Flesh-Eating 'Screwworm' Parasites Are Headed to the U.S.

Officials in nine countries are trying to get a handle on the New World screwworm, a fly whose larvae eat the living flesh of livestock.
The pest is marching northward at an alarming rate and has now moved some 1,400 miles from southern Panama to southern Mexico in about two years. Screwworms are disastrous for ranchers, whose cattle can become infected when the flies lay eggs in cuts or wounds, after which their resulting larvae burrow, or screw, into that flesh. The northernmost sighting is currently about 700 miles south of the U.S. border.
Since the insect breached biological containment in Panama's province of Darién in 2023, it has moved through Central America and is now found as far north as the Mexican state of Oaxaca. Thousands of animals have been infected, and officials have reported dozens of human cases in Costa Rica, Nicaragua, Honduras and Mexico this year.
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As the fly spreads northward from the narrow Darién Gap in Panama and up the funnel of Central America, it becomes harder to control. Agricultural departments suppress fly populations by releasing millions of sterile male flies per week into the environment throughout Central America. These males are raised in a facility in Panama jointly run by that country's agricultural department and the U.S. Department of Agriculture. Because female screwworms mate only once in their lifetime, this population of infertile males reduces the size of the next generation of flies. Consistent application of this sterile insect technique eradicated the screwworm from the U.S. in 1966 and from regions north of the Darién Gap in 2006.
That invisible wall holding the screwworm back has crumbled, however. 'I don't know how it got away so quickly,' says Maxwell Scott, an entomologist at North Carolina State University, who studies genetic methods to control populations of the fly. 'There had to be some movement of infested livestock, particularly through the middle [of Central America].... It just moved too fast,' Scott says about the swift speed of the screwworm spread.
On their own, the flies can usually fly no more than about 12 miles in their monthlong lifetime, says Sonja Swiger, an entomologist at Texas A&M University. But the screwworm larvae can travel great distances while developing inside (and gnawing on the flesh of) their hosts. A new generation reaches sexual maturity every week to two weeks, and females can lay up to 2,800 eggs over the course of their lifespan, according to the California Department of Food & Agriculture.
Most people aren't at risk of screwworm infections, which are rare compared with those in livestock. But cases have appeared in Central America since the breach of the Darién Gap. Nicaragua first detected the parasite in livestock in March 2024; by February 2025, health officials there confirmed 30 human cases. Costa Rica saw 42 confirmed cases between January and May 2025 and at least two deaths, according to the country's health ministry. Honduras has reported 40 human cases and three deaths, according to the public health network EpiCore, while Guatemala reported its first human case in May. The Mexican Ministry of Health has confirmed eight human cases.
In humans, infection with fly larvae is known as myiasis. Those who are most at risk for screwworm myiasis are people who work closely with livestock or who sleep outdoors. Treatment involves removing the larvae, sometimes with surgery.
Screwworms haven't made it back into the U.S. yet. How quickly this might happen depends on whether agricultural officials can hold the line in Mexico or push the fly southward. On May 27 U.S. Department of Agriculture officials announced $21 million in funding to retrofit a fruit fly production plant in Metapa, Mexico, to produce sterile screwworm flies. When operational, the plant will churn out between 60 million and 100 million additional flies a week to help suppress the breeding population in Mexico.
While the sterile insect technique is likely to remain the key tool in the arsenal against screwworms for years to come, new genetic methods of insect control could eventually come to bear against the problem. In May ethicists and entomologists, including Scott, wrote in a paper in Science that the screwworm is a good candidate for complete elimination with gene drive technology, which involves genetic engineering to ensure that a deadly mutation will be included in an animal's sperm and egg cells and thus will be passed on to the next generation. The loss of screwworms does not seem to substantially affect the ecosystem, the researchers wrote, and death by the insect is painful and slow.
'The extent of the public health threat posed by screwworm is not certain, but any flesh-eating insect that caused occasional human mortality in the Global North would almost certainly be marked for suppression if not eradication,' the researchers wrote.
Thus far, these genetic techniques have largely been explored to target malaria-carrying mosquitoes, though scientists at the Pasteur Institute of Montevideo in Uruguay are now working to develop a gene drive strain for the screwworm, Scott says. It will take time to make progress, but if the researchers succeed, the resulting mutant could spell the end of the screwworm not just in North and Central America but in South America, too. 'This is a very rapidly developing field,' Scott says.

