[RADIOLAB INTRO]

LATIF NASSER: Hey, I'm Latif. This is Radiolab. And today I have with me a Sarah.

SARAH QARI: Okay, no echo or anything?

LATIF: Nope.

SARAH: Okay, great.

LATIF: As in Radiolab reporter/producer Sarah Qari.

LATIF: Can you hear me without an echo?

SARAH: Oh, yeah. No, you sound great.

LATIF: And ...

SARAH: Sarah's here in the studio.

LATIF: ... I have a Sarah.

SARAH ZHANG: Hello.

LATIF: Hi!

SARAH ZHANG: Hi! Nice to talk to you.

LATIF: Sarah Zhang, who writes for The Atlantic, and is a wildly prolific science journalist.

SARAH ZHANG: Oh, thank you.

LATIF: Like, I feel like you cover COVID. I know you cover autoimmune diseases. I know you cover Ozempic.

SARAH ZHANG: I'm sort of lucky enough to cover just, like, whatever I'm interested in.

LATIF: But we called her up because a while back, she noticed something weird happening to the deer in the Florida Keys.

SARAH ZHANG: I don't know if you know about the Key deer. They're, like, really cute. They kind of like wait at bus stops, and people like to feed them.

LATIF: Okay.

SARAH ZHANG: And so back in 2016 ...

[NEWS CLIP: ... calling a very grave situation for this endangered species.]

SARAH ZHANG: ... people suddenly started noticing there were all these, like, ugly, like, gristly wounds on these deer.

[NEWS CLIP: Gaping holes or wounds in their neck or in their—in their head.]

SARAH ZHANG: Literally, you could see down to the bone.

[NEWS CLIP: Gruesome, painful.]

LATIF: Ooh!

[NEWS CLIP: It's a very sad sight.]

SARAH ZHANG: So it was—you know, it was kind of like a "weird thing happening in Florida" story. [laughs]

LATIF: Yeah.

SARAH ZHANG: But ...

[NEWS CLIP: Volunteers are teaming up to treat affected animals.]

SARAH ZHANG: ... what it turned out was that there was an infestation of an insect called ...

[NEWS CLIP: The New World screwworm.]

[NEWS CLIP: New World screwworm.]

SARAH ZHANG: ... the New World screwworm. Also known as flesh-eating worm.

LATIF: Oh!

SARAH ZHANG: And, like, I had never heard of screwworms before.

LATIF: Yeah, me neither.

SARAH: Yeah. No.

SARAH ZHANG: I would imagine most Americans have never heard about screwworms before. And the reason we don't know about it is because it's been eradicated from our country.

LATIF: Huh!

SARAH ZHANG: I was like, what? Like, that's crazy. I've never heard of, like, an insect being, like, completely eradicated from our country.

SARAH: And as Sarah started to dig into this ...

SARAH ZHANG: It got even crazier and crazier.

SARAH: What she found was the kind of amazing story of one of the biggest environmental interventions that humanity has ever undertaken. And the story of this worm, this honestly nightmarish, flesh-eating parasite that despite those efforts is right now today back in the news.

[NEWS CLIP: We're on screwworm watch, and we are ready. And ...]

SARAH: Forcing public health officials and ranchers and ethicists ...

GREGORY KAEBNICK: Here we are in the sixth major extinction that we humans are causing.

SARAH: ... to ask maybe one of the biggest questions that we as beings on this planet can ask.

GREGORY KAEBNICK: Would it ever be okay to bring about the extinction of a species?

LATIF: Oh boy.

SAM KEAN: Yeah. But I mean, these are about the worst parasite on Earth. I cannot imagine anything worse than these guys.

LATIF: So one of the first people we called when we got into this whole screwworm thing was the author named Sam Kean.

SAM KEAN: My name is Sam Kean. I am a science writer.

LATIF: Because it turns out he, like Sarah, at some point had fallen into the screwworm hole.

SAM KEAN: Yeah. I had never heard of them. And I started googling, and then I went to Google Images, and I immediately regretted doing that. [laughs]

LATIF: And according to Sam, the reason that none of us had ever heard about what he called the worst imaginable parasite on Earth is because of something that in 1970, the New York Times called ...

SAM KEAN: "The single most original thought of the 20th century."

SARAH: And it turns out that thought was born in the brain of a man named ...

SAM KEAN: Edward Knipling.

SARAH ZHANG: Edward Knipling.

SARAH: So as you know, Latif, when we first learned about this, I got obsessed and started doing some digging, and learned that Knipling has unfortunately passed away. But ...

[ARCHIVE CLIP, interviewer: Dr. Edward F. Knipling, we're—we're happy to be here in your home today, and thank you for participating in this oral history.]

SARAH: ... I found this whole trove of interviews that he did back in the day.

[ARCHIVE CLIP, interviewer: If you could start, please, by telling us when and where you were born.]

[ARCHIVE CLIP, Edward Knipling: I was born in Port Lavaca, Texas, March 20, 1909.]

SARAH: Knipling grew up on his family farm in Texas. And ...

[ARCHIVE CLIP, Edward Knipling: In those days, farming was a very difficult occupation.]

SARAH: Honestly, mostly because of the—what was I supposed to say after that?

[ARCHIVE CLIP, Edward Knipling: The screwworm.]

SARAH: Oh. Mostly because of screwworms.

SARAH ZHANG: Yeah. So it was—it was a problem for ranchers. Actually, I think this is a quote I remember. It was "Screwworms used to strike fear in the hearts of ranchers all throughout the Southern United States."

LATIF: Can we actually—could you just tell me what a screwworm even is?

SARAH ZHANG: It is a fly. It has kind of like ...

LATIF: Why do they call it a worm if it's a fly?

