in your yard and see what could collect rainwater when it rains. Right. So that can even be discarded toys like your toy truck that is sitting in the yard, and it can collect water. Right. So those are the Aedes mosquitoes. 

Now the Culex mosquitoes, they like a little bit more larger water bodies. So, this is when your pool turns green, for instance, if you're a large pool in their backyards and it turns a little bit green because you didn't shock it in time before the temperatures were going up. That's what Culex mosquitoes like to lay their eggs. Or if you have a big and chlorinated pool, for instance, like a baby pool or something. Those are the places where Culex mosquitoes like to lay eggs. 

Now there is a lot of places that you can't control because they're the places outside of your yards. Then if you can't control those breeding sites, what you can do is you can protect yourself from mosquitoes that come and bite you. And one of the best ways is to make sure that you're not outside when mosquitoes are at their peak activity. So, during dusk, for many mosquitoes, it's good to just move inside if you have an air-conditioned place or close the windows and the doors. If you can't do that, then the best is to wear long-sleeved shirts and trousers so that you don't have any exposed or minimize your exposed skin. 

And then lastly, what you can do is use some ointments or treatments on your skin to repel mosquitoes. If you need to be outside. And the best one is containing DEET. That's DEET. However, this is also a harsh chemical. So, this is something that you don't want to use unless it's really necessary. For instance, you travel to Africa, and you visit a place where there is a lot of malaria. That's when you want to use that. If it's just in your backyard. There are some other treatments that you can use too that have, for instance, citronella oil or eucalyptus oil. And you can look on the CDC website. It has a whole list of products that actually do work because there's a lot of products that don't work. Also on the market.

Dr. Biology:

Oh, and that reminds me.  You're actually working on a game right now.

Silvie:

I am.

Dr. Biology:

I have to say, we're actually working on.

Silvie:

Exactly. [laugther]

Dr. Biology:

And I should say many of our colleagues are working on the game. So can we talk a little bit about the game?

Silvie:

Yes. This is our mosquito hunter game. And I'm very excited about it. It's going to be a game that you can play on your phone or on your tablet and you go outside and you're going to scan your backyards or your park or any outdoor place, really. And you're going to look for potential breeding sites for mosquitoes. So, for instance, you find a flowerpot. 

So, now you can point your camera at the flowerpot and the game is going or the app is going to tell you this is most likely a breeding site for mosquitoes. And now you can earn points to try and get rid of that breeding site, for instance, by tipping it over or moving it under a place where it can't collect any rainwater or irrigation water. The aim of the game is to teach you how to recognize mosquito breeding sites, potential mosquito breeding sites, and what you can do about those.

Dr. Biology:

Right. That's the best way to reduce this is to reduce the number of mosquitoes is to reduce the number of places they can reproduce. 

Silvie:

Exactly.

Dr. Biology:

Now Silvie on Ask A Biologist, there are always three questions every scientist gets. You can't leave until you get these three questions.

Silvie:

[laughter] Okay.

Dr. Biology:

All right. The very first one. When did you first know you wanted to be a scientist?

Silvie:

I think I knew from since I was very, very young. When I was really young, I wanted to be a car mechanic. But that very soon got replaced by I wanted to be, um. I don't even know what the English word is. In Dutch, we say 'laborant’. Someone who works in the laboratory. And this is since I was six or seven years old and I still remember in primary school, we had to write a story about what do you want to be when you grow up? And the best ones got picked to be read out in church. And mine got picked. 

And I really remember this vividly. And my story was I wanted to work in a laboratory. This was the late eighties, and HIV was a big problem. So, I wanted to find a cure for HIV. Win the Nobel Prize. And then give all that money to the poor people. So, that tells you what kind of kid I was. So, not working on HIV. [I] haven't won the Nobel Prize just yet, but that was the dream when I was about eight years old.

Dr. Biology:

Wonderful. What a story. And now I get to be really evil because I'm going to take your science away from you.

Silvie:

Okay.

Dr. Biology:

No laboratory, no teaching. I'm going to take that away. No game development. I might even let you develop your game.

Silvie:

[laughter] You're harsh.

Dr. Biology:

I am. Is there something you've always wanted to do or would do if you had the abilities or the capabilities? If you couldn't do your science right.

