How a Favored Migrant Route Became Invasive Species Superhighway to US

Newsweek

6 days ago

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, D.C.The 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.