SARAH ZHANG: Well, flies are also maggots, right? Or flies come from maggots, so the maggot phase is a worm.

LATIF: Okay. Okay. Okay.

SARAH ZHANG: So basically what they do is that if you have a little, like, nick, a little wound ...

SAM KEAN: Even something as small as a tick bite.

SARAH ZHANG: ... these flies would lay their eggs in those wounds.

SAM KEAN: Roughly 400 eggs at a time.

LATIF: [laughs] Oh my God!

SARAH ZHANG: And then these maggots, essentially, would come out. They kind of look like a small white thread.

SAM KEAN: They have this kind of horrifying mouth with two sharp teeth, and a little ridge on their body that sticks out exactly the way the threads on a screw do, and would twist themselves down.

SARAH ZHANG: Kind of burrow themselves into the flesh like a screw. And they would eat the flesh.

SARAH: Ugh!

SARAH ZHANG: Yeah, it's ... [laughs]

SAM KEAN: And they are extremely hard to get out once they have locked in.

[ARCHIVE CLIP, Edward Knipling: A screwworm would get into the navel of calves when they were born, or ...]

SARAH ZHANG: And so back to young Edward Knipling ...

SAM KEAN: He got probably the worst job on the farm, which was to pick the screwworms out of the family's cows.

[ARCHIVE CLIP, Edward Knipling: And that was a very unpleasant task. [laughs]]

SAM KEAN: Have to try to yank them out of these animals that obviously aren't happy about this.

LATIF: Is there a tool that you use? Like, what do you ...

SAM KEAN: I'm guessing tweezers or just fingers.

LATIF: Screwdriver! You gotta use a screwdriver.

SAM KEAN: That was his job. And it introduced a lifelong hatred of screwworms in his heart.

LATIF: Huh!

SAM KEAN: You know, understandably so.

SARAH: But at the same time, you know, he has sort of a scientific bent of mind. He's curious.

SAM KEAN: He was actually kind of fascinated by insects.

SARAH: Over time he became a bug nerd.

SAM KEAN: Definitely a bug nerd.

SARAH: Even named his cats after insects.

SAM KEAN: One after a mosquito and one after a boll weevil. Anthonomus and Qlax.

SARAH: And when he grew up ...

SAM KEAN: Grew up to be an entomologist. But also kind of in the back of his mind, he was always thinking, "I have to figure out a way to control screwworms. I want to put a stop to them somehow."

SARAH: And as it happens ...

[ARCHIVE CLIP, Edward Knipling: The first job I had ...]

SARAH: ... in the late '30s, he got a job at the USDA.

[ARCHIVE CLIP, Edward Knipling: ... as an entomologist, working on the screwworm. That was the Bureau of Entomology and Planned Quarantine at that time.]

SARAH ZHANG: And then World War II breaks out.

SARAH: And the military enlists him in developing ...

[ARCHIVE CLIP, Edward Knipling: Insecticides and repellents for use by the—for the armed forces.]

SARAH ZHANG: He actually ends up helping develop DDT.

LATIF: Oh! Yeah. Heard of that one.

SARAH ZHANG: And this is actually sort of part of his journey, which is that he saw insecticides can be really effective, but they can also be really devastating for the environment.

LATIF: Mmm.

SARAH: So after the war, when he gets back to his screwworm job, he's just thinking about this problem.

SARAH ZHANG: Like, how can we figure out a way to control insects that does not require spraying lots of poisons?

SARAH: And his way of trying to figure that out is watching screwworms mate.

SARAH ZHANG: [laughs] Just watching a lot of insect sex.

LATIF: As one does. Like, why—yeah, why?

SARAH ZHANG: I think he was just trying to understand these pests, right? To think, like, what could we do about it?

LATIF: Huh.

SARAH: And so one day, a colleague of Knipling's is, you know, watching the screwworms have their sexy times, and he makes this observation.

SARAH ZHANG: That sounds like maybe not that important or not that interesting.

SAM KEAN: But that kind of hinted at something, he wasn't quite sure what exactly.

SARAH ZHANG: Which is that females only mate once.

SAM KEAN: Whether or not they get pregnant, they get one shot to have intercourse and try to have eggs.

LATIF: Hmm!

SARAH: I really feel for these females. Like, this is such a high stakes—you have to have the best sex of your life in that one time.

SAM KEAN: Yeah, that's it.

SARAH: But Knipling, he's looking at this and he's like, "Wait."

SARAH ZHANG: If he could just do something to all of the males, right? Like, if he could just make the males sterile.

SARAH: And then if he could trick the female screwworms into mating with these sterile males ...

SARAH ZHANG: The females aren't gonna lay any eggs that are viable.

SAM KEAN: That would essentially take those females out of circulation for reproduction purposes.

LATIF: Hmm.

SARAH: Hmm. Their one shot would just be doomed to fail, basically.

SAM KEAN: Yes, exactly.

SARAH ZHANG: So just, like, somehow just flood the zone with sterile males is the idea.

SARAH: These poor females already had it so hard, and now he's just, like, ruining the dating pool for them.

SARAH ZHANG: It's a—it's a little diabolical from the fly's perspective.

SARAH: But lucky for these screwworm ladies ...

SAM KEAN: It wasn't like that was a very practical idea.

SARAH: Because how would you even pull that off?

SAM KEAN: You know, how do you mass sterilize a bunch of insects?

SARAH: But then he finds a paper.

[ARCHIVE CLIP, Edward Knipling: An article, a scientific magazine ...]

SARAH ZHANG: Basically, since World War II, there's sort of like a lot of interest in, like, what can we do with radiation?

LATIF: Oh, of course. Right. Yeah.

