How Scientists Contain And Control Flesh-Eating Screw Worms : Short Wave Sarah Zhang wrote about it for the Atlantic: a decades-long scientific operation in Central America that keeps flesh-eating screw worms effectively eradicated from every country north of Panama. Sarah tells the story of the science behind the effort, and the man who came up with it.

Email the show at

America's 'Never-Ending Battle Against Flesh-Eating Screw Worms'

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript


You're listening to SHORT WAVE...


SOFIA: ...from NPR.

It was 2016 in the Florida Keys, and a little species of endangered deer called the Key deer weren't acting right.

SARAH ZHANG: People just start noticing that these deer seem to, like, be having these wounds on their heads.

SOFIA: That's Sarah Zhang, a reporter with The Atlantic. She says the deer looked like they'd been injured by something. There were gashes on their heads.

ZHANG: They were kind of, like, walking down the street, like, really shaking their heads like they were trying to shake something off.

SOFIA: Warning - right here, it is about to get gross - like, real gross.


UNIDENTIFIED REPORTER #1: Kim Gable (ph) is a Big Pine Key resident and has seen the deer in her area.

KIM GABLE: You'll see gaping holes or wounds in their neck or in their head, and you'll see them shaking their head.

ZHANG: You would see a deer. Like, almost half its head would be missing, and you could see down to the skull. And eventually, these deer start dying. And, you know, it becomes kind of this big mystery of why are they dying, especially with these really, really awful wounds on their heads.

SOFIA: Local scientists looked into it. And eventually, tests confirmed the deer were infected with a parasite. Its scientific name is C. hominivorax, which means man eater. It's more commonly known as the screwworm.

ZHANG: The screwworm is actually the maggot of a fly. And this fly only lays its eggs in the wounds of live animals. And its maggots, the screwworm, only eat live flesh, so they're flesh-eating screwworms.

SOFIA: They sound lovely, Sarah. They sound lovely.

ZHANG: (Laughter) They're your best friend, except not.

SOFIA: As they eat, the wound grows, attracting more flies, which create more maggots, which turn into more flies. You get it. They can spread really fast, so local authorities in Florida took some drastic measures.


UNIDENTIFIED REPORTER #2: We're talking at least 50 of the 1,000 Key deer at the National Key Deer Refuge have been euthanized.

SOFIA: On the roads out of town, checkpoints were set where anyone traveling with a pet needed to stop.


UNIDENTIFIED REPORTER #3: So those with pets leaving the Keys have to get their animals checked out to make sure they are clear of this deadly parasite.

SOFIA: The goal, which they achieved, by the way, was to protect as many deer as possible and keep the screwworm from leaving the Keys. See, this outbreak was kind of a fluke. Screwworms used to devastate livestock in the U.S., but they were eradicated more than 50 years ago. And that's because, to this day, the USDA maintains an international screwworm barrier along the border between Panama and Colombia, the thinness strip of land connecting North and South America.

ZHANG: And every week, planes loaded with sterilized screwworms fly over this patch of land and release millions of sterilized screwworms every week. And this has been going on for decades.

SOFIA: It's the biggest parasite control operation you've never heard of - at least I haven't.

This is wild. Sarah, I am embarrassed to say I had no idea.

ZHANG: No. I also had no idea until 2016 when I started writing about screwworms.


SOFIA: This episode - Sarah Zhang with the story of how this massive screwworm operation works and the guy who figured out the science behind it.


SOFIA: I'm Maddie Sofia, and this is SHORT WAVE, the daily science podcast from NPR.

The transcontinental screwworm barrier - yeah, that's basically what it's called - has been in place for 50 years - longer, Sarah Zhang says, than many of the people who maintain it have been alive. They work for a joint commission of Panama's agricultural department and the USDA. Sarah went to check out this whole operation on the border between Panama and Colombia, where scientists make it rain worms every single week. But before they can do that, they have to grow them.

ZHANG: So it's literally a factory. It looks like one. It smells like one. We can talk about that later.

SOFIA: Sorry, people. We are actually going to talk about it right now.


SOFIA: See, screwworms eat meat. So Sarah says one of the first things you smell when you walk into the screwworm factory is their food.

ZHANG: Dried blood, dried milk and dried eggs. It's kind of reconstituted into this, like, brown sludge, which I'm sorry to say it looks exactly like sewage - like, has the same consistency and color. So that's what it reminded me of. And it smells about as awful as you can imagine it will smell. So...

SOFIA: So these stinky little flesh-eaters are raised from maggots to pupae. They're sterilized using radiation, and then they're knocked out cold and loaded onto planes.

ZHANG: So the plane itself kind of has this, like, cold refrigerator box. And once the plane gets in the air, it kind of starts spitting out these flies, like, at a regular rate per nautical mile. And once they hit the warm air - I went in August, so this was very warm and very humid. But these cold flies hit the warm air, they wake up and they fall down to the ground and they start mating.


