America’s Never-Ending Battle Against Flesh-Eating Worms

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“The history of the screwworm eradication program. This program is the single most important reason Texas has so many deer.


NOTE: this article was originally published to TheAtlantic.comIt was written by Sarah Zhang.


The Florida Keys are a place where deer stand next to children at school-bus stops.


They lounge on lawns. They eat snacks right out of people’s hands. So when the deer began acting strangely in the summer of 2016, the people of the Keys noticed. Bucks started swinging their heads erratically, as if trying to shake something loose.

Then wounds opened on their heads—big, gaping wounds that exposed white slabs of bone. Something was eating the deer alive.

That something, lab tests would later confirm, was the New World screwworm, a parasite supposed to have been eradicated from the United States half a century ago. No one in the Keys had ever seen it. If you had asked an old-time Florida rancher though, he might have told you boyhood stories of similarly disfigured and dying cattle. In those days, screwworms found their way into cattle through any opening in the skin: the belly buttons of newborn calves, scratches from barbed wire, even a tick bite. Then they feasted.

Screwworms once killed millions of dollars’ worth of cattle a year in the southern U.S. Their range extended from Florida to California, and they infected any living, warm-blooded animal: not only cattle but deer, squirrels, pets, and even the occasional human. In fact, the screwworm’s scientific name is C. hominivorax or “man eater”—so named after a horrific outbreak among prisoners on Devil’s Island, an infamous 19th-century French penal colony in South America.

For untold millennia, screwworms were a grisly fact of life in the Americas. In the 1950s, however, U.S. ranchers began to envision a new status quo. They dared to dream of an entire country free of screwworms. At their urging, the United States Department of Agriculture undertook what would ultimately become an immense, multidecade effort to wipe out the screwworms, first in the U.S. and then in Mexico and Central America—all the way down to the narrow strip of land that is the Isthmus of Panama. The eradication was a resounding success. But the story does not end there. Containing a disease is one thing. Keeping it contained is another thing entirely, as the coronavirus pandemic is now so dramatically demonstrating.

To get the screwworms out, the USDA to this day maintains an international screwworm barrier along the Panama-Colombia border. The barrier is an invisible one, and it is kept in place by constant human effort. Every week, planes drop 14.7 million sterilized screwworms over the rainforest that divides the two countries. A screwworm-rearing plant operates 24/7 in Panama. Inspectors cover thousands of square miles by motorcycle, boat, and horseback, searching for stray screwworm infections north of the border. The slightest oversight could undo all the work that came before.

The insect is relentless in its search for hosts. Those who fight it must be relentless too.

This past August, I went to Panama to meet the people who maintain the screwworm barrier. The Keys outbreak was long over by then, quelled within months by the release of sterile screwworms from Panama. As startling as it was to Floridians, it had been just a small, gruesome blip in the history of the screwworm.

A transcontinental screwworm barrier has been in place for 50 years—longer than many of the people who now maintain it have been alive. They work for a joint commission of Panama’s agricultural department and the USDA known as COPEG, or the Comisión Panamá–Estados Unidos para la Erradicación y Prevención del Gusano Barrenador del Ganado. The day before I landed at Tocumen International Airport, two small COPEG planes had taken off and released their precious loads of screwworms over the Panama-Colombia border.

More screwworm flights were scheduled for the next day, a Wednesday, and Thursday and Saturday and Monday and so on and on. “We will be here for a long time,” a COPEG staff member in Panama told me with evident pride. “We should be here for the next 100 years.”

In the early days of the eradication effort, USDA scientists were not so certain of success—or longevity. They had to bootleg money from other programs because they didn’t have enough funding. In press interviews, they worried about what laughingstocks they’d be if their “idiotic insect-sex scheme” failed and, God forbid, became an extremely mockable symbol of government waste.

