
Is there a defence against the New World Screwworm?
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Charmaine Cozier
Welcome to the inquiry from the BBC World Service. I'm Charmaine Cozier. Each week, one question, four expert witnesses and an answer. Early June 2026, a three week old calf born on a ranch in Texas is attracting international attention. The young animal is infested with a flesh eating parasite which hasn't been active in the US for years. Now it's back. By mid June, 11 more infestations are confirmed in cattle, goats and sheep on different ranches in Texas. There's also an infested dog in the neighboring UN state of New Mexico. The name of the parasite is as bizarre as its behavior. So this week we're asking, can the New World Screwworm be stopped?
Narrator/Host
Part 1 the Screwworm Lowdown.
Grace Van Hoy
The New World Screwworm is a species of parasitic fly and it belongs to a really large group of flies that we call the flies and they are present all over the world and we have several species in the Americas, but this particular species is really unique.
Charmaine Cozier
Grace Van Hoy is a veterinarian and professor at the UC Davis Wild School of Veterinary Medicine in California. In the US
Grace Van Hoy
So the New World Screwworm is quite a unique fly among the blowflies themselves. So all of that big family of flies because it actually prefers to lay its eggs on living tissue which is quite different from all of the other blowfly species that we see in the Americas and throughout the world that prefer to lay their eggs in dead or what we call necrotic tissue.
Charmaine Cozier
Blowflies can have shiny blue, black or green backs. The New World screwworm has an extra trait.
Grace Van Hoy
So this fly, in addition to having really brilliant kind of metallic sheen on the back, has orange eyes and then three stripes on its back as well. So those are the distinctive features that help parasitologists and veterinarians recognize this specific fly species.
Charmaine Cozier
Why is it called a screwworm?
Grace Van Hoy
So the name screwworm came about because of the general behavior of the maggots themselves. So life cycle of a fly, they're all the same. So the, the adult female egg, and those eggs will kind of rapidly turn into maggots. And that's what we recognize as the fly larvae. And those maggots will actually continue to almost drill down and eat through live tissue. And so a lot of people will describe that as screwing into the tissue, which is the genesis of its name. And often they're attracted to wounds, they're attracted to mosquito bites, to tick bites, to castration sites, or especially, as we've become quite aware recently, to the umbilical cord or the navel in newborn animals. That's quite an attractive place.
Charmaine Cozier
That's how the three week old calf got infested in that first case in Texas. They also lay eggs near eyes, nostrils and mouths.
Grace Van Hoy
And then from there they're going to develop into maggots very quickly. So within a day or two, and if the animal can't get them off, can't access them, lick them off, scratch them off, they'll stay there and eat for about a week and then they'll drop from there into the ground and pupate. And eventually over about two or three weeks, they'll actually emerge from the ground or the soil as an adult fly. And as those wounds get bigger and bigger, they're going to attract more and more of those flies to lay eggs in there. And so we may see several generations or several different generations from different flies on a single animal. But it's important to note that this fly is non discriminate. It will lay its eggs in any warm blooded creature.
Charmaine Cozier
But if they're attracted to anything that's warm blooded, does that include us humans?
Grace Van Hoy
It sure does, Charmaine. So often people will notice wounds, will clean wounds on their own. But there is a chance that people who don't have regular access to healthcare or resources to treat their wounds would be at risk of Becoming infested. And it's something that need to think about. This is not just an animal health concern, but a public health concern as well.
Charmaine Cozier
Human deaths from infestations are rare. What's the role of the New World screwworm in the ecosystem? How does it regulate wildlife?
Grace Van Hoy
There are some parasites or flies or insects that actually play a role in nutrition for many animals. And so things like amphibians or birds will actually eat them and get nutrition. But really this parasitic fly's only job is to push its own agenda. And so it doesn't have a big role in our ecosystem as a source of nutrition or as a source really of disease transmission other than itself and to infest animals and make a big population.
Charmaine Cozier
There's also a different species called the Old World screwworm. That parasite is found in parts of Africa and Asia.
Grace Van Hoy
But our New World screwworm is considered endemic. So endemic generally means that it's a common found species in a region. And this is endemic in South America and parts of Central America towards South America. But it has not been considered endemic in North America, so Canada and the United States for quite some time, many, many decades.
Charmaine Cozier
And our next expert witness has the strange story behind the reason for that long absence.
Narrator/Host
Part two the sterile insect technique
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so
Narrator/Host
I think it's the 1860s that it's a French naturalist who identifies the New World screw worm, gives it the Latin name whose translated meaning means literally man eater, a man eating fly. But for a long time, there wasn't a scientific consensus among entomologists that that was a specific species or that it was really just a blowfly.