SOFIA: Not so bad, right? You wake up from a deep, peaceful sleep. You're floating gently down through the warm jungle air. You land, maybe make meaningful eye contact with another screwworm across the bar, and you just do what comes naturally.


SOFIA: Now, the key to this whole operation is the portion of the sterilized screwworms that are male. And that's because the female screwworm mates only once her whole life.

ZHANG: Which means that...

SOFIA: Not a great life.

ZHANG: ...If you could somehow make her mate with a male who is sterile, you could make sure she never has any more screwworms, she never has babies.

SOFIA: Males will mate multiple times, so that's what makes male birth control the key.

ZHANG: Yeah. Basically, you're just, like, saturating an environment with sterilized men.

SOFIA: Right. Well, I mean, OK.


SOFIA: The scientist who came up with this scheme, which, by the way, doesn't involve chemicals or pesticides that would harm other species, was a man named Edward Knipling.


EDWARD KNIPLING: I was born in Port Lavaca, Texas, March 20, 1909.

SOFIA: That's him in an interview recorded shortly before his death in 2000. Growing up on a farm in Texas, one of his jobs was to constantly check cows and pigs for screwworms.


KNIPLING: You can imagine getting in a hogpen with a sow that has screwworms in it and then trying to handle the...


KNIPLING: ...Handle the sow...


KNIPLING: ...And treat it for screwworms.

SOFIA: Good times, Ed. Good times.


KNIPLING: That was something that we had to do.

SOFIA: Years later, as a young entomologist at the USDA, Knipling wanted to figure out a way to control screwworms. He finally got an idea for how to do that after World War II.

ZHANG: You know, obviously, one of the things that happened in World War II is that we used the atomic bomb. And scientists who've worked with radiation are going around trying to warn people about the dangers of radiation. And one of these scientists is a fruit fly scientist who had actually looked at what happens if you kind of bombard fruit flies with radiation, and it turns out they become sterile. And Knipling hears about this, and he kind of has a light bulb moment and says, oh.


KNIPLING: I thought, well, here's a way that maybe we could sterilize screwworms. So I wrote a letter to...


SOFIA: Knipling got some colleagues and fellow scientists on board. It took months of experiments, zapping these little dudes to figure out the sweet spot where they're sterile...


KNIPLING: Without affecting their normal sexual behavior and competitiveness.

SOFIA: If you're curious, it's about 5 1/2 days after the larva turns into a pupa.

ZHANG: So this is when the adult flies' testes are developing. So if you bombard them with radiation at that exact time, it'll make it so that they can't produce any eggs or sperm but they can still fly around and look relatively normal to the wild screwworms.


SOFIA: Once Knipling and his colleagues figured this out, the U.S. spent decades scaling up a program that all but eliminated the screwworm in parts of the South and West, where it had been a huge problem for decades.

ZHANG: You know, it starts kind of in Florida, then goes to Louisiana and Texas and Arizona and California. And it works, but it's - you know, as we know, it's a long border between Mexico and the U.S.

SOFIA: Continuously producing enough flies to control the population over such a wide area was almost impossible.

ZHANG: So then this idea is hatched to, let's move the border further south, and let's move it to a part where there's less land to cover.

SOFIA: By the 1970s, the screwworm barrier had moved to Mexico. And by the 1980s, thanks to cooperation and coordination between the U.S., Panama and all the countries in between, it was moved even farther south to where it is now. The fact that it has existed effectively there for almost 50 years, Sarah says, is the result of a whole lot of trust and cooperation between the U.S. and a bunch of different countries in Central America.

ZHANG: It's the kind of thing where if you had one country not agree, like, how would it have worked, right?


ZHANG: It really was the case where you needed to get everyone in this entire area, Central America, on board.

SOFIA: So, yeah, it took one guy with a passion for zapping worm testes to come up with an idea that to this day saves American farmers over an estimated billion dollars per year and keeps countless wild animals and domestic pets safe. But it took a way bigger effort to make that idea into a reality. And here is a lesson Sarah says we can apply to what's happening right now.

ZHANG: This was a year ago. I was, you know, going around asking everyone who worked on the program, like, do you think this program could exist today? Could it start today from scratch? And uniformly, people were like, no. Like, how would you get all these countries to agree on this?

SOFIA: Containing a disease is one thing; keeping it contained is another thing entirely.


SOFIA: Sarah Zhang wrote about screwworms for The Atlantic. A link to her article, where you can read more about the massive operation to contain them, is in our episode notes.

This episode was produced by Brent Baughman, fact-checked by Berly McCoy and edited by Viet Le. I'm Maddie Sofia. Thanks for listening to SHORT WAVE from NPR.

Copyright © 2020 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.