A flesh-eating parasite that the United States spent decades eradicating, and even longer trying to keep at bay, has now shown up in Texas.
Federal officials confirmed this week that New World screwworm, a fly whose larvae burrow into living tissue, had been found in a 3-week-old calf in Zavala County in Southwest Texas. It is the state’s first confirmed detection since the early 1980s, and the first in US livestock in several decades. This infestation marks a new stage in the parasite’s northward resurgence through Central America and Mexico that began in 2023.
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. But the unwelcome infestation in Texas could be a serious test for ranchers and animal agriculture in the US. Beef prices are already near record high, and if screwworm spreads beyond this single detection, it could push prices and ripple through the economy.
Texas officials are now trying to answer the most urgent question: Was this a single stray case, or a sign that adult screwworm flies are already in the area?
A spokesperson for the Texas Animal Health Commission (TAHC) told Vox that 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, such as cattle, horses, and pets, cannot be moved out of this zone unless they are inspected.
Texas has been watching for this moment. The TAHC told Vox it has had fly traps along the Texas-Mexico border since July 2025, which has since collected over 54,000 suspicious flies. None of them were confirmed to be New World screwworm.
But the detection of this case in Zavala County has moved the state from precautionary work to containment. The TAHC told Vox that sterile flies are being deployed through ground release chambers where the infestation was detected, and aerial dispersal was expected to follow. The idea is to flood the area with sterilized flies, so wild screwworms mate without producing offspring — it’s the same strategy that the United States used to eradicate the parasite decades ago. Texas had already been doing these precautionary aerial sterile-fly drops over South Texas since late January, but after this case, officials said those releases were now being redirected toward the 20-kilometer response zone around the detection site.
What went wrong
While the containment efforts are on, it’s still unclear how the calf got infected. The TAHC told Vox that 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 the calf had no movement history, 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, also said that if the livestock itself was not moved up from Mexico, “then the fly had to be here.”
That doesn’t mean that screwworms are established in Texas. Scott said it is possible the case came from a single female fly, and US Department of Agriculture says there have been no further detections so far. But it does mean that the US is no longer preparing for a hypothetical threat.
In Mexico, screwworms-related export restrictions have cost cattle exporters more than $1.3 billion, according to the country’s National Agricultural Council. And in Texas alone, 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 US has a history of eradicating screwworms before, and for years it kept the parasite at a distance 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 vastly larger than it was when the US first eradicated the parasite. And the sterile-fly supply is limited. Scott, the NC State entomologist, said that 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 that ceiling, including by renovating a facility in Metapa, Mexico, and building new production capacity in Texas. Newer genetic engineered strains, including a male-only fly known as Novofly, could also make existing plants more efficient by producing only the sex that actually suppress the wild population. But those tools still need regulatory approval and field testing before they could be deployed.
The response is unfolding after a bruising year for the agencies and programs that manage animal disease. More than 15,000 USDA employees accepted the Trump administration’s incentives to leave the department, while the Animal and Plant Health Inspection Service, the USDA agency responsible for animal and plant health, lost more than 1,300 staff that included veterinarians and animal health personnel. The “Department of Government Efficiency,” which is 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, an agriculture trade publication, reported that the terminated work included screwworm monitoring in Central America.
It is unclear whether those cuts have affected the current response in Texas or the US’s broader ability to track northward movement of screwworm, but it sharpens the question: whether the US has enough surveillance, staffing, and sterile-fly capacity to meet a fast-moving animal health threat.
What exactly is a New World screwworm?
The New World screwworm is a parasitic fly found today across parts of South America and the Caribbean, Central America, and Mexico. They have shiny blue-gray bodies and look similar to house flies that swarm your local dumpster. But unlike those ordinary flies, screwworm flies love fresh wounds.
Female screwworm flies are attracted to warm-blooded animals, and lay their eggs in open cuts or natural openings like ears or nostrils. Each female can lay up to 200 eggs at a time, which hatch some 12 to 24 hours later. Upon hatching, the larvae twist into flesh like corkscrews tearing deeper as they feed, causing extreme pain and tissue damage. Their scientific name, Cochliomyia hominivorax, translates roughly to man-eater, and their common name, screwworm, capture their horror: a spiral larva that feeds on living flesh.
Missed cases can allow the flies to reproduce and spread, making an outbreak much harder to contain.
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. But missed cases can allow the flies to reproduce and spread, making an outbreak much harder to contain.
What makes screwworms particularly brutal is they only consume living flesh. A single infested wound can attract more flies, leading to repeated infestations in the same animal. Infestations in humans are excruciating and disfiguring, but rarely fatal with treatment. In animals, untreated cases can be devastating, causing severe wounds, blood loss, secondary infections and sometimes death.
But there’s an Achilles’ heel: Female screwworms mate only once in their life — a unique biological quirk that has underpinned the US’s control strategy for decades.
