The New World Screwworm: A Flesh-Eating Fly Returns

If you’ve never heard of the New World screwworm, consider yourself lucky. This isn’t your typical backyard pest—it’s a flesh-eating fly that once terrorized livestock across the American South and is now making an alarming comeback just south of our border.

The screwworm (Cochliomyia hominivorax) has a horrifying life cycle. Female flies seek out open wounds on warm-blooded animals—cattle, horses, and yes, occasionally humans—and lay their eggs there. When the eggs hatch, the maggots don’t just feed on dead tissue like most flies. Instead, they tunnel into living flesh, burrowing deeper as they grow. This process, called myiasis, can kill a full-grown cow in just one to two weeks.

America’s First Victory

Back in the 1930s through 1960s, the screwworm was a nightmare for ranchers across the South. Cattle losses mounted into the billions, and the threat to both livestock and human health became unbearable. But American scientists came up with an ingenious solution: the Sterile Insect Technique.

The concept was brilliantly simple. Scientists would mass-produce male screwworm flies, sterilize them with radiation, then release millions of them into the wild. Since female screwworms only mate once in their lifetime, if they mated with a sterile male, their eggs would never hatch. Do this on a large enough scale, and you could crash entire wild populations.

It worked. By 1966, the screwworm was declared eradicated from the continental United States. By 1982, the southeastern states were permanently screwworm-free. The success was so complete that the program expanded throughout much of Latin America, with a “barrier” established in Panama to prevent northward migration.

But the screwworm never disappeared entirely. It remained endemic in South America and parts of the Caribbean and Central America, occasionally breaking through our defenses. In 2016, an outbreak in the Florida Keys required a swift sterile fly response to eliminate it by early 2017.

The Current Crisis

Since November 2024, everything has changed. Wild screwworm populations have been pushing north through Central America and Mexico at an alarming pace, breaking past containment lines that have held for decades.

Between November 2024 and May 2025, Mexican authorities detected over 1,400 cases across multiple states. Some of these cases were found just 370 miles from the U.S. border in Veracruz—far closer than anyone wants to see them.

The response has been swift and dramatic. On May 11, 2025, the U.S. suspended all livestock imports from Mexico. Livestock ports that were scheduled to reopen remained closed, and monitoring along the southern border was intensified. These measures have remained in place through July as outbreaks continue near the border.

Why This Threatens Everything

The screwworm’s return poses a massive threat for several reasons. First, its life cycle is devastatingly efficient. A single female can lay 300 to 500 eggs near a host’s wound. The larvae hatch within a day and spend about a week burrowing deeper into living tissue. Without quick treatment, they can kill a fully grown cow in just weeks.

Second, while the larvae need warm weather to survive (they can’t make it through temperatures below 46°F), much of the American South stays hot enough during summer months for them to complete their full life cycle. Climate change is only making this worse, potentially expanding their viable range further north.

The economic stakes are staggering. The cattle and beef industries represent a massive component of the U.S. economy, and experts estimate losses could exceed $100 billion if the screwworm becomes established again. Even smaller outbreaks require mass inventories, quarantines, and treatments—all expensive and disruptive.

There are also serious health concerns. While rare, humans and pets can become hosts. Wildlife like deer, feral hogs, and birds can serve as reservoirs, making eradication much more complicated than just protecting livestock.

Fighting Back

The U.S. response has been multifaceted and expensive. The livestock import suspension from Mexico continues as outbreaks persist near Veracruz and Oaxaca, uncomfortably close to Texas.

More importantly, the USDA has begun ramping up sterile fly operations reminiscent of the mid-century success story. They’re building an $8.5 million sterile fly production and release facility at Moore Air Base near Edinburg, Texas, aiming for operational status by late 2025. They’re also spending $21 million to upgrade a sterile fly factory in Metapa, Mexico, boosting capacity by 60 to 100 million males per week.

The Panama facility continues producing about 100 million sterile flies weekly, and there are plans for a permanent U.S. sterile fly factory to support long-term defenses. The goal is to flood border regions with sterile males, preventing new generations from forming.

Surveillance has been dramatically enhanced across Arizona, New Mexico, Texas, and even Upper Midwest states like Minnesota. Officials emphasize vigilance even in cooler zones, recognizing that warmer summers and livestock movement could allow the screwworm to establish itself far from the border.

The Human Factor

What makes this situation particularly challenging is the human element. The screwworm doesn’t respect borders, and effective control requires cooperation between the U.S., Mexico, and Central American partners. Critics argue the U.S. import suspension penalizes ranchers, while Mexican President Claudia Sheinbaum insists Mexico is fully compliant with containment protocols.

There’s also the risk of complacency. The 2016 Florida outbreak required swift cross-agency response and cost millions to contain. Even a short local outbreak can spiral quickly without proper coordination.

What’s at Stake

Several factors are making this threat more serious than previous incursions. Global warming is expanding habitable zones farther north each year. Cross-border trade and wildlife movement heighten transmission risk. Food security concerns are intensifying amid ongoing supply chain stresses.

The Sterile Insect Technique remains our best defense, but only if deployed swiftly and at scale. The science is proven—it saved America once and can do so again. But it requires sustained investment, international cooperation, and rapid response capabilities.

For Texas alone, losses could approach $1.9 billion. Nationwide, the figure could soar past $100 billion. These aren’t just numbers—they represent real impacts on food prices, rural communities, and America’s agricultural security.

What You Can Do

Ranchers and pet owners need to stay alert for warning signs: animals shaking their heads, appearing distressed, foul odors, or visible maggots in wounds. Any suspected cases should be reported immediately to state animal health officials or USDA APHIS.

Wounds should be treated promptly with anti-parasitic treatments if maggots are found. Anyone with imported livestock should monitor for screwworm symptoms and follow quarantine regulations.

Can We Win Again?

The answer is yes, but only if we mount the right defense quickly. Success depends on several key elements: effective border SIT operations from Texas through Mexico, permanent sterile fly capacity in both nations, ongoing surveillance with rapid outbreak response teams, and sustained cross-border cooperation.

The screwworm was beaten before through science, determination, and international cooperation. With the right response, it can be beaten again. But the window for action is narrowing, and the stakes have never been higher.

The next few months will be critical. Either we contain this threat now, or we face the prospect of fighting a much larger battle on American soil. The choice is ours, but time is running out.

Pest Free Living