SARAH: And this paper was by a geneticist who was saying that ...

[ARCHIVE CLIP, Edward Knipling: It was possible to sterilize fruit flies by exposure to x-rays.]

SAM KEAN: And I think he sort of jumped off of his couch and said, "Oh my God!"

[ARCHIVE CLIP, Edward Knipling: Maybe we could sterilize screwworms.]

SAM KEAN: Like, this could be the solution to the problem.

LATIF: Okay, so what—yeah, what does he—what does he do?

SARAH: Well, so the thing he needs is a bunch of X-ray machines.

SAM KEAN: But he decides he is not going to go public and try to get any funding for this.

[ARCHIVE CLIP, Edward Knipling: We knew that if the media got a hold of that, they could make quite a deal out of this.]

SARAH: I mean, remember he's working for the government. Like, taxpayer money's at stake here.

SARAH ZHANG: What happens if the press gets hold of this? And then we're just totally ridiculed for, like ...

[ARCHIVE CLIP, Edward Knipling: Wasting money or something.]

SARAH ZHANG: ... watching insect sex all the time.

SARAH: Not in a weird way. [laughs]

SARAH ZHANG: Yeah.

[ARCHIVE CLIP, Edward Knipling: We were rather cautious about that.]

SAM KEAN: So he decides he wants to do this kind of on the down low.

SARAH: And basically, he gets one of his colleagues ...

SAM KEAN: To take a bunch of screwworms, and sneak them into a nearby military hospital and use the X-ray machines there.

SARAH: Flesh-eating worms in a hospital. Great.

SAM KEAN: Yeah, exactly.

SARAH: How does he get the screwworms in?

SAM KEAN: Unfortunately, we don't have many details about what he did to actually get inside there. I picture, you know, him, like, hanging out at the loading dock at night and, like, grabbing the door right before it shuts or, you know, like Watergate, like, taping the lock or something like that, sneaking through the hall. I believe they did this at night, too. He had to be kind of clandestine about it, sneak around a little bit.

SARAH ZHANG: And they are literally, like, you know, putting these flies through an X-ray machine and being like, hey what happens?

SARAH: But the problem with shooting a bunch of radiation at a bunch of flies ...

SARAH ZHANG: Is that you create all these mutations, a bunch of random mutations. So you might just kill the fly. That's kind of a problem.

LATIF: Right.

SARAH ZHANG: So they found that—they figured out the right dose, but they also figured out exactly when to put them through radiation. It's when the flies' testes are developing, right? Because that's, like, really what you really want to knock out. So that happens to be between 5.5 and 5.7 days.

LATIF: So it's like they've got it to within hours.

SARAH ZHANG: Yes, yes.

SARAH: And when they do that? Bingo.

SARAH ZHANG: It seems to work.

SARAH: But ...

[ARCHIVE CLIP, Edward Knipling: Of course, the next thing was would they perform in a natural population?]

SARAH: ... it's not enough to just do it in a lab.

SAM KEAN: So then he decides it's time for a real-world test. And he found an island off Florida.

[ARCHIVE CLIP, Edward Knipling: On the island of Sanibel.]

SAM KEAN: Sanibel Island.

SARAH: He shows up there with some of the radiated screwworms from his lab, releases them on the island.

SAM KEAN: And it did not work.

SARAH: Like, the population of screwworms stayed pretty much the same.

SAM KEAN: The experiment failed.

LATIF: Hmm.

SAM KEAN: And Knipling was glad he hadn't said anything or, you know, gone after public money.

SARAH: So yeah, does he just resign to his failure?

LATIF: That's the end of the story, yeah?

SAM KEAN: No, not the end of the story.

[ARCHIVE CLIP, Edward Knipling: We were kind of stymied what to do for a year or so. And then I got a letter from a veterinarian on the island of Curacao.]

SAM KEAN: A Dutch island called Curacao.

LATIF: Where is Curacao?

SAM KEAN: Off the coast of Venezuela.

[ARCHIVE CLIP, Edward Knipling: He wrote a letter.]

SAM KEAN: And said, "The goats are being ravaged. I know you study this. Can you help us in any possible way?"

[ARCHIVE CLIP, Edward Knipling: And I thought, well, this is just the place that we're looking for.]

SARAH: And this time, Knipling wants to go all out.

SAM KEAN: They were not going to take a chance that there would be too few flies.

SARAH: Like, no more handful of flies from his rinky-dink lab.

SAM KEAN: They're gonna set up an industrial facility for making these flies.

SARAH: And, like, carpet Curacao.

SAM KEAN: And really overwhelm them with the sterile males to make sure that they were doing everything they could to give this experiment a chance of success.

LATIF: Hmm!

SARAH: So in this factory that they set up ...

SAM KEAN: They came up with a formula for the food that they were going to feed them.

LATIF: Okay.

SARAH: And it was ground horsemeat.

SAM KEAN: That they soaked in blood, and they would let that get putrid.

LATIF: Wow!

SAM KEAN: And then they would douse it with formaldehyde.

LATIF: Oh wow!

SARAH: Okay! [laughs]

SAM KEAN: So yeah. And it was cheap enough to get, you know, so they—they knew they could produce a lot of this stuff pretty quickly.

LATIF: Okay, all right.

SARAH: Why the formaldehyde?

LATIF: Yeah.

SARAH: I don't know why that's such a ...

SAM KEAN: Apparently they just liked the formaldehyde, I don't know.

SARAH: Oh wow!

LATIF: It's just a little extra kick. It's like hot sauce for them. [laughs]

SAM KEAN: So then from about March 1954, they are producing 170,000 of these sterile adults per week.

SARAH: They are feeding them this slurry, though at this point without the formaldehyde.