Charmaine Cozier
Thomas Rath is an associate professor in the history of modern Latin America at University College London in the uk. He wrote a book called the Dread Plague and the Cow the Politics of Animal Disease in Mexico and the world. In 1933, two scientists published research confirming the New World Screwworm as a distinct type of blowfly.
Narrator/Host
So it was a problem before it was sort of identified as a specific species. But that was important because it meant that if this was a specific species, then perhaps governments and scientists and ranchers could come up with specific techniques to counter it. And that's something that a lot of ranchers in particular in the United States were very interested in.
Charmaine Cozier
Infestations continued in the Southwest, including Texas. One of the most severe outbreaks happened there in 1935, when an estimated 180,000 livestock died. The devastation spread southeast to Florida after infested live cattle were transported there. At that time, remedies focused on treating wounded animals and not on deterring the injury causing parasite. Then, in the 1950s, a game changing practical control method was ready to be deployed. It's called the sterile insect technique.
Narrator/Host
I find it really, really interesting and it sounds a little bit like something out of science fiction, but in fact it is real. Essentially, there's three elements to this. You need to mass produce the insects that you don't want in a huge factory, in cages, in controlled conditions, feeding them the kinds of things that they want. In the case of screwworm, it's lots of protein, it's lots of liquefied meat and blood and some other things as well. They sort of add to the mix. The next thing you do is you sterilize them with a carefully calibrated dose of gamma radiation, which sterilizes the insects, means that they can't reproduce and the
Charmaine Cozier
final step is to release them.
Narrator/Host
So crates of sterilized insects by the million kind of loaded onto aircraft and then dispersed over particular places where the wild non sterile screwworm is prevalent. And the theory is that you do that in such great numbers that you overwhelm the wild population.
Charmaine Cozier
Here's another thing you need to know. The female New World screwworm can only mate once. So if that happens with an infertile fly, then eggs won't turn into maggots.
Narrator/Host
Eventually the population dies out or reaches such low numbers that it's effectively eradicated as a problem.
Charmaine Cozier
The sterile insect technique took years to develop. It emerged from the discoveries of two scientists working for the US Department of Agriculture.
Narrator/Host
They test it first on a small island off the coast of Florida in the early 1950s. And then there's a larger really important test that takes place on the Dutch island of Curacao in 1953. And it basically works. In a matter of months, they eradicate screwworm on Curacao. And so that's the first really important kind of pilot project for this technique.
Charmaine Cozier
Then it was time for the big rollout.
Narrator/Host
The first project in the mainland United States begins in Florida. Between 1957 and 1959, roughly they kind of quarantine off a particular section of mainly Florida and some other surrounding states. They drop sterile flies by the billion there over two years and decide that that has been effective by 1959. The geography of eradication then moves west into the US Southwest and particularly into some really important cattle producing states like Texas. So that unfolds through the early 1960s and is basically complete by about 1966, at which point the United States declares itself SC screw worm free. At that point you've got almost like a curtain of sterile flies that are being constantly released across the US Mexico border. Then from 1972 through to the early 2000s, gradually that border is pushed southward, down through Mexico, down through Central America, and then it's moved down to Panama.
Charmaine Cozier
The New World Screw Worm was eventually contained in the southern end of Central America, in a dense stretch of jungle between Panama and Colombia.
Narrator/Host
One of the reasons why it was seen as sort of important and advantageous to do that is that the US Mexico border is really long and really expensive to create enough flies for this border to work. The Darien Gap in Panama is much narrower, much cheaper and much easier to place to create this border.
Charmaine Cozier
But it didn't mean infestations were over. In the US there were outbreaks in southwestern states in 1972, Texas in 1982 and Florida in 2016. There was also a surprise outbreak in Libya, where the fly is not endemic. The U.S. mexico and the United nations worked together on the response, basically following
Narrator/Host
the model from the Americas in Libya from 1989 to 1991. And Mexico actually plays quite an important role in that campaign. All the sterile flies for that campaign are produced in the factory in southern Mexico and then shipped over and dispersed in Libya. And it was seen as a very important problem to stop in Libya and stop as soon as possible. And that's why a lot of resources were put into this, because the New World Screw Worm was not present or known in any other part of Africa. So if it had gained a foothold, so to speak, in Libya, it would have spread very rapidly down through Sub Saharan Africa and become a major economic problem.