The man who came up with the scheme, and believed in it most passionately, was Edward F. Knipling, a USDA entomologist who, in the 1930s, spent long hours watching screwworms mate. As a boy, he had waged constant war against insect pests on his family’s Texas farm. “Every plant that we grew,” he later said, “there was some type of insect that was causing damage.” Screwworms infected the farm’s cows and pigs, and Knipling remembered them as one of the worst pests. He had to climb into the hog pens to smear medicine on the wounds of uncooperative sows. “That was a very unpleasant task,” he recalled some eight decades later, in an interview shortly before he died.

Adult screwworms are actually flies, with big red eyes and metallic blue-green bodies. After mating, the females lay their eggs in open wounds, and the resulting larvae eat through a ring of surrounding flesh. Once sufficiently engorged, the larvae drop off the wounds to pupate, emerging as a new generation of flies. As Knipling watched screwworms churn through their life cycle in his government laboratory, he made an observation whose importance he could intuit but not yet put to use: Female screwworms mate only once in their entire life. If a female screwworm mates with a sterile male, she will never have any offspring. So if the environment could somehow be saturated with sterile males, Knipling surmised, screwworms would very quickly mate themselves out of existence.

But Knipling did not know how to sterilize male screwworms. In any case, the U.S. was entering World War II, and his expertise was needed elsewhere. Knipling was reassigned to a lab in Florida, where his team perfected incredibly effective pesticides such as DDT that protected troops from insect-borne diseases. DDT helped the U.S. win the war, but it would later devastate the environment from overuse. This experience shaped the rest of Knipling’s career. He devoted himself to sterilizing insects, a way to control pests without pesticides.

After the war, Knipling went back to studying screwworms with his fellow entomologist Raymond Bushland. Scientists now knew, from the horrific consequences of atomic bombs dropped on Japan, that high doses of radiation damage human tissue and cells. When a colleague introduced Knipling to research on the sterilization of other flies by radiation, he wondered: Could radiation sterilize screwworms?

The entomologists managed to get access to an X-ray machine at a nearby military hospital where one of Bushland’s old Army buddies worked. Every Thursday afternoon, Knipling and Bushland put their screwworms through the X-ray machine, experimenting with different developmental stages and radiation levels. They needed screwworms that were damaged enough to be sterile but not so damaged that they could not attract a mate in the wild. The best time to irradiate, the two found, was 5.5 to 5.7 days into the pupal stage, when the adult fly’s ovaries and testes were developing and thus most sensitive to radiation.

The radiation worked. These screwworms indeed turned sterile, but Knipling and Bushland still needed to prove that irradiated males could actually mate with fertile females in the field. In 1951, a USDA team began releasing sterile male screwworms on Sanibel Island off the coast of Florida. The screwworms persisted, so the team in turn persisted, releasing sterile males on the island for two more years. It still didn’t work. The team thought that fertile males were probably flying over from the mainland. So in 1954, the scientists tried again on a more remote island: Curaçao in the Dutch Caribbean. This time, they succeeded. Screwworms disappeared from the island within months.

From here, a quietly audacious project to reengineer the environment for livestock got underway, ultimately changing the lives of cattle, deer, and humans throughout the North American continent.

When the Florida Cattlemen’s Association caught wind of the success on Curaçao, it immediately recognized the potential closer to home. The group lobbied state and federal officials for a bigger undertaking. And in 1957, the USDA began a campaign to wipe out the flesh-eating parasites east of the Mississippi River. When that succeeded two years later, ranchers in Texas, Louisiana, New Mexico, Arizona, and California started clamoring for their own eradication program. Screwworms were so widespread in those states that they had shaped cowboy culture: Long days of “riding the range,” for example, were dedicated to finding signs of screwworm infection.

For the USDA, though, the West proved a challenge on a different scale. The screwworm-eradication program had to build a new plant in Mission, Texas, to produce as many as 200 million sterile flies each week. Because screwworms prefer to eat living flesh, feeding the plant’s flies was a grisly logistical puzzle. Initially, USDA scientists gave the screwworms a mixture of warm ground beef and blood, but beef was expensive. At various times, they substituted cheaper meats such as horse, whale, pig or cow lung, and even nutria—an invasive rodent that was then taking over Louisiana. By 1962, the flies at the Mission plant were consuming 240,000 pounds of meat and 10,000 gallons of whole blood every week. In the earliest days of sterile-screwworm testing, the flies stank so badly that airlines refused to ship them. Workers learned to spray the boxes with cologne.