SAM KEAN: That took 40 tons of the slurry to get that job done.

LATIF: Wow!

SAM KEAN: And it was a smashing success. The population plummeted after they started releasing these flies. His big idea had worked.

SARAH: Back in the United States ...

[ARCHIVE CLIP, Edward Knipling: Livestock people, especially.]

SARAH ZHANG: Ranchers catch hold of this.

[ARCHIVE CLIP, Edward Knipling: They came to us.]

SARAH ZHANG: And they're like "Wait, we really want this."

SAM KEAN: Eventually the clamor from ranchers got so big, Knipling decided, well, I think the time is right ...

SARAH: To try rolling out his screwworm strategy in the US. And so armed with some funding from the USDA ...

[ARCHIVE CLIP: This new technique for insect control is eliminating screwworms.]

SAM KEAN: The first big push started in about 1957.

SARAH: In Florida.

SAM KEAN: And after about two years of dedicated work, they had eliminated them ...

SARAH: Not just in Florida, but everywhere ...

SAM KEAN: East of the Mississippi.

LATIF: That is huge!

SARAH ZHANG: And then it kind of keeps going west.

SARAH: The Texas cattleman's association hears about it.

SAM KEAN: I mean, it was a Texas-sized problem that they had.

LATIF: [laughs]

SARAH: They want in too, so Knipling and his team ...

SARAH ZHANG: End up building a really big factory in Texas.

[ARCHIVE CLIP: Screwworm eradication headquarters at Mission, Texas. Here, millions of screwworm flies are being reared each day.]

SARAH ZHANG: All these, like, metal machines.

SARAH: Sarah told us about a similar factory she went to in her reporting.

SARAH ZHANG: Many different rooms, sort of like all at different temperatures and different humidities for each phase of the life cycle.

SAM KEAN: I could not have imagined it smelled good.

SARAH ZHANG: I'm sorry to say this, but what it reminded me of was the smell of a used tampon.

LATIF: Okay. It makes sense.

SARAH ZHANG: Blood that's kind of a little bit bad maybe.

SARAH: From these factories they start releasing flies multiple times a week.

LATIF: So how do they do that?

SARAH ZHANG: By airplane.

[ARCHIVE CLIP: These planes are being loaded with sterile male screwworm flies in Mission, Texas.]

SARAH ZHANG: Basically, they would fly these little, like, prop planes and release these flies in the air.

SAM KEAN: They took them up in refrigerated boxes. They had to buy, essentially, cases of perfume and cologne and dump them on the boxes before the pilots would allow them in the airplane. And then they would essentially open the hatch and just dump them out and let them fall down.

SARAH: Oh wow!

SAM KEAN: And at this point ...

SARAH: The Knipling strategy is working so well, screwworms are disappearing from all of the Southwest.

SAM KEAN: They started marching their way down to the border.

LATIF: Yeah.

SAM KEAN: Pushing it down, pushing it down.

SARAH: Eventually, people were like, "Well, screwworms obviously don't respect national borders."

[ARCHIVE CLIP, Edward Knipling: If we're really going to deal with this screwworm problem, that we would have to enlarge the program.]

SARAH: What if we just kept going?

[ARCHIVE CLIP: This operation can now expand into Mexico.]

SAM KEAN: You know, get them out of Mexico. Then go down, country by country through Central America.

LATIF: That is so much ground to cover.

SAM KEAN: Yep.

SARAH: And so all through the '70s, '80s, '90s ...

SARAH ZHANG: There are all these kind of international agreements.

SAM KEAN: Frankly, this was a tough sell in some places, because the US had a history of meddling ...

LATIF: Oh yeah.

SAM KEAN: ... in Central America.

LATIF: Right.

SAM KEAN: Especially in this time.

SARAH: But despite that, and despite political turmoil ...

SAM KEAN: Revolutions and coups and things like that.

LATIF: Holy!

SAM KEAN: The disgust over screwworms was enough ...

SARAH: That all seven countries in Central America came to the table.

SAM KEAN: And they started marching down, you know, a dozen miles at a time or so, just working down year by year, all the way down to the border between Panama and Colombia.

SARAH: And in 2006, all of North America is declared screwworm-free.

LATIF: Wow!

SARAH: And that so-called single most-original thought in the 20th century changes the face of the entire continent.

LATIF: Amazing!

SARAH ZHANG: And to this day—you know, at the time I was reporting it, like, there are still—there's still a factory in Panama that's still producing millions of screwworms, growing millions of screwworms every day, flying a plane, I think it was three times a week, and maintaining, this like—basically this, what they called of it, like a—a sterile insect barrier.

SARAH: The idea is that that area where Panama meets Colombia ...

SARAH ZHANG: The Darien Gap, which I think it's been used a lot for other reasons.

LATIF: That's like the most dangerous area for migrants, right?

SARAH ZHANG: Exactly.

SARAH: But it's also one of the narrowest parts of the Americas. It's only about 60 miles wide.

LATIF: Okay. So it's like a natural chokepoint.

SARAH: Right. And because of that, maintaining that sterile screwworm barrier only costs ...

SAM KEAN: $15 million per year, which is a pretty reasonable price compared to the huge cost if screwworms were just kind of running wild.

SARAH ZHANG: Yeah, so when I was reporting on this back in 2020, the money saved was estimated to be over $1 billion a year.

LATIF: Wow!

SARAH ZHANG: So $1 billion saved compared to just $15 million cost.

LATIF: Wow, so that's pretty good return.

SARAH ZHANG: And, you know, obviously it was more expensive when you were, you know, in the active eradication phase.

LATIF: Right.

