The New World screwworm, scientifically known as Cochliomyia hominivorax—literally ‘man-eater’—is a parasitic fly whose larvae feed on living flesh. For 70 years, the United States has waged an aerial war against this pest, which poses a significant threat to animals, including cattle, pigs, deer, dogs, and even humans. A successful eradication program pushed the screwworm down to Panama, creating an invisible barrier to protect North America. However, recent events signal a concerning resurgence. This article delves into the unfolding crisis, its potential impact, and the measures needed to combat this gruesome parasite.
The screwworm’s return could have devastating consequences for livestock and public health. The economic implications for the cattle industry, already facing high beef prices due to drought, are particularly alarming. Understanding the factors contributing to the screwworm’s comeback and the strategies to contain it is crucial. This article explores the history of screwworm eradication, the reasons for its resurgence, and the steps being taken to prevent a full-blown crisis in the United States.
From the initial breakthroughs in the 1950s to the present-day challenges, this article covers the key aspects of the screwworm saga. We will examine the sterile fly technique that led to near eradication, the breach in the Panama barrier, and the ongoing efforts to develop new control methods. The insights from ranchers, scientists, and officials provide a comprehensive overview of the current situation and the path forward. This is a race against time to protect livestock and livelihoods from a pest thought to be a thing of the past.
The Sterile Fly Technique: A Decades-Long Success Story
In the 1950s, the U.S. Department of Agriculture (USDA) pioneered an innovative approach to combat the screwworm: the sterile fly technique. This method involves raising screwworms in factories, sterilizing them with radiation, and releasing millions of these sterile adults over infested areas. The sterile flies then mate with wild populations, resulting in infertile eggs and a gradual decline in the screwworm population. This technique proved remarkably successful, leading to the eradication of screwworms from the United States and much of North America.
The sterile fly program was extended south into Mexico and eventually to the Darién Gap, the jungle region straddling the Panama-Colombia border. A barrier was established there in 2006, maintained by weekly releases of millions of sterile screwworms. This strategy effectively cordoned off the screwworm-free north from the south. As Sarah Zhang reported from Panama several years ago, this never-ending battle kept the threat of screwworms far from America.
The success of the sterile fly technique hinged on consistent, large-scale production and distribution of sterile flies. The USDA invested heavily in fly factories and aerial release programs, ensuring that the sterile flies outnumbered the wild population. The program’s effectiveness also relied on careful monitoring and rapid response to any outbreaks. This proactive approach kept the screwworm at bay for decades, allowing the cattle industry to thrive without the constant threat of infestation.
“The sterile flies proceeded to, well, screw the continent’s wild populations into oblivion,” Zhang noted, highlighting the effectiveness, if somewhat crude, of the program. The long-term commitment to this strategy demonstrated the importance of sustained efforts in pest control and disease eradication. However, the recent resurgence of screwworms indicates that vigilance and adaptability are essential to maintain these gains.
Breach of the Barrier: The Screwworm’s Alarming Resurgence
In 2022, the seemingly impenetrable barrier at the Darién Gap was breached. Cases of screwworm infestation in Panama, primarily in cattle, surged from dozens annually to 1,000, despite the ongoing release of sterile flies. This sudden spike signaled a breakdown in the containment strategy. The parasite began its northward journey, initially at a slow pace, but accelerating rapidly by 2024. Alarmed experts started sending warnings as the screwworm advanced through Central America.
By May 2025, the screwworm had traveled 1,600 miles through eight countries, reaching Oaxaca and Veracruz in Mexico, just 700 miles from the Texas border. This rapid advance prompted the U.S. to suspend live-cattle imports from Mexico, a drastic measure to protect the U.S. cattle industry. The situation has raised concerns among ranchers and officials alike, highlighting the vulnerability of the existing control measures.
Wayne Cockrell, a Texas rancher and chair of the cattle-health committee for the Texas and Southwestern Cattle Raisers Association, expressed his dread over the screwworm’s impending return to Texas. He emphasized that the current sterile-fly production capacity is insufficient to halt the parasite’s advance or push it back to Panama. His concerns reflect a growing sense of urgency within the cattle industry, which faces the prospect of adapting to a new reality after decades of screwworm-free existence.
Cockrell, who is too young to remember the pre-eradication era, has heard the horror stories of how every cut, scratch, and newborn calf’s navel could become a fatal wound. The potential re-establishment of the screwworm in the U.S. could undo decades of progress and require another lengthy and costly eradication campaign, potentially lasting decades.
The Funnel Effect: Why Central America Is Key
Central America’s unique geography plays a crucial role in the screwworm’s spread. The region is shaped like a funnel, with its narrowest point in Panama. This is why the initial eradication efforts focused on establishing a barrier there. Maintaining a 100-mile barrier in Panama is economically more feasible than securing the 2,000-mile U.S.-Mexico border. However, with the screwworm now beyond a narrow isthmus in southern Mexico, the challenge has grown exponentially.
The wider the front of the screwworm war becomes, the more sterile flies are needed to contain it. The fly factory in Panama has ramped up production to its maximum of 100 million flies per week, all of which are being dispersed over Mexico. However, during the first eradication campaign in the 1980s, planes dropped 150 million flies weekly over the isthmus in Mexico. When the front was even farther north, a factory in Mexico produced as many as 550 million flies weekly. These factories have long been shut down.
The Texas and Southwestern Cattle Raisers Association is urging the USDA to build a new sterile-fly plant in the U.S. capable of producing hundreds of millions of flies. While the USDA is working closely with Mexico to reestablish a biological barrier, the process of building a new factory could take years. Texas lawmakers have introduced the STOP Screwworms Act to expedite the process, but time is of the essence.
