We’ve learned time and again that animals can give diseases to humans. We’ve seen this happen with coronaviruses, the flu, Ebola — basically most major disease outbreaks in recent memory. But, of course, the reverse is true too: Humans can give viruses, including the novel coronavirus, to animals. FiveThirtyEight’s senior science writer Maggie Koerth wrote about this on the site earlier this week, and she joined PODCAST-19, FiveThirtyEight’s coronavirus podcast, to discuss her work further. The episode and a lightly edited transcript follow.
Anna Rothschild: So, to start off, which animals do we know can contract COVID-19?
Maggie Koerth: So, over the course of the last year, there’s been a lot of research on this. And some of it has been just naturalistic — this is a transference of SARS-CoV-2 that happened — some of it is stuff that’s coming from laboratory experiments on cell lines, and some of that is coming from direct animal experiments. But what we are sort of figuring out is that there are quite a few animals that actually are susceptible to SARS-CoV-2, to contracting COVID-19, from us. That includes your minks, and ferrets, kind of similar to those minks from Denmark that got COVID last year. You’ve got rabbits. You also have the cat family, so everything from lions and tigers to your domestic kitty cat living in your living room, are all susceptible to COVID. In fact, cats seem to get it pretty easily from us. You also have nonhuman primates. So this has been an issue for gorillas as well.
Anna Rothschild: So, just how sick do these animals get?
Maggie Koerth: Some of them get very sick. I mean, minks were dying from this, and some of them have very few symptoms at all. What we’ve seen sort of with the domestic cats, for instance, has been, you know, maybe a runny nose, but not necessarily even showing any symptoms, just carrying it around and transmitting it from cat to cat.
Anna Rothschild: So, this may seem like kind of a silly question. But if certain animals aren’t getting so sick, why should we worry so much about them actually contracting this disease?
Maggie Koerth: Well, to illustrate that, I will point back to the fact that Asian bats don’t necessarily get very sick with COVID-19. But they were carrying around all of these coronaviruses, including the precursor viruses to SARS-CoV-2. A population that doesn’t get very sick but gets this virus pretty readily is a population where a virus can begin to mutate and change and either jump back to humans or begin to make its hosts more sick and other hosts more sick.
Anna Rothschild: Right, it’s actually pretty similar to what we are saying about people who don’t get vaccinated. Just because the risk to you is fairly low — say, you’re young and don’t have any preexisting conditions — doesn’t mean that the virus can’t mutate inside of you if you get it and, you know, start off a cascade of new infections that are actually more dangerous.
Maggie Koerth: Right, every place that a virus has an opportunity to divide, to reproduce, is an opportunity for its genetic information to be copied. And every time your genetic information gets copied, well, that is where mutations happen. And most of the time those mutations honestly do not matter. But the way that evolution works is that sometimes they do.
Anna Rothschild: Do we have real-world examples — maybe not from COVID-19 but from past viral infections that have jumped from humans to animals — of the viruses kind of changing as they get passed back and forth between humans and animals?
Maggie Koerth: We do. Flu is actually really fascinating for this. And we know a lot of this stuff about flu because this is something researchers have been focused on studying for a very, very long time, because flu, unlike COVID-19, seems to infect agricultural animals, things like chickens and domestic ducks and pigs. So, if you look at this pig situation, and this is one of the most fascinating stories I picked up while reporting, the 2009 swine flu pandemic, is something that some of you may remember, originated on these giant pig farms in Mexico, where people were in close contact with pigs and it was getting sort of passed over from the pigs to people. At that time, when it first emerged, this particular variant of flu was not present in pigs anywhere else in the world. Today, after that pandemic has happened and people have spread it all over the planet, there is not a place on earth that farms pigs that does not have descendants of that 2009 swine flu strain in the pigs. And it is not because the pigs have been traveling all over the place.
Anna Rothschild: You mean they’re not globe-trotters?
Maggie Koerth: Oh, man.
Anna Rothschild: Sorry.
Maggie Koerth: No, no, please, I respect that and shoot your shot. Yes, they were not globe-trotters. This is people, this is people spreading this virus to pigs. So, now you have these variants that are descended from that 2009 pandemic in these pigs, and they’re starting to change again and come back into us. And there’s been at least 400 cases of kids who raise pigs for state fairs in the U.S. picking up new mutated strains of the 2009 pandemic, back from pigs to them again.