SARAH ZHANG: But the maintenance cost, you know, is really not that much money for a government program. And it's just kind of an example of, like, how when with basic science you don't always know exactly where it will go, but in this case, it ended up, like, creating this continent-wide multinational collaboration that has been going on for decades and decades. And the fact that this all kind of started with someone, like, watching screwworms mate in the lab and having an epiphany is kind of remarkable.

SARAH: Now there have been a couple of small outbreaks here and there over the years, including the Florida one.

LATIF: The deer. Right.

SARAH: The Key deer, 2016 one. Yeah.

LATIF: Mm-hmm.

SARAH: But they've all gotten tamped down pretty easily. Like, it didn't take all much to beat those back. But when we come back from break, all kind of things are gonna break loose—news, biological barriers and maybe also Latif's heart.

LATIF: This is Radiolab. I'm Latif Nasser, here with producer Sarah Qari.

SARAH: Hello.

LATIF: And we've been talking to journalist Sarah Zhang and writer Sam Kean about screwworms.

SARAH: Right. The skin-crawly, flesh-eating parasite that we all forgot about because Knipling and his sterile flies drove it down south.

LATIF: All the way to a biological demilitarized zone at the very bottom of North America, aka the southern border of Panama.

SARAH: Yeah, but in 2023 ...

[NEWS CLIP: Ahora, tenemos una nueva ...]

SARAH ZHANG: A massive spike in screwworm cases, from, like, 25 cases a year to, like, thousands.

SAM KEAN: And since then ...

SARAH ZHANG: It's only gotten worse.

[NEWS CLIP: The parasitic fly is moving north.]

SAM KEAN: They have been steadily marching north.

[NEWS CLIP: There's been outbreaks of this deadly insect.]

SARAH ZHANG: They've been found in Costa Rica.

[NEWS CLIP: Nicaragua.]

SARAH ZHANG: Honduras.

[NEWS CLIP: El Salvador, Guatemala.]

SARAH ZHANG: They're even in Mexico.

[NEWS CLIP: Only 700 miles from the US border.]

SARAH: And it turns out it's basically getting closer every day.

[NEWS CLIP: Less than 400 miles.]

[NEWS CLIP: 300 miles from Texas and the southern border.]

SARAH: It's gotten as close as ...

SAM KEAN: 70 miles or so from the US border.

LATIF: Whoa!

SAM KEAN: This is an ongoing breaking news story.

LATIF: So what—what happened? Like, how did they get past the biological wall?

SAM KEAN: There are a couple of theories.

SARAH ZHANG: One theory is that maybe the strain of screwworm that they were growing at the factory was not as effective anymore. I don't know. Maybe the ladies out there caught onto what was going on, and weren't mating with them or something.

LATIF: Yeah.

SAM KEAN: Just general disruptions due to COVID. It might have weakened the production a little bit.

LATIF: Or maybe, Sam says, given that we live in a more and more interconnected world ...

SAM KEAN: Because it does still exist in South America.

LATIF: ... where people and products and animals are moving from place to place, it was just a matter of time before these worms found their way through.

SAM KEAN: So the best guess is that people were smuggling cattle that were infected.

LATIF: Yeah.

SAM KEAN: And then it just got out.

SARAH: But regardless, as of today, right now, the screwworm ...

SARAH ZHANG: It's sort of like knocking on our doorstep.

[NEWS CLIP: It's not a matter of if the deadly pest gets to the US, but when.]

SARAH: And surprisingly enough, in this time of cuts to science funding and all that, and, you know, the idea of building political will around something that everyone has pretty much forgotten seems dubious.

[NEWS CLIP: The issue has alarmed Washington.]

SARAH: It actually does seem like the US government is paying attention.

[NEWS CLIP: The U.S. Department of Agriculture has snapped into action.]

SARAH: And really, just like in Knipling's time, pretty much because of the cattle industry.

[NEWS CLIP: Billions of dollars of losses a year.]

LATIF: Hmm, of course.

[NEWS CLIP: It could truly crush the cattle industry, as well as other livestock industry in Texas.]

SARAH: So the USDA has started to set up screwworm traps along the US-Mexico border.

[NEWS CLIP: Shut down the southern border ...]

SARAH: They have agents on horseback.

[NEWS CLIP: The so-called tick riders.]

SARAH: ... patrolling the border for stray cows.

LATIF: Huh!

SARAH: They're pouring money into screwworm research. And of course, they are rebuilding Knipling's fly factories.

SAM KEAN: They're now building a new facility down in Texas.

SARAH: Another one in Mexico. All told, probably in the next couple years, we will be producing and releasing something like 500 million flies per week.

LATIF: Wow!

SARAH: And we probably will have spent, like, hundreds and hundreds of millions of dollars.

LATIF: Do you see this as like the modern cow factory farm system is, like, unsustainable, and, like, this is just like one other Band-Aid to keep that system financially profitable or something? Like, I could see an argument that's like, oh, this is just to help, like, big rancher businesses.

SARAH ZHANG: Yeah. Well I think in a literal sense, like, this program was created to help ranchers. [laughs] That is, in fact, the expressed purpose of the USDA, so I think I wouldn't—I wouldn't even argue with that premise. But this does affect wild animals, too.

LATIF: Right.

SARAH ZHANG: In Central America, like, you used to see, like, howler monkeys fall out of trees because they had become so disfigured and sick from getting infested by screwworms.

SARAH: And beyond that, I mean, actually, you know, this time around, with this effort, it's not just the USDA that's involved in this program.

MEGIN NICHOLS: So I'll try and make the case on the human side of things. Number one ...

SARAH: So I talked to this public health official named Megin Nichols, who is at the CDC.

MEGIN NICHOLS: And my current role is incident manager for our New World screwworm response.