Silvie:

Well, the first thing that comes to mind, and this has always been my backup plan, I guess, is I would love to have bed and breakfast somewhere out in Scotland, for instance, or in the Pyrenees, somewhere where hikers come round, and they can spend the night move on. That sounds like a nice. Yeah, just a nice, completely different lifestyle. But still being in touch with nature. Right. 

So, I can see that and my other alternative plan, I guess is I took a creative writing course as part really for improving my scientific writing. I think scientific writing is very often dry and can be improved by just telling the story a little bit more. So, I wanted to do a creative writing course to really improve my scientific writing. But while taking that cause, I really enjoyed just writing stories and I never really done anything with it. But I guess if you would be harsh and take it all away from me, maybe while I wait for those hikers to come round in my bed and breakfast, I can work on writing just a novel.

Dr. Biology:

Right. Could. Could blend the two.

Silvie:

Yeah. There you go.

Dr. Biology:

What about the car mechanic?

Silvie:

Yeah. No, I'll leave that to the experts. [laughter]

Dr. Biology:

All right. The third question, what advice would you have for a young scientist or perhaps someone that actually started out a different career and decided that they don't want to be the auto mechanic? Or maybe that bed and breakfast isn't working out for them, and they want to get into science. What advice would you have?

Silvie:

Gosh, I have so many. But I guess the main thing is you want to be curious, right? You need to ask questions, but I think it needs to go further than that. And be very critical in your thinking. So never take an answer for. Okay. That's done and dusted. Let's go to the next question because science is always evolving. It's always changing. And I think even a lot of scientists sometimes can fall into the trap of thinking they know the answer. So, my biggest advice is to play devil's advocate always with all the things that you have found to what if what I just discovered isn't true? What would my biggest critics say? And I think that's always just a very good approach to take. And being very self-critical on what you read, what you discover yourself.

Dr. Biology:

Right. And we're not always saying that what you learned or what we know is completely wrong. It's usually that we find out there's more to the story...

Silvie:

Exactly.

Dr. Biology:

..than we thought.

Silvie:

Right.

Dr. Biology:

So an example is we often will say, oh, we'll turn this gene on or this gene off, and we will cure this or fix that. The problem is it's usually not just one gene.

Silvie:

Right.

Dr. Biology:

It can be numerous genes because science, a lot of people think well, in particular, they think there's a book on the shelf that has all the answers.

Dr. Biology:

Right.

Dr. Biology:

And there isn't a book with all the answers.

Dr. Biology:

And it isn't that we find out that the answers we have are completely wrong. Sometimes we find there's more detail to it.

Silvie:

And usually, the story is just more complex than we thought. So even for malaria, we thought if we want to get rid of malaria, we need to get rid of these two species of mosquitoes. And now we're starting to get rid of those mosquitoes, or at least they're in lower numbers. And what we find is, oh, wow, there's this whole group of mosquitoes that don't come in the house. So, we never put a trap there. So, we didn't know they existed, but they bite outside. And so now we just know more. So, we need to target those mosquitoes.

Dr. Biology:

Well, with that final thought, Sylvie, I want to thank you for being on Ask A Biologist.

Silvie:

Yeah. Thanks very much for having me.

Dr. Biology:

You have been listening to Ask A Biologist and my guest has been Silvie Hyben an evolutionary biologist and faculty member in the School of Life Sciences. She’s also a researcher in the Center for Evolution and Medicine at Arizona State University. 

Now be sure to check out our episode notes and transcript for additional links – including one to our insect lifecycle story called – Metamorphosis – Natures Ultimate Transformer. And in case you didn’t know, there’s more than one-way insects go through their life cycle. AND for those who listen to us using our app that shows chapter links and images we will also include images there.

The Ask‑a‑Biologist podcast is produced on the campus of Arizona State University and is recorded in the Grass Roots Studio housed in the School of Life Sciences, which is an academic unit of the College of Liberal Arts and Sciences.

And remember ‑ even though our program is not broadcast live, you can still send us your questions about biology using our companion website. The address is askabiolgist.asu.edu. or you can just Google the words Ask a Biologist 

As always - I’m Dr. Biology – and I hope you are all staying safe and healthy.