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Newsweek

3 hours ago

• Newsweek

How a Favored Migrant Route Became Invasive Species Superhighway to US

Based on facts, either observed and verified firsthand by the reporter, or reported and verified from knowledgeable sources. Newsweek AI is in beta. Translations may contain inaccuracies—please refer to the original content. The Darién Gap, long a perilous route for millions of migrants traveling north toward the United States, has also become a pathway for the northward spread of the New World screwworm—a parasitic fly now challenging decades of containment efforts. The flesh-eating fly, whose larvae burrow into the wounds of living animals, has surged north through Central America since 2022, recently reaching the Mexican states of Oaxaca and Veracruz—roughly 700 miles from the Texas border. Scientists and federal officials now warn that the screwworm could cross into the U.S. as early as this summer if containment efforts falter. For decades, the United States and Panama maintained a bi-national barrier to block the parasite at the Darién, using a program that dropped millions of sterilized screwworm flies over the jungle to prevent breeding. That system began unraveling in 2022, amid pandemic-era supply-chain disruptions and record human and animal movement through the region. Aerial view showing migrants walking through the jungle near Bajo Chiquito village, the first border control of the Darien Province in Panama, on September 22, 2023. The clandestine journey through the Darien Gap usually lasts... Aerial view showing migrants walking through the jungle near Bajo Chiquito village, the first border control of the Darien Province in Panama, on September 22, 2023. The clandestine journey through the Darien Gap usually lasts five or six days, at the mercy of all kinds of bad weather. More than 390,000 migrants have entered Panama through this jungle so far this year, far more than in all of 2022, when there were 248,000, according to official Panamanian data. More Photo by Luis ACOSTA / AFP) (Photo by LUIS ACOSTA/AFP via Getty Images Between 2021 and 2024, more than 1.2 million migrants crossed the Darién, according to data from Panama's National Migration Service and UNHCR. The crossings peaked at 520,000 in 2023 before falling to 302,000 in 2024. U.S. officials believe that mass movement contributed to operational breakdowns in the region's screwworm control infrastructure. "There are many challenges in this containment and eradication program," Phillip Kaufman, a professor of entomology at Texas A&M University, said in an interview with Newsweek. "Producing sufficient numbers of sterile flies and getting them released in the correct places and at the right time is critical. If the flies move further north than the isthmus in southern Mexico, it becomes more and more challenging to contain them." The screwworm, scientifically known as Cochliomyia hominivorax — roughly translated as "man-eater"—was eradicated from the U.S. by 1982 using the Sterile Insect Technique (SIT), which involves releasing sterilized male flies to prevent reproduction and collapse the population. The parasite targets livestock, wildlife, pets — and occasionally humans — by burrowing into open wounds. Infected animals are once again being found in areas previously considered screwworm-free, triggering emergency responses from both governments. Human and Animal Migration a Factor Though the vast majority of screwworm cases are in livestock, human infections have surged in the wake of the parasite's northward spread. Nicaragua reported 30 human cases by February 2025. Costa Rica saw 42 confirmed cases between January and May, including two deaths. Honduras recorded 40 cases and three deaths, while Guatemala and Mexico have also reported infections. In humans, the infestation—known as myiasis—typically occurs in individuals with untreated wounds, particularly those living in rural areas or sleeping outdoors. Treatment often requires surgical removal of larvae and follow-up care to prevent secondary infection or tissue loss. While the root causes of the screwworm's northward march remain under study, scientists suspect a combination of human migration, illegal animal movement and weather patterns. Migrants walk by the jungle near Bajo Chiquito village, the first border control of the Darien Province in Panama, on September 22, 2023. The clandestine journey through the Darien Gap usually lasts five or six... Migrants walk by the jungle near Bajo Chiquito village, the first border control of the Darien Province in Panama, on September 22, 2023. The clandestine journey through the Darien Gap usually lasts five or six days, at the mercy of all kinds of bad weather. More LUIS ACOSTA/AFP via Getty Images "Additional movement of animals and humans has helped facilitate the expansion of the flies' location," Sonja Swiger, a professor and extension specialist at Texas A&M AgriLife, told Newsweek. "This includes both the moving of infested animals unknowingly and by disrupting