SARAH: Because the CDC is worried about this outbreak, too. And the thing that Megin pointed out is that ...

MEGIN NICHOLS: These flies like to infest a wound.

SARAH: I mean, you know, wildlife or cows aside, screwworms will happily lay their eggs in pretty much any warm-blooded animal—including humans.

MEGIN NICHOLS: And it can be a pretty small wound, like as small as a little cut or a bite on the skin.

SARAH: I—I read that they can also get in through just like bodily openings. Is that true?

MEGIN NICHOLS: It is true. They are often attracted to mucus membranes, so eyes, nose ...

SARAH: Ah!

MEGIN NICHOLS: ... ears, urogenital ...

LATIF: Oh!

SARAH: Yikes.

MEGIN NICHOLS: So, yes. And during this outbreak, there have been over 1,000 human cases, and the number continues to grow daily.

SARAH: And to be fair, you know, like, not to fearmonger here, this is not a pandemic level human health crisis. Most people that take precautions will be totally fine.

MEGIN NICHOLS: But I also think very much about this on an individual level.

SARAH: And the kinds of individuals that would be the most susceptible to a serious screwworm infestation, like folks who can't get medical help right away. Or people who are unhoused.

LATIF: Hmm.

SARAH: Or have weakened immune systems, or even, like, kids with a lot of scrapes.

MEGIN NICHOLS: They're the last ones that need to deal with something like this.

SARAH: And on the off chance that it does happen, it is kind of a horror show.

MEGIN NICHOLS: One of the most impactful images that I have seen related to 2025 was patients that were lining the hallway of a medical treatment facility in a country that is currently part of this outbreak, and the patient was holding a bowl in which they were basically sneezing out and pushing out larvae from their nasal passage.

SARAH: Oh!

LATIF: Oh, God!

MEGIN NICHOLS: And other people who were lining this hallway basically waiting to be seen because they were also needing to get the maggots out of their body.

SARAH: Oh my gosh!

LATIF: Oh!

LATIF: Okay, all right. I now—I see the point, that this is more than just the cattle industry.

SARAH: Yeah.

LATIF: But I do now wonder, if it's just gonna keep coming back? Like, is this just an inevitable cycle that we're now stuck in? Like, we're just gonna have to keep doing this?

SAM KEAN: Yeah, you kind of are on a treadmill.

LATIF: You're on a treadmill, yeah.

SAM KEAN: Using Knipling's technique. Yeah.

SARAH: Yeah. But I did, as I was reporting on this, come across a different way that people are starting to think about screwworm control.

SAM KEAN: So another option that is controversial, but scientists have been talking about it ...

SARAH: This is something that Sam mentioned, too.

SAM KEAN: ... is something called a "gene drive."

LATIF: Okay, so what is that?

SAM KEAN: It kind of goes around the normal Mendelian laws of genetics.

SARAH: Okay, so first of all, let's assume that you are a scientist that has modern-day genetic technologies available to you, and you introduce a killing gene into the screwworm population.

SAM KEAN: You could make it lethal to be a female. So it could trigger something where females just don't get out of the egg stage.

LATIF: It's like a little time bomb in the cell.

SARAH: Yeah, exactly. And typically, if you were to try to do that, genetics would work its normal way, and that killing gene would only get inherited 50 percent of the time. And in every successive generation it would become less and less common, and you'd have to keep reseeding that gene into the population.

LATIF: Okay.

SARAH: But, this is where gene drives have a little extra trick.

SAM KEAN: In a gene drive, there's closer to a hundred percent chance that the gene will get passed down.

LATIF: Hmm.

SARAH: You can basically ensure that it'll get passed down every time.

LATIF: Hmm.

SARAH: And so it's kind of like a set it and forget it kind of thing, like you introduce the killing gene and then you just let it have its way with the screwworm population.

SAM KEAN: The screwworm's gonna be out there reproducing and spreading that gene by themselves very quickly.

SARAH: And what that really means is, eventually ...

SAM KEAN: It's gonna wipe them out.

SARAH: Not just, you know, beating screwworms back down to Panama but, like, truly and wholly eradicating screwworms.

LATIF: Extincting them.

SARAH: Extincting them off the face of the planet forever.

LATIF: I just—well, like, I—I don't want us to have to live with these, but I don't want us to kill them forever. That feels wrong. That feels like we're doing something untakebackable that is not—that we should not be doing.

SARAH: Yeah.

GREGORY KAEBNICK: Yeah. So a ...

SARAH: But let me bring in one more person here.

GREGORY KAEBNICK: So I'm Greg Kaebnick. I am a research scholar at the Hastings Center.

SARAH: Because I think he feels a lot of the same way.

LATIF: Hmm.

GREGORY KAEBNICK: I mean, here we are, in the early stages of the sixth major extinction, one that we humans are causing. And I mean, in a way, this seems like, gosh, joining forces with the other side in some way.

SARAH: Hmm!

LATIF: Yes. Yes, exactly.

SARAH: Well, but at the same time, Gregory has seen this potential use of gene drives coming on the horizon, and really wanted to think about it more. So what he decided to do is get together a panel.

GREGORY KAEBNICK: I thought, I'll turn to some environmental ethicists.

SARAH: Ecologists.

GREGORY KAEBNICK: Conservation biologists.

SARAH: Geneticists, entomologists.

GREGORY KAEBNICK: Some people with public health background.

SARAH: No one from the Texas Cattle Ranchers Association.

LATIF: We already know what they think.

GREGORY KAEBNICK: I thought it would be particularly interesting to bring together people who would be, by and large, kind of predisposed to want not to take out species.

SARAH: So ...