How the US beat screwworms
Screwworms once terrorized the American South and the Western US, and killed 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.
But starting in the 1950s, USDA scientists found a way to use the fly’s biology against itself. If they could find a way to get the female flies to mate with sterile mates, they could stop the flies’ population in its tracks. And that’s how the sterile insect technique (SIT) was developed.
The SIT is fairly straightforward: Rear huge numbers of screwworms in a lab and sterilize the pupae through radiation (a discovery from the post-war atomic age when scientists realized they could make flies infertile without killing them). Then these freshly sterilized pupae are packed onto twin-engine planes, timed so the flies hatch in the air. These flies are then sprayed out over the forest and ranchlands by the millions. They wake in warm air and do what flies do: They mate. Those pairings then produce nothing. If you do that at a sufficient scale and for a long enough time, the population will eventually collapse.
The first eradication program in the American Southeast ran through the 1950s followed by a larger push across Southwest, costing roughly $42 million in total. Ranching groups pushed the USDA for eradication, Texas cattlemen even wrote letters to USDA urging the agency to expand SIT. And unlike today’s debates around genetically modified mosquitoes, screwworms never stirred much controversy. The technique was targeted, pesticide-free, and spared other insects, which is why it was an unusually “green” pest control, said Max Scott, a professor of entomology at NC State University. By 1966, the fly was gone.
The technique then was adopted in Mexico and parts of Central America, pushing the flies all the way to a narrow band of dense rainforests between Panama and Colombia called the Darién Gap. The Pan-American Highway famously stops there, the region is sparsely populated, treacherous to cross, and light on livestock. It’s exactly the kind of chokepoint where a biological “firewall” can hold.
Since 1998, a US-Panama program called Commission for the Eradication and Prevention of Screwworm (COPEG) has held the line at the Darién Gap. Planes drop off millions of sterile flies each week, and inspectors patrol the frontier town (not the deep Darién itself) to spot infestations, pluck out maggots manually, and treat wounds with insecticides — because SIT only works if you also knock down active infestations.
The program costs about $15 million annually and is funded mostly by USDA, with Panama contributing a small share. “It was one of the greatest achievements of the USDA in the 20th century,” Scott said.
But, in 2023, the firewall cracked.
Smuggling of cattle through Central America seeded fresh outbreaks in new regions, and climate shifts — higher temperatures and humidity — aided their spread. By spring 2025, Mexico was reporting detections as far north as Oaxaca and Veracruz, a stretch of land far wider and difficult to contain than the narrow Darién. COPEG has been running flat out, turning out around 100 million larvae each week. But even at maximum capacity, the plant can only do so much. The screwworm front continued to advance, and has now reached continental US.
What happens next
The response underway in Texas — animal movement restrictions, fly and animal surveillance, sterile-fly releases — is the standard screwworm playbook, and it may be enough if the Zavala County case remains to just one calf.
The harder question is what happens if more cases appear.
For now, the United States is relying on the basic strategies that worked decades ago, while racing to rebuild the capacity that made it work.
SIT only works when sterile males vastly outnumber fertile wild males. Scott said earlier eradication programs often aimed for a 9-to-1 or 10-to-1 ratio of sterile to fertile males, because lab-reared flies that have been sterilized 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, and males are the ones that suppress reproduction.
That could become the bottleneck if the response has to expand. During the eradication campaign in Mexico, Scott said, officials had access to a plant producing roughly 500 million flies a week. That kind of capacity may not be necessary if Texas stamps out this case quickly. But Texas is vast, and the larger resurgence in Mexico and Central America has not gone away.
There’s also a trade-off. The sterile flies that are now being released in Texas are redirected from the Panama plant, and those flies would have otherwise been used in northern Mexico. That may be necessary to constrain the Zavala Country case. But the more flies officials have to pull north, the fewer flies they have to push back the broader front of screwworm moving through Mexico.
That capacity is coming but not immediately. USDA is renovating a facility in Metapa, Mexico, that is 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.
And new genetic tools could eventually help too. Scott’s lab helped develop a male-only screwworm strain, called NovoFly, that could make sterile-fly production much more efficient. Instead of producing male and female flies, a plant using this strain could produce only the males needed for population control, effectively doubling the useful output of existing facilities.
But Novofly isn’t here yet. Scott said his lab developed the strain around 2018, and that it has spent years in storage because there was no urgent plan to use it. Now it is moving through EPA review, but it would need US approval, as well as approval from Panamanian regulators and field testing, before they could be deployed in the real world.
For now, the United States is relying on the basic strategies that worked decades ago, while racing to rebuild the capacity that made it work. The new few weeks will determine whether Texas is dealing with a contained incursion or something more serious.
Update, June 5, 1:25 pm ET: This story was originally published on September 7, 2025, and has been updated with the latest information about screwworm in Texas.