For the past 70 years, the United States has been engaged in a relentless aerial war against the New World screwworm, a parasitic menace known scientifically as C. hominivorax, which translates ominously to “man-eater.”
These larvae are notorious for inflicting horrific wounds on a variety of animals, including cattle, pigs, deer, and even humans, by chewing through their flesh.
In the 1950s, the U.S. Department of Agriculture (USDA) spearheaded a continent-wide initiative to eradicate this parasite.
By raising screwworms in factories and using radiation to sterilize them, they released millions of these sterile flies weekly across the United States, then further into Mexico and eventually throughout North America.
This aggressive program significantly reduced screwworm populations, and by 2006, authorities had established an invisible barrier at the Darién Gap, a dense jungle area between Panama and Colombia, deploying planes to release millions of sterile flies weekly.
However, in 2022, the barrier was breached.
Reports from Panama indicated a dramatic surge in screwworm cases among cattle, jumping from dozens to 1,000 in just a year, despite the continued dispersal of sterile flies.
By 2024, the parasite began its rapid northward advance, alarming many observers and stakeholders in Central America.
As of this month, the infestation has traveled 1,600 miles through eight countries, reaching Oaxaca and Veracruz in Mexico, with only 700 miles left to reach the Texas border.
In response, the U.S. has suspended live cattle imports from Mexico.
Wayne Cockrell, a Texas rancher and chair of the cattle-health committee for the Texas and Southwestern Cattle Raisers Association, expressed deep concern, stating that the return of the screwworm to Texas is a matter of when, not if.
The current sterile-fly program is unable to keep up with the parasite’s advance, let alone push it back down to Panama, according to Cockrell.
At 60 years old, he is too young to have witnessed the devastation of screwworms himself but has heard harrowing stories from those who lived through it when every cut or scratch posed a potential death sentence for cattle.
If screwworms re-establish themselves in the U.S., Cockrell warns that it may take decades to eradicate them again, echoing the time it took for the initial eradication.
The years of successful screwworm containment have unraveled in just two years.
Geographically, Central America resembles a funnel that narrows significantly at the Darién Gap, making it easier to manage screwworm populations.
The USDA not only sought to eradicate screwworms out of altruism but also due to economic pragmatism, as forming a screwworm barrier at the 100-mile Darién Gap was much less costly than maintaining one at the expansive 2,000-mile U.S.-Mexico border.
However, the recent surge in cases as the parasite advances beyond the isthmus in southern Mexico marks a critical turning point for containment efforts.
As the battle against screwworms expands, the need for sterilized flies grows as well, placing an even greater strain on the already limited supply.
The fly production factory in Panama has ramped up from its standard output of 20 million flies weekly to a maximum of 100 million, now all dispatched to Mexico.
During the original eradication campaign of the 1980s, planes were releasing as many as 150 million flies a week.
Furthermore, at the peak of the program, when the infestation was farther north in Mexico, a factory could produce up to 550 million flies weekly—a far cry from the current output.
Given this increase in demand, the Texas and Southwestern Cattle Raisers Association has urged the USDA to establish a new fly production facility in the U.S.
A USDA representative confirmed that they are working closely with Mexico to set up a biological barrier and will reevaluate production capacity if the fly spreads further.
Meanwhile, several Texas lawmakers have introduced the STOP Screwworms Act, aimed at directing the USDA to develop a new sterile-fly factory, but establishing such a facility could take years.
Cockrell emphasizes the urgency of the matter, stating, “The facility needs to start tomorrow.”
The cattle industry in the U.S. is not prepared for the screwworm’s potential return.
Cockrell highlighted several concerns: fundamental treatments for screwworm infection are no longer licensed in the U.S. due to decades of lacking need, and ranches have slimmed down their workforce, reducing the number of cowboys from 50 to just five.
Moreover, standard practices like branding and ear tagging can make the animals more susceptible to infection.
If screwworms reappear, Cockrell asserts that the cattle industry must adapt quickly to this new reality, as their return may lead to a further escalation of beef prices, which are already climbing due to drought conditions.
The exact mechanism that led to the breach of the barrier in 2022 remains somewhat unclear.
However, the pandemic largely disrupted supply chains at the Panama fly factory and interfered with routine cattle inspections that could have raised alarms earlier.
The once-impenetrable Darién Gap became a bustling route for migrants, further complicating pest management.
Initially, the screwworm advanced gradually through Panama and Costa Rica, but once in Nicaragua, it accelerated, moving swiftly through Honduras and Guatemala to reach Mexico within ten weeks in 2024.
This rapid movement was attributed to illegal cattle smuggling.
According to Jeremy Radachowsky, director for Mesoamerican and the Western Caribbean at the Wildlife Conservation Society, illegal cattle trade contributed significantly to the fast propagation of the parasite.
In Central America, an estimated 800,000 cattle are raised illegally in nature reserves and smuggled to Mexico by various means, dramatically speeding up the screwworm’s spread.
Radachowsky observed that the emergence of new screwworm cases has followed the known routes of smuggling operations.
This constant movement of infected cattle complicates efforts for re-eradication, likening the scenario to trying to drain a pool while “the spigot’s still open.”
Decades of freedom from screwworms had lulled many in the ranching community into forgetting the potential threat.
As one U.S. official with insights into the matter remarked, “We were so successful that literally people forgot.”
Timely inspections, infection reports, and movement restrictions are just as crucial for eradication as the release of sterile flies.
Scientists have suggested innovative control methods involving genetic modifications, which are still in developmental stages.
Max Scott, an entomologist from North Carolina State University, conducted research into creating a male-only strain of screwworm to reduce the number required for dispersal but saw funding for this initiative end last summer.
Despite setbacks, Scott has established a collaboration with researchers in Uruguay who are exploring a gene drive to develop sterile screwworms.
Uruguay views this initiative favorably, as the country never benefited from prior screwworm eradication efforts, situated deep within screwworm territory in South America.
Former USDA scientist Steven Skoda reminisced, “We used to dream of a world totally free of screwworm.”
Unfortunately, this dream never reached realization, and the barrier that once protected North America has now crumbled.
Cockrell fears that some of his colleagues in Panama may never witness the complete eradication of screwworms within their lifetimes.
image source from：https://www.theatlantic.com/science/archive/2025/05/screwworms-outbreak-united-states/682925/