GREGORY KAEBNICK: We gathered at Arizona State.

SARAH: In May, 2024.

GREGORY KAEBNICK: For a day and a half of presentations and discussion.

SARAH: They considered a couple of the top nasty species.

GREGORY KAEBNICK: Like the mosquito species that is the main vector of malaria.

SARAH: And one of them was screwworms.

GREGORY KAEBNICK: Mm-hmm. Yes.

SARAH: So the idea was to hash out some of the pros and cons for either side.

GREGORY KAEBNICK: You know, keep, eliminate, and sort of report out our findings.

SARAH: And given the group of people that were sitting at the table, they started out with some of the arguments that you might expect.

GREGORY KAEBNICK: We need to think about the value of species.

SARAH: Yeah, what do you mean when you say that?

GREGORY KAEBNICK: Yeah. I mean, there'd be different ways in which a species could be valuable.

SARAH: Like, first off, there's some very practical ecological considerations.

GREGORY KAEBNICK: I mean, the main question is: what's it doing in the wild? You know, is it a pollinator? Turns out, screwworm is something of a pollinator, but ...

SARAH: Is it?

GREGORY KAEBNICK: Yeah, it is.

SARAH: Oh my gosh!

GREGORY KAEBNICK: But it's probably not a very important pollinator.

LATIF: [laughs]

SARAH: Okay.

GREGORY KAEBNICK: There are lots ...

SARAH: Bees do it better?

GREGORY KAEBNICK: Yeah. [laughs] The bees probably win.

SARAH: Then you have to consider whether some other animal needs to eat it.

GREGORY KAEBNICK: Probably sometimes.

SARAH: But turns out it's not really a key food source for any other animal.

GREGORY KAEBNICK: Is it—does it play an important role as a predator in keeping other species in check?

SARAH: I mean, obviously in this case it is yes, a predator. But according to Gregory, the animals it eats ...

GREGORY KAEBNICK: Are already under pressure from human forces. So if anything, you want to ease the burden as much as you can on those species.

LATIF: Hmm.

SARAH: Okay, but they did also consider some more abstract values that a species might have.

GREGORY KAEBNICK: You know, aesthetic ...

SARAH: Aesthetic? Interesting.

GREGORY KAEBNICK: ... educational, historical, scientific. I think that's the list.

SARAH: Or maybe we put it in terms of screwworm.

LATIF: Yeah. In my mind, I was thinking about it like, if you were going around the way you did, and you opened it up to all your colleagues, and then sitting there was a giant screwworm ...

SARAH: [laughs]

GREGORY KAEBNICK: [laughs]

LATIF: ... who was gonna make the case to save itself, yeah, like, what's the—what's the best case that that screwworm would make?

GREGORY KAEBNICK: [laughs] That's the hard case.

SARAH: [laughs]

LATIF: Aesthetic, clearly. [laughs]

GREGORY KAEBNICK: Right.

SARAH: Obviously.

GREGORY KAEBNICK: Educational. Like, don't mess with nature, kids.

SARAH: [laughs]

SARAH: You know, jokes aside though ...

GREGORY KAEBNICK: I mean ...

SARAH: ... Gregory says, you know, people like him do actually put a lot of stock in the sort of intrinsic value of a species.

GREGORY KAEBNICK: It's this ingenious development in and of its own right.

SARAH: You know, anything that's around today has been honed by millennia of evolution, and in that way every species is ...

GREGORY KAEBNICK: A marker of the creative, natural forces that sustain life, that brought us into being. I mean, I feel like you have to try to put it in a practically poetic way in order to really convey what people are thinking.

SARAH: So Gregory's group, after about a day and a half of discussing all this ...

GREGORY KAEBNICK: We're all kind of exhausted, and we're trying to, like, put it all together.

SARAH: And Gregory says, okay, we're gonna go around the room one by one.

GREGORY KAEBNICK: Sort of put everyone on the spot and see what they thought. And we did so, and by the time we'd gone around, everyone had—most people had—I think maybe everyone had actually said, "Yeah, screwworm looks like a pretty good candidate."

LATIF: Really? That it would be okay to wipe this species out, off the planet forever?

GREGORY KAEBNICK: Yeah.

SARAH: How did that feel? Was that awkward?

GREGORY KAEBNICK: Well, I mean, it was—it was this, like—it was kind of like this serious moment. Everyone was sort of sighing about it.

LATIF: Uh-huh?

GREGORY KAEBNICK: Like, "Oh, I really don't want to—I really don't want to vote for this, but it looks like maybe the case can sometimes be strong enough."

LATIF: But isn't there sort of, like—I mean, this just feels like, you know, an idea that always comes up with ecosystems, like, we think we know and then we don't. Like, there's always the knock-on effects that we never see coming, and we thought we knew everything but then it surprises us but there it is. Like, does it—does it not feel like hubris to assume that we know everything that could potentially go wrong?

GREGORY KAEBNICK: Yeah. But we did, in fact—at least up until very recently—get rid of screwworm in southern North America and throughout Central America. And, you know, the sky didn't fall.

SARAH: Hmm.

GREGORY KAEBNICK: Other species didn't wink out because they weren't able to eat New World screwworm anymore. So we have a little bit of a kind of preliminary ...

SARAH: Like, experimental data.

GREGORY KAEBNICK: Yeah, kind of. That suggests that it's probably not really that important ecologically.

SARAH: So losing it, you know, doesn't seem to do any harm. And then on the other side ...

GREGORY KAEBNICK: Part of what was so persuasive about it was that the death that it causes is exquisitely awful.

SARAH: I mean, with screwworms, did you guys consider alternatives to eradication?

GREGORY KAEBNICK: Like, if there were some other kind of treatment for it, some sort of medicine that you could take.