The Mission plant closed in 1981, when screwworm rearing moved to Mexico and then later to Panama. Today, the screwworm-production facility in Panama is located on an old sugarcane plantation, about 20 miles east of the country’s capital. The roof is painted a smooth mint green, and the facility is the program’s most modern yet. But screwworms still have primordial urges: They still have to feed on animal remnants. So when I stepped inside the facility last August, the first thing I noticed was the smell—foul, with a metallic tang. My brain involuntarily matched it to bloody tampon.

Screwworms are no longer given raw meat, but their keepers make sure they get animal protein in other ways. The current diet is a more economical mix of powdered blood, milk, and egg—reconstituted and then thickened with cellulose into a dark-brown sludge. I watched workers pipe it into trays, the diet gushing like sewage.

The rearing facility itself is a windowless maze of concrete. Each room is maintained at a prescribed temperature and humidity, optimized for a particular stage of screwworm development. The larvae, for example, hatch in a room heated to 102 degrees Fahrenheit, which mimics the body temperature of an infected animal. Once hatched, they are wheeled to cooler rooms, where they crawl out of their food and become pupae. My glasses fogged up multiple times as we moved through rooms hot and cold, dry and humid, following the screwworm’s life cycle.

In the 102-degree hatching room, staff pulled out a tray of food to show me a “feeding pocket.” When screwworms eat, they like to pack themselves tightly together like cigarettes in a carton, their mouths shoved down into the food and their white tails wriggling in the air. I had come across pictures of feeding pockets in animals’ open wounds before. Now I forced myself to look at the writhing brown mass, imagining this protein sludge to be flesh yielding to thousands of relentless mouths.

“For me, beautiful,” said Sabina Barrios, the head of screwworm production, gesturing at the feeding pocket. What she saw were healthy screwworms that would grow big and fat. In a few days, they would turn into pupae, also big and fat, that could be sterilized with radioactive cobalt 60. And a couple of days after that, they would emerge as flies, vigorous and ready to mate. A hundred and fifteen people work to keep the plant operating 24 hours a day, every day, to produce 20 million of these flies a week.

COPEG does more than produce sterile flies. It also runs a network of screwworm-inspection posts and offices that reaches into the most remote parts of the country. Early the next morning, I set out for the Darién, Panama’s easternmost and least populated province, with Pamela Phillips and John Welch. Both are scientists who have spent decades working on screwworm eradication; Phillips is now COPEG’s technical director, and Welch is its former technical director and current screwworm-program liaison.

Phillips and Welch work for COPEG through the USDA, and they had to inform the U.S. embassy that they were going to the Darién. We also had to avoid “red zones,” areas in the province that the U.S. government considers too risky for its employees to enter at all. (COPEG’s Panamanian inspectors still work in those parts.) FARC guerrillas used to roam the Darién, and drugs from South America still pass through here on their way north.

Welch is a bearded Texan with no fewer than two screwworm tattoos. He learned to speak Spanish after joining the program in 1984, and despite his very American accent, he launched into easy jokes with everyone we met. He hired Phillips to analyze satellite imagery for screwworm habitats in 1994. After 25 years of working together, their dynamic resembles that of an old married couple. “I’m biologically 67,” Welch told me. “Mentally, still a teenager,” Phillips added. As scientists, they have both found in screwworms a formidable intellectual challenge. As nature lovers, they enjoy the opportunities for travel that come with the job too.

Welch may be an adolescent at heart, but his back doesn’t handle these trips as well as it used to. The road to the Darién is so festooned with potholes that Phillips had to frequently swerve into the opposite lane to avoid them. When we crossed into the province, we stopped at a checkpoint to show our passports to the Senafront, Panama’s border police. COPEG has a checkpoint here too, mere feet away—but for animals. Cattle leaving the Darién are unloaded one by one and inspected for wounds, which are painted with a blue-green anti-parasite medication to kill any screwworms.