the operations of COPEG to release SIT flies in the area." A Department of Agriculture (USDA) spokesperson told Newsweek the disruption was caused "both by the movement of infested animals unknowingly and by interference with COPEG's operations to release sterile flies in the area." The USDA said it is working with regional governments to strengthen veterinary inspections and education. But officials noted that controlling illegal livestock movement often falls under national security, making enforcement more complex. "We have raised this concern directly with officials in each affected country," a USDA official told Newsweek. "While regional governments acknowledge that uncontrolled animal and human movement undermines containment efforts, monitoring and controlling this activity is complex." Recent enforcement appears to be having an effect. Under President Donald Trump's return to office in early 2025, crossings through the Darién fell sharply. Only 408 people crossed in February, and just 194 in March, effectively emptying camps along once-overrun migration routes, including Lajas Blancas. Risk of Reintroduction U.S. officials suspended live animal imports from Mexico in May after screwworm detections in cattle. The USDA has invested $21 million to retrofit a Mexican fruit-fly production facility in Metapa to manufacture an additional 60 to 100 million sterile screwworms weekly. That would double current output from a long-operating facility in Panama, which has been running at capacity. "Should the screwworm continue moving north, having access to many more flies than the existing facility in Panama can produce will be important to protect U.S. agriculture," a USDA spokesperson told Newsweek. "We are exploring possible options for increasing our capacity to produce these sterile insects, which could include a facility here in the United States." Secretary of Agriculture Brooke Rollins looks on as President Donald Trump signs executive orders in the Oval Office of the White House on April 9, 2025, in Washington, D.C. Secretary of Agriculture Brooke Rollins looks on as President Donald Trump signs executive orders in the Oval Office of the White House on April 9, 2025, in Washington, U.S. government now plans to open a second sterile fly production center in Texas by the end of 2025. Located at Moore Air Base, less than 20 miles from the Mexican border, the $8.5 million facility would eventually produce up to 300 million flies per week. Agriculture Secretary Brooke Rollins said on Wednesday the move is necessary to "protect the American cattle herd from the devastating threat of New World screwworm." Texas alone houses 12 million cattle and calves—more than any other U.S. state—part of a thriving livestock industry valued at $15 billion. If screwworms re-enter Texas, the economic losses could be staggering. A USDA model projects that a statewide outbreak could cost producers $732 million annually and trigger $1.8 billion in broader economic costs. Nationwide, screwworm eradication since the 1960s has saved the livestock industry over $1.5 billion per year. "These are not speculative numbers—they reflect the hard-earned lessons from the last century," Kaufman, the entomology professor, said. A Long Road to Re-Eradication Despite new investment, containment is already slipping. "It is going to take a long time to put this genie back in the bottle," Andrew Short, chair of the entomology department at the University of Florida, said in an interview with Newsweek. "We do not have good means of monitoring for screwworm aside from looking for already-infected animals." Short called for urgent improvements. "We desperately need better trapping and surveillance methods to complement animal inspections," he added. The sterile insect technique remains a cornerstone of eradication efforts, but experts say new technologies are in development to improve its efficiency. One potential breakthrough is a genetically modified strain that eliminates female flies during production, allowing for twice the output of sterilized males. Cows are seen standing in a feedlot on June 14, 2023 in Quemado, Texas. Cows are seen standing in a feedlot on June 14, 2023 in Quemado, Texas."This is done by preventing the development of the female flies when they are mass produced," Kaufman explained. "The result is near doubling of the production of flies in the sterile fly production facility." The USDA said it would consider building a companion fly-breeding center alongside the new Texas facility, joining existing sites in Panama and Mexico. Together, these could generate as many as 500 million sterile flies per week to outnumber and outcompete wild screwworms. Despite the costs of containment, USDA economists estimate that maintaining the eradication program delivers annual economic benefits exceeding $1 billion—far outweighing the expense of operations like the COPEG fly factory in Panama. In contrast, responding reactively to an outbreak, rather than preventing it, could drain public and private resources for decades.