Anna Rothschild: Geez, that’s so crazy. Again, this might seem like sort of a callous question, and I sort of suspect I know the answer, but for certain animals, like bats, which humans don’t always have the best relationship with, why don’t we just eliminate that reservoir of disease? Why not just kill those animals so that the diseases can’t just jump back into humans?
Maggie Koerth: So, we can really easily, if necessary, kill an entire herd of pigs on a farm. Nobody wants to do that. But it’s a thing that you can do if you’re trying to shut down the spread of disease. That’s really, really hard to do with animals that we don’t have that kind of control over. So, I think a good example of this is that there was a species of bat that used to be extremely common in the American Northeast, one of the most common bats in that region, and people never figured out where most of these bats were spending the winter. People were studying these bats, people ran into these bats all of the time, and these bats still had these secrets that we just never knew. So you can’t kill off something if you don’t know its life. And even if you could do that, it doesn’t necessarily work.
So, I talked to this guy who — his research is studying vampire bats. And he’s very defensive about the vampire bats, because like, he kind of had this kind of sad space of like, “Well, nobody loves them, and no one cares about them. And there’s all these conservation projects to, like, save bats all over the world. And I study the bats that people are actively trying to kill.” And this is because these vampire bats — these species of bats that actually do drink blood — they spread rabies. And so there are a lot of Central and South American countries where these bats live, where there have been projects to cull them off to stop the spread of rabies among farm animals that they feed on. And one of the really interesting things that they found from that is that it does not necessarily reduce the spread of rabies to kill off all the bats in an area. In fact, it can make rabies cases go up. And they think that that is because when you wipe out an entire population of bats indiscriminately, you’re knocking out the ones that were also resistant to that virus. And then you’re leaving an ecological niche where new bats from someplace else who might not have been resistant to that virus can now sort of flow in and start doing their thing and it doesn’t necessarily actually stop the spread of disease.
Anna Rothschild: I just want to say, for what it’s worth, I actually really love bats. And vampire bats are really cool. They share blood with each other. They’re actually really good sharers. And they’ll even share it with, like, genetically dissimilar members of their group. So they’re kind of nice.
Maggie Koerth: I mean, let’s be clear, bats are adorable.
Anna Rothschild: I think so too.
Maggie Koerth: If you’ve ever wanted a snuggly-looking little mouse-fox thing that can fly … oh, my God, who hasn’t?
Anna Rothschild: I can think of some people probably who haven’t …
Maggie Koerth: They’re wrong.
Anna Rothschild: We now know that these animals can get COVID. What can we do to keep these animals safe?
Maggie Koerth: So, what we can do to keep these animals safe is honestly limiting our contact with them, and making sure that we are treating them as fellow creatures that we can spread disease to. The virus got into us, probably from bats. Those bats don’t have the brain space to sit around and have conversations about how they should be treating us. But we do. And now that responsibility is with us to make sure it doesn’t get spread to other animals.
Anna Rothschild: What are the next steps with this research?
Maggie Koerth: So, scientists are still sort of trying to figure out which bats in North America might be susceptible to this. So far, the one that they’ve actually done a live animal study on turned out not to be. And they’re still doing research on what animals are susceptible to it more broadly as well. But I think a big part of what the next steps are is just being cognizant that this is something that can happen. So, you know, limiting contact between animals that have had contact with humans that have had COVID, limiting our contact with animals when we know we are sick. Those are things that are definitely the next steps in this process.
Anna Rothschild: Well, this is clearly an evolving story. So please keep me posted as you learn more. But for now, Maggie, thank you so much for speaking with me. This was great.
Maggie Koerth: Thank you so much for having me on.
Anna Rothschild: That’s it for this episode of PODCAST-19. If you have a question you’d like us to answer on the show, email us a voice memo at email@example.com. That’s firstname.lastname@example.org I’m Anna Rothschild. Our producer is Sinduja Srinivasan. Chadwick Matlin is our executive producer. We actually filmed this episode of the podcast, so if you’d like to watch, head over to FiveThirtyEight on YouTube. Thanks for listening. See you next time.