Not too far past this checkpoint, the highway in the Darién simply ends. The only way to travel farther is by river. We, along with several Panamanian staff, put all our things in waterproof trash bags and got into a small COPEG boat. We arrived first in the town of Garachiné, a small collection of mostly cinder-block houses, then climbed into a COPEG pickup to jostle along the uneven dirt road to the smaller town of Sambu. During the hour-long drive, we passed more horses and riders than cars. Butterflies erupted by the road in regular bursts. But the land here is far less wild than it used to be. What was once rainforest and swamp is now pastureland. “People bring cattle here to fatten them up,” Phillips said.

Cattle ranching has been expanding in the Darién, and COPEG inspectors must travel down ever more roads and rivers to visit ever more farms. At a farm between Garachiné and Sambu, a farmer greeted us with his infant boy in tow, having recognized the COPEG logo—a retro-looking atomic fly—on our car. Inspectors stop by regularly to check the animals and chat with the farmers, reminding them to be vigilant of screwworms. A COPEG veterinarian, Manuel Sánchez, showed me the handwritten log recording several years of visits to this farm. Staff members visit the highest-risk farms in this region at least once a month, medium-risk farms every four months, and the rest once a year. The small COPEG outpost in this zone is responsible for a total of 224 farms. There are 12 other COPEG field posts like it in the Darién.

We were late leaving Garachiné, and the high tide that allows boats to move through the area’s shallow rivers and estuaries was on its way out. If the tide got too low, we would have to spend the night in Garachiné, where the COPEG office keeps a stack of old mattresses handy for stranded employees. We decided that the few hundred feet of mud and shallow water that separated us from the boat were traversable. But the mud gave way like quicksand, swallowing our feet whole. I pulled out my left leg, then my right leg, and then my left again. Phillips almost lost her shoe. We eventually made it to the boat, wet and oozing mud.

When the USDA began expanding the screwworm program into Central America, it dispatched scientists to all sorts of remote places. They collected new screwworm strains, studied the habitat in each country, and when screwworms were thought to be eradicated from a region, verified that they were gone. The work took Welch to Mexico, Panama, Costa Rica, Belize, Guatemala, Honduras, Nicaragua, the Dominican Republic, Colombia, Argentina, Uruguay, Jamaica, Aruba, and Cuba’s Guantánamo Bay. He and his colleagues hiked up and down mountains, sometimes in pouring rain, to check traps baited with a chemical attractant called Swormlure. On easier days, they sat for hours next to rotting liver, waiting for its odor to attract live screwworms. Sometimes they slept in cars and on hut floors. Sometimes they camped, falling asleep to the sounds of wind and water.

The U.S. government’s decision to eradicate screwworms in Central America was ultimately about money. Protecting American livestock by dropping sterile flies over the narrow 50-mile Isthmus of Panama is cheaper than maintaining a barrier, even a virtual one, along the 2,000-mile U.S.-Mexico border.

The U.S. had officially declared victory over the screwworm in 1966, but the barrier of sterile flies it established on the U.S.-Mexico border quickly proved ineffective. Ranchers in the U.S. Southwest continued to see outbreaks. With Mexico’s cooperation, the eradication front moved south to the Isthmus of Tehuantepec, where Mexico narrows to 120 miles. The two countries split the bill based on the value of the livestock that would benefit in each country: 80 percent U.S., 20 percent Mexico.

In 1985, USDA scientists proposed moving the barrier south again—to the even narrower Isthmus of Panama, where it would be cheaper to maintain. But it would require convincing the governments of seven more countries to agree to—and help pay for—screwworm eradication.

The late 1980s were turbulent times for Central America. Both state-sponsored and guerrilla attacks on civilians were widespread. Other parts of the U.S. government were intervening in the region as part of Cold War politics. “A lot of people speculated I was with the CIA,” says John Wyss, a retired USDA scientist and administrator who spent long periods in Central America working on the screwworm program’s expansion. Negotiations between USDA officials and their Central American colleagues took place against a backdrop of violence. Another USDA scientist, in his memoir, recalled negotiating with Honduran government officials about screwworms—only to learn two weeks later that the building where they had met had been blown up.