USA Today

a day ago

• USA Today

'Killer bees' keep spreading (and killing) across the US

Scientists say the bees' temperament is to ward off predators. But unsuspecting humans will find them to be aggressive, spiteful and terrifying. Africanized honey bees, often called "killer bees," are now found in 13 states and are slowly spreading northward, attacking people, livestock and pets along the way. In the last three months alone, a man mowing his property died after a bee attack, three people were taken to the hospital after tree trimmers disturbed a colony, hikers ran a mile to get away from an agitated hive and a woman and three horses were swarmed by bees spooked by a lawn mower. The horses later died from "thousands" of stings, their owner said. Scientists say the bees' temperament is a defense mechanism to ward off predators – and note that because honey bees die after stinging, they are sacrificing themselves to protect their colony. But to unsuspecting humans, killer bees sure seem aggressive, spiteful and terrifying. They've been known to follow their victims for up to a mile – even following cars and trucks – and can sting through regular beekeeping gear. In the attack on the Texas horses, owner Baillie Hillman said, "they didn't give up." As parts of the west become warmer and drier, it's only going to get worse as the bees' preferred climate moves northward. There's no national database of deaths related to bee stings, but a 2023 study by the Centers for Disease Control and Prevention found that on average 72 people died a year from hornet, wasp and bee stings. "In Texas, every year there's at least four big (Africanized bee) attacks that make the news," said Juliana Rangel, a professor of apiculture (beekeeping) at Texas A&M University, where they're widespread in the wild. "Killer bees" first made headlines in the 1990s when they began to appear in the United States, spawning alarming news stories and a surge of horror movies such as Deadly Invasion, The Swarm, The Savage Bees. Today, people in the South and the Southwest are learning to live with them, but the danger remains, especially when the highly defensive bees first appear in an area or when people accidentally run into a colony. Africanized honey bees don't survive in areas with cold winters and don't like high levels of rain, making for natural cutoffs in areas where temperatures fall, Rangel said. However in the coming decades, climate change means the bees could potentially move into southeastern Oregon, the western Great Plains and the southern Appalachian mountains, according to research published in 2014. "By 2050 or so, with increasing temperatures, we're going to see northward movement, mostly in the Western half of the country," said Rangel. How dangerous are Africanized honey bees? Africanized honey bees don't have more, or more potent, venom than Western honey bees. "The stings feel the same," said Rangel. What differentiates them is the numbers a colony sends out to attack. While each bee can only sting once before it dies, a colony of Africanized honey bees will send out proportionally more bees in a defensive response, leading to more stings. "If I'm working around one of my European honey bee colonies and I knock on it with a hammer, it might send out five to ten individuals to see what's going on. They would follow me perhaps as far as my house and I might get stung once," said Ellis. "If I did the same thing with an Africanized colony, I might get 50 to 100 individuals who would follow me much farther and I'd get more stings. It's really an issue of scale," he said. Africanized honey bees are also much more sensitive to potential threats. "You could be mowing a lawn a few houses away and just the vibrations will set them off," Rangel said. There are multiple reports of people cutting into colonies when trimming trees, clearing brush or doing landscaping. In some cases tree trimmers set off an attack merely by throwing a rope over a branch. In toxicology, researchers speak of the LD50, the lethal dose required to kill 50% of those exposed to it, said Rangel. For honeybee stings, it's calculated at about 9 stings per pound of weight. So half of people who weigh 150 pounds might die if they got 1,350 stings. That's unlikely with Western honey bees, but can happen with Africanized bees. In 2022, an Ohio man suffered 20,000 bee stings when cutting tree branches. He survived but only after being put in a medically-induced coma. What states have Africanized honeybees? At least 13 state have reported Africanized honeybees. In the south, southern Florida has the highest numbers. "From Tampa to Orlando to Daytona, south of that a large percentage of the feral bee population are of African decent. North of that we don't have much," said Ellis. Africanized bees have also been reported in Alabama, Georgia, Louisiana and Mississippi, though they don't seem to thrive in those wetter environments as much. The bees have found a natural home in the Southwest, in part because the landscape is much like the arid and semi-arid parts of Africa they're originally from. Feral colonies are common in southern California, Nevada, Arizona, New Mexico and