Charmaine Cozier
The New World Screwworm officially returned to Southern Mexico in November 2024. By mid June this year, there were just over 2,000 recorded active animal cases there. Our next expert witness is in the US State where the parasite headed next. Past episodes of the inquiry, like why are more people getting Lyme Disease? Or why does Ebola keep coming back? Are available wherever you get your podcasts. Tap like to subscribe.
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Narrator/Host
so
Philip Kaufman
we've been preparing for this since the fly crossed into Mexico in November of 2024 and we've had about 18 months to get ready for this and develop these response plans that are now being implemented.
Charmaine Cozier
Professor Philip Kaufman is head of the Department of Entomology at Texas A and M university in the U.S. i also
Philip Kaufman
serve as the Chair of the Texas A&M AgriLife New World Screwworm Task Force.
Charmaine Cozier
That task force brings together a range of specialisms to provide and coordinate research, knowledge and resources to protect livestock. A three week old calf was the first New World screwworm infestation in Texas since 1982. The response was rapid, so the rancher
Philip Kaufman
noticed the animal had an injury on it around noon and by 4pm they had collected the fly larvae from that animal and transported them to one of our USDA laboratories where they have some individuals who are qualified and determined that it was highly probable that that was the new world screwworm.
Charmaine Cozier
USDA is the U.S. department of Agriculture.
Philip Kaufman
They then flew those samples, which was the protocol at the time, from Texas to Iowa, where the national laboratory is, that's charged with proper identification, official identification, and later that evening, I think by 7:30 or so they had come back with a confirmation that it was indeed New World Screwworm. The USDA immediately routed some of the sterile flies from very southern Texas up to that area, which is probably a couple of hundred miles from where they had the flies for the normal dispersal and they were releasing flies on that ranch that night. And the next day our Texas Animal Health Commission, which is the state government agency that is responsible for kind of boots on the ground management, had their people in place. The next morning.
Charmaine Cozier
A 20 kilometer containment zone was set up there and around other infestation sites that emerged later. Movement restrictions were also put in place to allow the agency to check all of the animals within those zones and
Philip Kaufman
any animal that a rancher might want to move out of that infestation site, out of that area must be examined and treated and receives a certificate of movement. And we're doing that in order to try and contain these infestations because the fly itself doesn't move very far. It's animal movements that have historically allowed screwworm to move across Texas and other
Charmaine Cozier
states in the US USDA announced an immediate drop of 4 million infertile male flies a week onto the containment zones to find females who can only mate once. But prevention efforts were underway long before the first case was detected.
Philip Kaufman
So the sterile insect technique is the only proven way to eradicate this pest. And the USDA has been releasing sterile flies across Mexico for the past 18 months, trying to slow the northern progression of the fly.
Charmaine Cozier
Now it's touched down in Texas again.
Philip Kaufman
We may never know why it escaped the quarantine area in the Darien Gap on the eastern end of Panama. We are seeing that these are likely very few flies that have established themselves in Texas. And with the sterile releases that are happening, if we maintain our animal movement restrictions and our ranchers are cooperative with this, we should be able to even eliminate these infestations and return Texas to screw worm free within a few months.
Charmaine Cozier
Texas is the biggest cattle state in the US So the outbreak there raised wider concerns. It prompted the Canadian Food Inspection Agency to put temporary import restrictions on livestock, including horses, which were in affected areas 21 days before their planned border crossing date. There have also been questions about what the New World screwworm might mean for consumers.
Philip Kaufman
There is absolutely no food safety issue with this fly. It's a parasitic fly. And when an infestation is found, that animal is treated, the wound is cleaned if necessary, it's given antibiotics and makes a full recovery. So you will never see this fly in the food supply. It's not a disease. Now, if it were to get large enough and cause some other challenges, it's possible it could impact the cost of beef, which is already quite high in the US but we are quite a ways away from that. You know, we have a great many more tools available to us this time than we didn't have in the 1980s. A lot better animal drugs. We have communication tools like the Internet. We have a much more engaged public in many respects. But we also have other challenges, like higher wildlife populations that occurred because we eliminated the fly 44 years ago and the wildlife populations have surged. We also have a lot of feral dogs in South Texas and they could be harboring this fly.
Charmaine Cozier
And you actually met with the rancher that had the first case, is that correct?
Philip Kaufman
I did, and the animal is doing fine. He was showing me pictures of it on his phone and I was on a call with the Secretary of Agriculture and she said the animal is doing perfectly fine and should make a full recovery.
Narrator/Host
Part 4 A fly in the ointment.