The eradication effort did have enthusiastic local allies, Wyss told me: Livestock owners in every country loved the idea. The negotiations went slowly, but by 1994, all of the countries had signed cooperative agreements with the U.S.

In 2000, when Central American eradication was in its final phases, Wyss wrote a paper touting the program’s political achievements. “There is one tool that has played an important role since the beginning of the screwworm eradication program, one that is frequently overlooked or not even mentioned,” he wrote. “This tool is the cooperative agreement.” These agreements outline, in bureaucratic detail, how salaries will be paid, how property will be managed, and how the agreement itself will be amended. Panama was declared free of screwworm in 2006, and the sterile-insect barrier was erected. COPEG is now jointly headed by two director generals: Vanessa Dellis for the U.S. and Enrique Samudio for Panama.

The screwworm program costs $15 million a year, a small fraction of the estimated $796 million a year that it saves American farmers. (That estimate, from 1996, is equivalent to $1.3 billion in today’s dollars.) Still, the program is constantly looking for ways to cut costs. At the production facility, Phillips showed me prototypes of small, climate-controlled rearing cabinets, which could eliminate the need to heat or cool entire rooms. Biologists are also developing a genetically modified male-only strain of screwworms, which would require fewer flies to be raised and released. A cheaper program is a more sustainable one, and sustainability is essential.

Cornelia Li

Over the years, the success of screwworm eradication has inspired scientists to apply the sterile-insect technique to other agricultural pests such as the Mexican fruit fly, the Mediterraean fruit fly, and the pink bollworm. These programs have had varying degrees of success in the U.S., but none has expanded as widely as the screwworm effort. Screwworm infections, meanwhile, are still endemic in livestock and wildlife south of the sterile-insect barrier. South American countries, such as Uruguay, have at times inquired about eradicating the parasite. But for any country to go it alone makes little sense, given the porousness of national borders. And getting the entire South American continent on board is so colossal a financial and diplomatic undertaking that it hardly seems possible.

I wondered about this when I was in Panama, and I’ve been wondering about it now, while I’m confined to my apartment. Today, the world is struggling to contain a pestilence of a very different kind. Nations are closing their borders to fend for themselves, and the U.S. is entirely absent from its role in global leadership. Containment is about science, yes, but it’s also about politics. Screwworm eradication worked only because it had support from both.

In late 2016, when screwworms showed up in the Florida Keys, experts immediately suspected that they had come from Cuba. The U.S.-led eradication program had never reached the island—the two countries weren’t exactly on good terms in the ’80s and ’90s—so screwworms continued to prosper 90 miles from the Keys. Cuban screwworm experts still had to deal with the pests the old-fashioned way, by treating infections as they arose. And even they agreed that their island was the likely source of the Keys outbreak. “We asked them what did they think, and they laughed and said, ‘What do you think? We’re right here,’” Welch recalled.

That Cuban and U.S. entomologists could talk to one another at all was new. For decades, USDA employees were prohibited from communicating with their Cuban counterparts, even when they attended the same international screwworm meetings. Then, in late 2014, President Barack Obama relaxed U.S. restrictions on trade with Cuba and relations started to thaw. So even when DNA tests were ultimately inconclusive about the origin of the Keys outbreak, scientists in both countries began to talk about implementing an eradication program in Cuba, and a Cuban scientist traveled to Panama to visit the COPEG production plant.

But in August 2017, strange stories started to circulate about American diplomats in Cuba. They were falling ill, from mysterious and unconfirmed “sonic attacks.” In response, the U.S. withdrew its nonessential staff from the country and expelled two Cuban diplomats. Cuban scientists stopped replying to the Americans’ emails about screwworms.

On my last full day in Panama, I woke up before dawn to head to the airport, where I would board a small turboprop plane loaded with 2.1 million sterilized screwworms. I had wandered the airport’s passenger terminals with my carry-on just a few days earlier, but now I was whisked into a wing closed to commercial passengers. This was COPEG’s dispersal center.

