Sterile-fly fight begins after Texas screwworm find

Texas sterile-fly – Federal officials confirmed a New World screwworm fly was found in a 3-week-old calf in Zavala County, Texas— the state’s first confirmed detection since the early 1980s and the first in U.S. livestock in several decades. Texas moved quickly to contain the ris

The first case didn’t look like a headline. It started with a 3-week-old calf in Zavala County, in southwest Texas—until federal officials confirmed it carried New World screwworm.

Texas is now treating the discovery like a test of how fast the country can respond when a parasite that was once driven out begins moving again. This is the state’s first confirmed detection since the early 1980s, and the first in U.S. livestock in several decades. The infestation also marks a new stage in a northward resurgence through Central America and Mexico that began in 2023.

For ranchers and animal agriculture, the fear isn’t just the harm to one animal. Screwworm larvae burrow into living tissue. causing extreme pain and tissue damage. and missed cases can let the flies reproduce and spread. turning a single incursion into something much harder to contain. Beef prices are already near record high. and if screwworm spreads beyond the single confirmed detection. it could push prices higher and ripple through the broader economy.

Texas officials are trying to answer the most urgent question: was this a single stray case, or evidence that adult screwworm flies are already in the area?

A spokesperson for the Texas Animal Health Commission (TAHC) said officials had not confirmed any additional cases and were conducting ranch-to-ranch animal surveillance and fly surveillance around the infested zone. That zone covers about 12 miles around the detection site. Warm-blooded animals—including cattle, horses, and pets—cannot be moved out of this zone unless they are inspected.

Texas has been watching for signs of this moment for months. The TAHC told Vox it has had fly traps along the Texas-Mexico border since July 2025, collecting over 54,000 suspicious flies. None of them were confirmed to be New World screwworm.

After this Zavala County detection, Texas shifted from precaution to containment.

The TAHC said sterile flies are being deployed through ground release chambers where the infestation was detected. and aerial dispersal was expected to follow. The plan is to flood the area with sterilized flies so wild screwworms mate without producing offspring—a strategy the United States used to eradicate the parasite decades ago. Texas had already been doing precautionary aerial sterile-fly drops over South Texas since late January. but after this case. officials said those releases were redirected toward a 20-kilometer response zone around the detection site.

Even with containment efforts underway, the origin of the infection remains unclear. The TAHC told Vox it was not aware of any recent animal movement off the ranch where the calf was found, or any known link to Mexico or another affected area.

If there was no movement history, the biology points back to the presence of adult flies. Phillip Kaufman, an entomologist at Texas A&M University who has worked with state officials on screwworm response planning, said, “there certainly have to be adult flies in the area,” that laid eggs on it.

Maxwell Scott. an entomologist at North Carolina State University who studies screwworm control. said that if the livestock itself was not moved up from Mexico. “then the fly had to be here.” Scott also stressed that the case does not automatically mean screwworms are established in Texas. It is possible, he said, that the case came from a single female fly. Scott added that the U.S. Department of Agriculture says there have been no further detections so far.

But the practical shift is already visible: the U.S. is no longer preparing for a hypothetical threat.

Economically, the stakes extend beyond the immediate response. In Mexico, screwworm-related export restrictions have cost cattle exporters more than $1.3 billion, according to Mexico’s National Agricultural Council. In Texas. a widespread outbreak could drain as much as $1.8 billion a year from ranchers and the wider economy. according to a USDA estimate.

The situation is tied to what happened along the broader northern front. The U.S. has a history of eradicating screwworms and keeping them at bay through an “invisible sterile-fly barrier” near the Panama-Colombia border. But that barrier has cracked, and screwworm is now spread across a much wider front in Mexico and Central America.

Livestock production is also larger now than it was when the U.S. first eradicated the parasite. The sterile-fly supply is limited. Scott said the only current production plant in Panama is running at full capacity—24/7. 365 days a year—and producing about 100 million flies a week. only half of which are males. the sex that actually suppresses the population.

The USDA is moving to raise capacity. including by renovating a facility in Metapa. Mexico. and building new production capacity in Texas. Scott also described newer genetic engineered strains that could improve efficiency, including a male-only fly known as Novofly. Those tools, he said, still need regulatory approval and field testing before they can be deployed.

The response unfolding in Texas arrives after a bruising year for the agencies tasked with animal disease surveillance and control. More than 15,000 USDA employees accepted the Trump administration’s incentives to leave the department. The Animal and Plant Health Inspection Service. the USDA agency responsible for animal and plant health. lost more than 1. 300 staff—including veterinarians and animal health personnel. The “Department of Government Efficiency. ” officially scheduled to sunset next month. also listed an $84 million cut last year to a USAID grant that supported animal-disease surveillance and outbreak response. Agri-Pulse reported that terminated work included screwworm monitoring in Central America.

Officials haven’t said yet whether those staffing and funding shifts have affected the current response in Texas or the broader ability to track northward movement. Still, they tighten the question many ranchers will be asking now: does the U.S. have enough surveillance, staffing, and sterile-fly capacity to handle a fast-moving animal health threat?.

What exactly is a New World screwworm?

It is a parasitic fly found today across parts of South America and the Caribbean, Central America, and Mexico. The flies have shiny blue-gray bodies and look similar to house flies that swarm around dumpsters. But unlike ordinary flies, screwworm flies are drawn to fresh wounds.