SARAH: Gregory says if it was possible to, like, separate screwworms from the thing that they do or, like, the disease that they cause of flesh being eaten, and you—if you could attack that disease, then—then great, you know? Like, that's sort of the case with malaria and mosquitoes, where you can—you can attack the disease, or in that case the parasite, rather than the mosquito. But with screwworms, there's just no way to do that. Like, eating flesh is in their nature. It's what they do. There's just no way to make them, like, vegetarians or make them uninterested in flesh.

GREGORY KAEBNICK: The only way you get rid of the disease is by getting rid of the fly. At least at the time that we were meeting and writing, there was nothing else to do except take it out if you can. Eliminate the flies.

LATIF: Ugh, I don't know. I don't know. I want to agree with him, but I—I just—I don't know if that wins the day for me. Like, I don't disagree, but I just can't bring myself to agree.

SARAH: No, this is—you know, this is the part of the story where I feel like I agree with you. I don't know. I don't know what to do with the fact that I don't want screwworms to exist. You know, like, I think about, like, the people in South America. We just stopped at Panama? Like, I don't know, I don't want them to have to deal with screwworms and suffer and—but at the same time, this stuff gets scary.

LATIF: Yeah, it does.

SARAH: I guess one thing I will say, though, that Gregory did mention is that if we go down this path ...

LATIF: Yeah?
SARAH: ... theoretically, there is sort of an undo button here.

GREGORY KAEBNICK: You can definitely keep it somewhere else in a facility or, you know, frozen eggs or something. And if it were to turn out that ...

SARAH: Like, the cure for cancer was in the screwworm eggs, or ...

GREGORY KAEBNICK: ... it's really vital to the life cycle of this frog species in South America or something like that.

SARAH: Some ecological Rube Goldberg machine got set off because we got rid of all the screwworms.

LATIF: Yeah.

GREGORY KAEBNICK: It would be possible to achieve something like de-extinction of screwworm.

LATIF: To bring it back, to seed it. Fling it out of the planes the way we're flinging out the sterile ones.

GREGORY KAEBNICK: Yeah. And what we know from the outbreak in Central America now is that it can take off pretty well.

LATIF: Imagine the, like, public campaign where it would be like, "Hey, everybody! Remember how we—remember how we ..."

SARAH: "Remember how they used to exist?"

LATIF: Right. There's this one frog in Panama that this was really helpful for.

SARAH: Yeah.

LATIF: So we're just gonna drop these flesh-eating worm—we're just gonna rain them down over you and your home.

SARAH: Yeah. [laughs]

LATIF: So one last thing before we go. Back in 2016, Edward Knipling and his screwworm research won a Golden Goose Award. If you haven't heard of it before, the Golden Goose Award goes to US-government-funded science that sounds ridiculous but ends up changing the world. And Knipling's research was exactly that. Even years after it proved useful, members of Congress still ridiculed and scapegoated it ...

[ARCHIVE CLIP: Frivolous pork projects such as the screwworm research. $35 million.]

LATIF: ... as an example of government waste.

[ARCHIVE CLIP: Even though the screwworm has been eradicated in the United States.]

LATIF: This is now the fourth Golden Goose-inspired story that we have done.

SARAH: Nice.

LATIF: We did one about cone snails called Golden Goose. One about a bacterium called The Age of Aquaticus. And one recently about honeybees called Time Is Honey. And now this one. It's almost become like a sneaky little recurring series that if you ever meet someone who doesn't believe that the government should fund basic science, just play them one of these. We'll link them all in the episode description.

SARAH: This episode was reported and produced by me, Sarah Qari, with reporting help from Latif Nasser. Our fact-checker was Emily Krieger. Check out Sam Kean's podcast, The Disappearing Spoon. His episode about screwworms is linked in our episode description. Same goes for Sarah Zhang's latest story in The Atlantic. And if you are interested in hearing more about gene drives, check out our episode about it, the last 10 minutes of our CRISPR update.

LATIF: Thank you to James P. Collins, Max Scott, Amy Murillo, Daniel Griffin, Phil Kaufman, Katie Barnhill, Arthur Caplan, Ron Sandler, Yasha Rohwer, and our gaggle of friends at AAAS who administer the Golden Goose Awards: Aaron Keefe, Gwendolyn Bogard, Valeria Sabate, Meredith Asbury, and Joanne Padrón Carney.

SARAH: And last last thing, if you want more information on screwworm-infested areas, or how you can take precautions to be safe, there is a website you can check out. It is screwworm.gov.

LATIF: That's screwworm.gov. Sarah, I guess it's time for us to just screw off.

SARAH: [laughs] Indeed.

LATIF: And we'll catch you next week.

SARAH: All right!

[LISTENER: Hi, I'm Gabby. I'm from San Francisco. And here are the staff credits. Radiolab is hosted by Lulu Miller and Latif Nasser. Soren Wheeler is our executive editor. Sarah Sandbach is our executive director. Our managing editor is Pat Walters. Dylan Keefe is our director of sound design. Our staff includes: Jeremy Bloom, W. Harry Fortuna, David Gebel, Maria Paz Gutiérrez, Sindhu Gnanasambandan, Matt Kielty, Mona Madgavkar, Annie McEwen, Alex Neason, Sarah Qari, Rebecca Rand, Anisa Vietze, Arianne Wack, Molly Webster and Jessica Yung, with help from Gabby Santas. Our fact-checkers are Diane Kelly, Emily Krieger, Natalie Middleton, Angely Mercado and Sophie Sanahee.]

[LISTENER: Leadership support for Radiolab's science programming is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radiolab was provided by the Alfred P. Sloan Foundation.]

 

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