Four days a week, a white van that workers call the “Pupamobile” drops off coolers of sterilized pupae. The center then bursts into activity. A repurposed industrial pill-counting machine spits out 450 milliliters of pupae at a time onto aluminum trays, which are then stacked about 50 deep. Two days later, the adult emerge as flies. They are moved into a cold room, where they enter a dormant state; the sluggish flies are then loaded into a washing-machine-size metal box and loaded onto the plane.

I was there on Monday, which meant that two flights were scheduled that morning. The planes are used military aircraft, each customized with a continuously rotating dispersal machine that spits flies out of the plane’s belly at an adjustable rate per nautical mile. (In the old days, a technician threw out small cardboard boxes of flies by hand, but the boxes sometimes didn’t break open on impact and the flies died inside.) The dispersal machine is kept cold, so the flies won’t wake until they hit warm outside air. A few, however, always manage to escape into the cockpit.

One of the pilots came over to introduce himself as Michael Jackson. (Yes, he explained, his father was a big fan.) Jackson used to do medical evacuations, but he had switched to more regularly scheduled screwworm-dispersal flights, and I would be flying with him that morning. I climbed into his plane and noticed, with slight hesitation, that the only open seat was the co-pilot’s in the cockpit. He nodded. I was to sit there. As usual, a technician sat next to the screwworm box in the back, logging temperatures and confirming the dispersal machine’s proper function.

By the time we took off, at 6:50 a.m., the sun had burned off some of the clouds. “You are so lucky. We have good weather today,” Jackson said. Good weather meant good visibility. I watched as the sea of rust-red roofs around Panama City gave way to the emerald lushness of the Darién. I had never before flown so close to the ground: When we passed over towns where field inspectors had recently found screwworm infections, we dipped to just 1,500 feet to make sure the sterile flies would not be blown off course on their journey back to earth. We were low enough to pick out individual cattle, scattered like grains of rice on a green plain.

The wildlife were not as easy to spot from the plane, but their lives would also be altered by our flight. Welch had told me that howler monkeys in Panama sometimes fell from trees after screwworms ate out their eyes. That doesn’t happen anymore. Jaguars, sloths, tapirs, horses, coyotes, buffalo, rabbits, and squirrels up and down the North American continent are now spared from screwworms too. In the U.S., the main ecological consequence of eradication has been a dramatic increase in the wild-deer population, which once fluctuated with screwworm numbers. The parasite used to kill a large proportion of newborn fawns, whose unhealed belly buttons were open wounds. In the Keys, the recent screwworm outbreak became obvious during mating season, when males began fighting one another with their antlers. Their small, usually harmless nicks and cuts turned large and horrific once screwworms invaded them.

When the plane reached the Colombian border, Jackson notified air traffic control and continued on for 20 nautical miles. Every month, the program asks Colombia for permission to release screwworms within its borders, hoping to create a screwworm-free buffer zone that reinforces the continental barrier. We looped over the Darién and this thin slice of Colombia several times in four hours, until the plane had dispersed all 2.1 million flies onboard. The dispersal machine whirred behind us, spinning at its preset speed, pushing out clumps of flies at its preset rate. Screwworms that had spent their entire life crammed in a factory would now wake to find themselves falling thousands of feet through thin air. They would land, and then they would mate. That’s what their evolutionary instincts have primed them to do; that’s what the humans who sterilized them want them to do.

During the final hour of the flight, I looked intently out the window, hoping for a last glimpse of all the places I had been. We flew over the potholed highway into the Darién. We flew over the yellow ribbons of river that led to Sambu and Garachiné. We flew over COPEG’s production plant, with millions of screwworms feeding beneath its unmistakable green roof. The next day, I would fly through the same airspace, this time on a jetliner bound for America—where USDA-graded steaks sit on supermarket shelves, where deer leap across the landscape, and where the efforts of a distant group of people keep us safely ignorant of screwworms.


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