Female screwworm flies are attracted to warm-blooded animals and lay eggs in open cuts or natural openings like ears or nostrils. Each female can lay up to 200 eggs at a time, and the eggs hatch some 12 to 24 hours later. The larvae then twist into flesh like corkscrews as they feed, causing extreme pain and tissue damage.

Their scientific name. Cochliomyia hominivorax. translates roughly to “man-eater. ” and the common name “screwworm” reflects the spiral larva feeding on living flesh. After feeding for up to a week. the larvae wriggle back out of the wound and drop to the ground. where they pupate in the soil before emerging as adult screwworm flies—ready to repeat the cycle.

Most infestations—including livestock cases like the one in Zavala County—are treatable when caught early. Missed cases can allow the flies to reproduce and spread, making an outbreak much harder to contain.

In humans, infestations are excruciating and disfiguring but rarely fatal with treatment. Human infestations from these flies are rare in the United States, and the Centers for Disease Control and Prevention says there have been no locally acquired human cases reported in the country.

Animals can pay the highest price. Untreated cases can be devastating, causing severe wounds, blood loss, secondary infections, and sometimes death. A single infested wound can also attract more flies, leading to repeated infestations in the same animal.

There is one biological quirk that has made control possible for decades. Female screwworms mate only once in their life. That characteristic underpins the U.S. strategy.

How the U.S. beat screwworms before

Screwworms once terrorized the American South and the Western U.S., killing millions of dollars’ worth of cattle each year. By the mid-20th century, the fly was costing America’s ranchers up to $100 million annually.

Starting in the 1950s, USDA scientists developed a way to use the fly’s biology against itself. The sterile insect technique—SIT—works by getting female flies to mate with sterile mates so the population stops reproducing.

The SIT process is straightforward in concept: huge numbers of screwworms are reared in a lab. sterilized through radiation. and packed into twin-engine planes. These sterilized pupae are timed so the flies hatch in the air. Officials then spray the flies over forest and ranchlands by the millions. They mate, and the matings produce nothing.

In the American Southeast, an eradication program ran through the 1950s and was followed by a larger push across Southwest, costing roughly $42 million in total. Ranching groups pushed the USDA for eradication, and Texas cattlemen wrote letters urging the agency to expand SIT.

By 1966, the fly was gone.

The technique was then adopted in Mexico and parts of Central America. pushing screwworms into a narrow band of dense rainforests between Panama and Colombia known as the Darién Gap. Since 1998. a U.S.-Panama program called the Commission for the Eradication and Prevention of Screwworm (COPEG) has held the line there. Planes drop off millions of sterile flies each week. and inspectors patrol to spot infestations. remove maggots manually. and treat wounds with insecticides—because SIT only works if active infestations are also knocked down.

COPEG costs about $15 million annually, funded mostly by USDA with Panama contributing a small share. Scott said, “It was one of the greatest achievements of the USDA in the 20th century.”

By 2023, that firewall cracked. Smuggling of cattle through Central America seeded fresh outbreaks in new regions. and climate shifts—higher temperatures and humidity—helped the spread. By spring 2025. Mexico reported detections as far north as Oaxaca and Veracruz. a stretch far wider and harder to contain than the narrow Darién. COPEG has been producing around 100 million larvae each week. but even at maximum capacity. Scott said the front continued to advance and has now reached continental U.S.

The response in Texas follows the standard screwworm playbook: animal movement restrictions, fly and animal surveillance, and sterile-fly releases. If the Zavala County case remains limited, officials may be able to stop the cycle quickly.

The harder question is what happens if more cases appear.

SIT only works when sterile males vastly outnumber fertile wild males. Scott said earlier eradication programs aimed for a 9-to-1 or 10-to-1 ratio of sterile to fertile males because lab-reared sterilized flies are not perfect competitors in the wild.

Right now. the main production plant in Panama is producing about 100 million sterile flies a week. but only about half are males. Males are the ones that suppress reproduction. Scott pointed to the Mexican eradication campaign, when officials had access to a plant producing roughly 500 million flies a week.

More flies could help—yet the response requires trade-offs. Scott said the sterile flies being released in Texas are redirected from the Panama plant and would otherwise have been used in northern Mexico. Pulling more flies north means fewer flies left to push back the broader front moving through Mexico.

USDA is trying to expand capacity. It is renovating a facility in Metapa. Mexico. expected to add tens of millions of sterile flies per week. and it is building new production capacity in Texas. The Food and Drug Administration has also issued emergency authorizations for some animal treatments. bringing more tools to prevent and treat infestations while containment is underway.

Genetic options could eventually add another layer. Scott said his lab helped develop a male-only screwworm strain called NovoFly. With this strain, a plant could produce only the males needed for population control, effectively doubling the useful output of existing facilities.

But NovoFly isn’t ready. Scott said his lab developed the strain around 2018 and it has spent years in storage because there was no urgent plan to use it. He said it is now moving through EPA review, but it would need U.S. approval as well as approval from Panamanian regulators and field testing before deployment.

So for now, Texas is relying on the basic strategy that once worked, while racing to rebuild the capacity that made it effective.

The coming weeks will determine whether Texas is dealing with a contained incursion or something more serious.

Update: The story was originally published on September 7, 2025, and has been updated with the latest information about screwworm in Texas as of June 5 at 1:25 pm ET.

New World screwworm Texas Animal Health Commission Zavala County sterile insect technique COPEG USDA Metapa Novofly cattle disease surveillance