especially Texas. They've also been seen in southwest Arkansas, southern Utah and about two-thirds of Oklahoma, said Szalanski. Where did killer bees get their killer instincts? There are around 20,000 species of bees on the planet and only 12 of them are honey bees. Of those, 11 are found in Asia and only one in Europe, the Middle East and Africa. It is called Apis mellifera, the Western honey bee, said Jamie Ellis, a professor of honey bee research at the University of Florida. There are multiple subspecies of Apis mellifera. It was one of these, the East African lowland honey bee (Apis mellifera scutellata), that was imported to Brazil in the 1950s. In the areas where East African lowland honey bees live, they have many predators so they've evolved to be more defensive, said Ellis. "They're not really aggressive, they're not just looking for a fight," he said. "They're just trying to communicate that, 'You are too close to us. We don't want you here – go away.'" That can feel pretty scary when they're coming after you, said Rangel. She's done research on bees in Belize, where people keep hives far from their homes and livestock and put on their protective beekeeping clothing in their trucks before getting out near a hive. "They can pursue you in your vehicle for a mile," she said "The only thing preventing them from killing you is the veil. It's like a cloud of bees that all want to sting you. It's scary." How did Africanized honey bees get to the United States? Africanized honeybees are a cross between Western honey bees, from Europe and the East African lowland honey bee. They first occurred in 1956, when a prominent Brazilian geneticist, Warwick Kerr, brought African bees to Brazil to hybridize them with Western honey bees from Europe, which didn't do well there. His plan was to breed bees that were better adapted to Brazil's climate while retaining the gentleness and high honey-producing traits of the Western honey bees. He didn't get the chance. A year later, 26 Africanized queen bees were accidentally released into a nearby forest, where they thrived, said Kirk Visscher, an emeritus professor of entomology at the University of California, Riverside. "The beekeeping industry in those areas is now far more vital than it was – they just have mean bees," he said. That hybrid between the two subspecies, called Africanized honey bees, began to make its way north, arriving in south Texas in 1990. "They spread a lot quicker than what people thought was possible. They never thought they could get from Brazil to Texas in a span of less than 40 years," said Allen Szalanski, a professor of entomology at the University of Arkansas. "But they did." The two bee subspecies are impossible to differentiate without either DNA analysis or careful observation of their wing vein patterns. It is their behavior that sets them apart. Most of the danger is from feral hives in the wild. Beekeepers carefully manage their hives to keep Africanized queens out. Because bees are so critical to agriculture, the movement of bees is regulated in many states. Most have a bee inspector to oversee the process. Currently a total of 38 states regulate the movement of honey bees within the United States and require a permit or health certificate to do so. Four states, Arizona, Kansas, North Dakota and Oregon, have no regulations on the movement of bees, according to research published in 2022. Another eight states have no regulations requiring permits or health certificates for the interstate movement of bees. Africanized honey bees aren't all bad Scientists who study bees emphasize that while Africanized honey bees are more defensive, they hold a lot of promise for overall honey bee health because they're more pest and disease resistant and are very genetically diverse. That's important because honey bees play a critical role in agriculture and the environment and have been subject to large die-offs in the past two decades. In 2017, one-third of the nation's hives perished. Breeding and even gene manipulation could result in better – and gentler – bees, said Ellis. "Although they have heightened defensive behaviors, science may be able to maximize their positive traits and minimize their negative traits," he said. What should you do if you encounter an Africanized honeybee hive? Because of their highly defensive nature, if you encounter aggressive bees, the best response is to leave – fast. "Get away as quickly as possible. Don't jump in water, don't swat with your arms. Just run away as as fast as you can," said Ellis. "You might want to pull your shirt up around your nose and mouth to protect against stings that could cause swelling of your airway." In general, the biggest threats are to tethered or penned livestock and pets that can't get away as well as humans using heavy equipment who accidentally get too close to a hive without being able to hear the angry buzzing of the bees. Once you've evacuated the area, call for professional help. "If you're in a rural area, it's very likely these bees have been Africanized," said Rangel. Nobody should try to deal with a colony on their own "just to save a few bucks," she said. "You don't want to mess with them."