“The facility needs to start tomorrow,” Cockrell emphasized, highlighting the urgency of the situation. The ability to produce and deploy sufficient numbers of sterile flies is critical to preventing the screwworm from gaining a foothold in the U.S. and reversing decades of progress.
An Unprepared Industry: Challenges and Adaptations
The U.S. cattle industry is largely unprepared for the screwworm’s return. Certain drugs used to treat screwworm infections are not licensed in the U.S., as they have been unnecessary for half a century. Additionally, ranches now employ fewer cowboys, making regular cattle inspections more challenging. Routine practices like branding and ear tagging also leave animals vulnerable to infection.
To face the screwworm, the cattle industry will need to adapt quickly. This includes adopting new preventative measures, such as applying insecticides to wounds and monitoring cattle more frequently. The industry may also need to re-evaluate its practices to minimize the risk of infection. The economic impact of the screwworm could be significant, potentially driving up beef prices already elevated due to drought.
Cockrell noted that the industry has become complacent due to decades of screwworm-free existence. This complacency has led to a decline in vigilance and preparedness. Re-educating ranchers and implementing updated control measures will be essential to mitigating the impact of a potential outbreak. The industry must also work closely with government agencies and researchers to develop and implement effective strategies.
The potential for increased beef prices due to screwworm infestations could further strain consumers already facing economic challenges. Addressing the screwworm threat is not only vital for the cattle industry but also for ensuring affordable food prices. The collective efforts of ranchers, government, and researchers are crucial to prevent a full-blown crisis.
Factors Behind the Resurgence: A Perfect Storm
The reasons for the screwworm’s resurgence are multifaceted. The coronavirus pandemic disrupted supply chains at the fly factory in Panama and hampered regular cattle inspections. The Darién Gap, once an impenetrable jungle, became a popular route for migrants, potentially facilitating the parasite’s spread. These factors created a perfect storm that allowed the screwworm to breach the established barrier.
The illegal cattle trade in Central America has also played a significant role. Jeremy Radachowsky, director for Mesoamerican and the Western Caribbean at the Wildlife Conservation Society, explained that approximately 800,000 cattle are illegally raised in nature reserves and smuggled to Mexico each year. This illegal trade has allowed the screwworm to spread much faster than it could naturally.
The line of new screwworm cases has followed known smuggling routes, indicating that infected cattle are being transported across borders. This constant northward movement makes re-eradication more difficult. It’s akin to trying to empty a pool while the spigot remains open, according to Radachowsky. Stopping the illegal cattle trade is crucial to containing the screwworm’s spread.
Decades of screwworm-free existence have also led to complacency among ranchers and officials. Inspections, timely reports of infection, and restrictions on cattle movement are vital for eradication, in addition to sterile fly releases. A U.S. official familiar with the situation noted that people had simply forgotten about the screwworm’s threat. Re-emphasizing the importance of these measures is essential to combatting the resurgence.
Advanced Solutions: Genetics and Gene Drives
Scientists have proposed more advanced methods for controlling the screwworm through genetics. Max Scott, an entomologist at North Carolina State University, conducted USDA-supported research to create a male-only strain that could reduce the number of flies needed for dispersal. However, funding for this research ended last summer. Scott has also suggested using gene drives, a controversial technique that could rapidly spread genetic material that makes females sterile into the wild population.
The USDA has shown limited interest in gene drive technology. However, Scott has partnered with scientists in Uruguay to study a gene drive for sterile screwworms. Uruguay is interested because it has never benefited from screwworm eradication, being located in South America, where the parasite remains prevalent. The potential of gene drives to revolutionize pest control is significant, but further research and careful consideration of ethical implications are needed.
Steven Skoda, a retired USDA scientist, expressed his dream of a world entirely free of screwworms. While eradication never reached South America, the current breach in North America’s barrier is a setback. The campaign to push screwworms from southern Mexico to the southern edge of Panama took 21 years. The current situation threatens to undo much of this progress.
The future of screwworm control may depend on integrating new technologies with traditional methods. Continued research, vigilance, and adaptability are essential to protect livestock and livelihoods from this persistent threat. The lessons learned from past successes and failures will guide the path forward in this ongoing battle.
Conclusion: A Call to Action
The resurgence of the New World screwworm poses a significant threat to the U.S. cattle industry and potentially public health. After decades of successful eradication efforts, the breach of the Panama barrier signals a critical turning point. The factors contributing to this resurgence include disruptions from the coronavirus pandemic, illegal cattle trade, and a decline in vigilance. Immediate action is needed to prevent the screwworm from gaining a foothold in the United States.
Key takeaways from this article include the importance of sustained sterile fly production, the need for enhanced cattle inspections and movement restrictions, and the potential of advanced genetic technologies for screwworm control. The U.S. cattle industry must adapt quickly to the new reality, implementing preventative measures and working closely with government agencies and researchers. The economic implications of a widespread screwworm outbreak could be severe, potentially driving up beef prices and straining consumers.
The STOP Screwworms Act represents a crucial step forward, directing the USDA to open a new sterile-fly factory. However, time is of the essence, and the facility must be established as quickly as possible. Collaboration between ranchers, scientists, and officials is essential to develop and implement effective strategies. The campaign to eradicate screwworms is far from over, and continued vigilance and innovation are needed to protect livestock and livelihoods.
Ultimately, the screwworm’s resurgence serves as a reminder of the ongoing battle against pests and diseases. Sustained commitment, adaptability, and investment in research and control measures are essential to safeguarding agricultural industries and public health. The lessons learned from the screwworm saga can inform future efforts to combat other threats and ensure a secure and prosperous future.
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