Rui Manuel Cardosa Pereira
The top priority by far is the increase of number of sterile flies. Available to target the population.
Charmaine Cozier
Rui Manuel Cardosa Pereira is head of the Insect Pest Control Section at the Joint FAO and IAEA Centre of Nuclear Techniques in Food and Agriculture based in Vienna, Austria. The centre established by the Food and Agriculture Organization of the United nations and the International Atomic Energy Agency assists countries in using nuclear science and the sterile insect technique to control major crop and disease transfer transmitting pests
Rui Manuel Cardosa Pereira
that took about 40 years to eradicate all southern US, Mexico and central America until Panama. And now the fly in about three years, a bit more than three years reinvade all the region. And there is a lack of knowledge by the new vets how to deal with the situation. Also there is no enough sterile flies available at the moment. The only facility that exists now in Panama it do not produce enough flies to retain the fly to go towards north. And because the production is only of about 100 million sterile flies per week.
Charmaine Cozier
So right now there's only one place in the world that can breed and sterilize New World screwworms. The US and Panama run it together. But more plants are in progress in Mexico and the state of Texas.
Rui Manuel Cardosa Pereira
The authorities from those countries are running and they know the urgency. What I know from for example, the refurbished facility in Mexico is expected to be full operating produce another 100 million by the end of 26 will be inaugurated very soon. And then the Texas facility is already starting its construction and the goal is that to start to produce flies in 27.
Charmaine Cozier
The long term aim for the Texas facility is 300 million a week. Because sterile male screwworms don't live for long.
Rui Manuel Cardosa Pereira
The lifespan is quite short. We are talking about a week. Which means that that is why the releases when occur, they are done once a week to cover all the entire area and to replace, let's say all the sterile flies that have been gone in that week period to keep the pressure of steriles in the area.
Charmaine Cozier
And is it possible to know how many sterile flies will be needed not only to sort of deal with outbreaks, but to keep things under control?
Rui Manuel Cardosa Pereira
Yeah, we are talking about the numbers that the initial eradication campaign uses which was about 500 million when they were dealing with the southern US states and later with Mexico in that period. That number of flies will be approximately what will be needed today because the situation is quite similar. The use of the sterile insect technique is not a silver bullet. It's not a standalone technology needs to be integrated with other control methods to turn it much more efficient.
Charmaine Cozier
So returning to our main can the New World screwworm be stopped? Yes, it was driven out of the US And Mexico and stayed away for years. Releasing millions of sterile flies to shrink the parasite population over time is just one part of the response. It also needs containment, medication, and, as our expert witness in Texas, Professor Philip Kaufman notes, rigorous human surveillance.
Philip Kaufman
It is a very important pest and we want people to to know it's here. We want people to look at their animals so that we can find it early. The earlier you find an infestation, the more likely the animal will recover fully and the more likely we are going to be able to eliminate it.
Charmaine Cozier
Thank you for joining us on the Inquiry from the BBC World Service. To find past episodes like Is Extinction a Thing of the Past? Or Can Brussels Crack the Gender Pay Gap? Look for the Inquiry podcast wherever you get yours, tap. Subscribe for Updates this episode was written and presented by me, Charmaine Cozier. The producer was Jill Collins, researcher Amelia Cox, editor Tom Bigward and technical producer Craig Boardman.
Rui Manuel Cardosa Pereira
Imagine being here. Scientists have made the atomic bomb.
Narrator/Host
The first one was dropped on a
Rui Manuel Cardosa Pereira
Japanese city this morning here. During the meeting, Mr. Mandela was informed
Philip Kaufman
of the government's decision regarding his release.
Narrator/Host
And here witnesses have spoken of a wall of water that swept coastlines in Sri Lanka, Indonesia.
Philip Kaufman
Hear from the people who were there.
Charmaine Cozier
He heard me praying while he was
Rui Manuel Cardosa Pereira
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Philip Kaufman
I'm gonna leave you here to tell the story. Knowing that you have a spy and that you're following a spy. It doesn't get any cooler than that. Witness History brings you first hand accounts.
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Merkel gets up and says I'm going and Tusk says I'm locking the door. Nobody's leaving. You're reaching a compromise or you're not leaving this room.
Philip Kaufman
Was it shopping carts that made you rich?
Rui Manuel Cardosa Pereira
Well, they didn't make me poor.
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Listen now.