Yahoo

a day ago

• Yahoo

An explosion of sea urchins threatens to push coral reefs in Hawaii ‘past the point of recovery'

The turquoise water of Hōnaunau Bay in Hawaii, an area popular with snorkelers and divers, is teeming with spiny creatures that threaten to push the coral reef 'past the point of recovery,' new research has found. Sea urchin numbers here are exploding as the fish species that typically keep their populations in check decline due to overfishing, according to the study, published last month in the journal PLOS ONE. It's yet another blow to a reef already suffering damage from pollution as well as climate change-driven ocean heat waves and sea level rise. Kelly J. van Woesik, a researcher at the North Carolina State University Center for Geospatial Analytics and a study author, first noticed unusually high numbers of sea urchins on snorkeling trips. 'I knew there was a story to be told,' she said. She and her fellow researchers used data from scuba surveys and images taken from the air to track the health of the reef. 'We found on average 51 urchins per square meter, which is among the highest recorded densities on coral reefs anywhere in the world,' van Woesik said. Sea urchins are small marine invertebrates, characterized by their spiny bodies and found in oceans around the world. They play a useful role in preventing algae overgrowth, which can choke off oxygen to coral. However, they also eat the reef and too many of them can cause damaging erosion. In Hōnaunau Bay, the coral is already struggling to reproduce and grow due to ocean heat and water pollution, leaving it even more vulnerable to the erosion inflicted by sea urchins. Its rate of growth has plummeted according to the study. Reef growth is typically measured by the amount of calcium carbonate — the substance which forms coral skeletons — it produces per square meter each year. The reef in Hōnaunau Bay is growing 30 times more slowly than it did four decades ago, according to the study. Production levels were around 15 kilograms (33 pounds) per square meter in parts of Hawaii, signaling a healthy reef, according to research in the 1980s. Today, the reef in Hōnaunau Bay produces just 0.5 kg (1.1 pounds) per square meter. To offset erosion from urchins, at least 26% of the reef surface must be covered by living corals – and even more coral cover is necessary for it to grow. Gregory Asner, an ecologist at Arizona State University and study author, said what was happening in this part of Hawaii was emblematic of the mounting pressures facing reefs throughout the region. 'For 27 years I have worked in Hōnaunau Bay and other bays like it across Hawaii, but Hōnaunau stood out early on as an iconic example of a reef threatened by a combination of pressures,' he said, citing warming ocean temperatures, pollution from tourism and heavy fishing. The implications of coral decline are far-reaching. Coral reefs are sometimes dubbed the 'rainforests of the sea' because they support so much ocean life. They also play a vital role protecting coastlines from storm surges and erosion. 'If the reef can't keep up with sea-level rise, it loses its ability to limit incoming wave energy,' said van Woesik. 'That increases erosion and flooding risk of coastal communities.' Kiho Kim, an environmental science professor at American University, who was not involved in the study, said the findings highlight the fragility of reef ecosystems under stress. 'Dramatic increases in any species indicate an unusual condition that has allowed them to proliferate,' Kim said. That imbalance can undermine diversity and reduce the reef's ability to provide essential ecosystem services including food security and carbon storage, he told CNN. Despite the challenges, researchers emphasize that the reef's future is not sealed. Local groups in Hōnaunau are working to reduce fishing pressure, improve water quality and support coral restoration. 'These reefs are essential to protecting the islands they surround,' van Woesik said. 'Without action taken now, we risk allowing these reefs to erode past the point of no return.'