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BBC World Service | June 30, 2026
Host: Charmaine Cozier
In this episode of The Inquiry, the BBC World Service probes into the resurgence of the New World Screwworm in the United States after several decades of absence. By assembling four expert witnesses, the episode explores the biology of the screwworm, historical eradication methods, the current outbreak’s management, and the challenges of sustaining control. The central question: Can the New World Screwworm be stopped?
(02:30–07:32)
Biology & Unique Behavior
Expert: Dr. Grace Van Hoy, Veterinarian & Professor, UC Davis
"Those maggots will actually continue to almost drill down and eat through live tissue... that's the genesis of its name." (Grace Van Hoy, 04:02)
Impact on Animals and Humans
Geographic Distribution
(07:42–13:49)
Discovery & Identification
Expert: Dr. Thomas Rath, Associate Professor of Modern Latin American History, University College London
Eradication via Sterile Insect Technique (SIT)
"You overwhelm the wild population... eventually the population dies out or reaches such low numbers that it's effectively eradicated as a problem." (Narrator, 10:38)
Past Outbreaks & Global Response
"If it had gained a foothold... it would have spread very rapidly down through Sub Saharan Africa and become a major economic problem." (Narrator, 13:07)
Recent Developments
(15:18–20:57)
Expert: Professor Philip Kaufman, Head of Entomology, Texas A&M & Chair, AgriLife New World Screwworm Task Force
Rapid Detection and Response
"The animal had an injury... by 4 pm they had collected the fly larvae... by 7:30... confirmation that it was indeed New World Screwworm... they were releasing flies on that ranch that night." (Philip Kaufman, 16:06–16:32)
Containment Measures
"Any animal that a rancher might want to move... must be examined and treated and receives a certificate of movement... because the fly itself doesn’t move very far. It's animal movements that have historically allowed screwworm to move..." (Kaufman, 17:36)
Eradication Efforts
"The sterile insect technique is the only proven way to eradicate this pest." (Kaufman, 18:15)
Challenges
"You will never see this fly in the food supply. It's not a disease." (Kaufman, 19:29)
Personal Touch
"He was showing me pictures... the animal is doing perfectly fine and should make a full recovery." (Kaufman, 20:42)
(21:04–24:54)
Expert: Rui Manuel Cardosa Pereira, Head, Insect Pest Control, FAO/IAEA Joint Centre
Production Bottlenecks
"There is no enough sterile flies available at the moment... only about 100 million sterile flies per week." (Pereira, 21:46)
Scaling Up
"The Texas facility is already starting its construction ... the goal is to start to produce flies in 27." (Pereira, 22:42)
Demands of Eradication
"The use of the sterile insect technique is not a silver bullet. It's not a standalone technology; needs to be integrated with other control methods..." (Pereira, 23:49)
Ongoing Surveillance
"We want people to look at their animals... The earlier you find an infestation, the more likely the animal will recover fully and... we are going to be able to eliminate it." (Kaufman, 24:54)
"It's a public health concern as well."
— Dr. Grace Van Hoy (05:54)
"The sterile insect technique is the only proven way to eradicate this pest."
— Prof. Philip Kaufman (18:15)
"That number of flies will be approximately what will be needed today because the situation is quite similar."
— Rui Manuel Cardosa Pereira (23:49)
| Time | Segment Title | Description | |--------|-----------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------| | 01:36 | Introduction | Host sets up the episode’s premise: screwworm outbreak in Texas after decades. | | 02:30 | Part 1: The Screwworm Lowdown | Dr. Grace Van Hoy explains screwworm biology, lifecycle, and risk to animals/humans. | | 07:42 | Part 2: The Sterile Insect Technique | Dr. Thomas Rath discusses historic eradication, SIT development, and global eradication efforts. | | 15:18 | Part 3: Re-invasion | Prof. Philip Kaufman details the Texas response, containment, and eradication efforts. | | 21:04 | Part 4: A Fly in the Ointment | Rui Manuel Cardosa Pereira explains current production limitations and the international response/logistical ramp-up. | | 24:54 | Conclusion | Summary: Success is possible with SIT, but requires integrated control and vigilance. |
Yes, eradication is possible—historically proven—but demands a multi-layered and coordinated approach:
Sustained investment in infrastructure (sterile fly production), education, and cross-border cooperation will determine the outcome of the current outbreak.
"The earlier you find an infestation, the more likely the animal will recover fully and the more likely we are going to be able to eliminate it."
— Prof. Philip Kaufman (24:54)
For past episodes like "Why are More People Getting Lyme Disease?" and "Why Does Ebola Keep Coming Back?", search for The Inquiry wherever you get your podcasts.