There is a particular cruelty to the way the New World screwworm works. Most parasites, most decay, most of the things we associate with biological horror operate on the dead, on flesh that has already surrendered. The screwworm is different. It chose, somewhere in its evolution, to target only the living. A warm-blooded animal. An open wound. A newborn calf's umbilical cord, still moist from birth.
The adult fly finds that wound and deposits its eggs, up to 400 at a time, inside the raw tissue. Within hours, they hatch. The larvae, creamy white and comma-shaped, begin to eat. Not dead flesh. Not decaying tissue. Living muscle. Living blood. They burrow inward, deeper with each hour, their spiral motion giving the creature its name, the screwworm, drilling itself into something still breathing.
As the wound grows, it attracts more flies. More eggs. More larvae. The animal, in agony, cannot stop it. Within days, if untreated, the infestation kills. And it has been doing this, somewhere in the Americas, since long before humans were around to name it.
What We Did About It, Once
For most of American agricultural history, the screwworm was simply part of ranching in the South and Southwest. Before eradication, it caused hundreds of millions ofdollars inn losses every year. Ranchers spent enormous time and money on treatment and prevention. Cattle, horses, dogs, wildlife, and occasionally people- the fly's scientific name, Cochliomyia hominivorax, is Latin for "man-eater," coined after a French naval surgeon encountered it on a prison island in French Guiana where it was eating prisoners alive- all fell victim to it through the warm months.
Then, in the 1950s, American entomologist Edward Knipling proposed something never tried at scale: instead of killing the flies, sterilise them. Release billions of sterile males into the wild. When they mate with wild females, and the female screwworm only mates once in its life, the eggs produced are infertile. The population collapses. Do it long enough, and the species vanishes from the region entirely.
It worked. The United States was declared free of screwworm in 1966. A sterile fly barrier was established at the Panama-Colombia border, 100 million sterile flies released per week into a narrow corridor of jungle, holding the screwwormsouthof itfor decades. It was one of the most successful pest eradication programmes in human history. That was then.
Where It Is Now
In 2023, the screwworm broke through Panama. In 2024, it crossed into Mexico. By November 2024, the outbreak was confirmed. By mid-2025, more than 20,000caseshad been identified in Mexico alone, advancing steadily northward. In July 2025, the USDA closed all southern border ports of entry to cattle, bison, and horses from Mexico, a trade closure that sent ripples through an American beef industry already strained by historic drought and high costs.
On June 3, 2026, it crossed anyway. A three-week-old calf in Zavala County, Texas. Larvae identified in its umbilical area. The first confirmed case of New World screwworm in the United States in sixty years. A second case followed in La Salle County. The surveillance maps began updating. The calls to state veterinarians began.
What a Rancher in Chiapas Already Knew
While government agencies were building response playbooks, Mexican farmers had been living with this for months. In September 2025, a rancher named Fidel Gutiérrezin Chiapas described what it meant on the ground. "They get the worms within two or three days of birth, and that complicates things because we have to come and keep treating them." He had lost one cow the summer before, a cost of over $1,000forhissmall farm.
For a small farmer, one cow is not an abstraction. One cow is rent, or school fees, or the difference between a year that works and a year that doesn't. The screwwormdoesn'tcare about any of that. It finds the wound, and it begins.
The human cases are rarer but not zero. In August 2025, a person who returned to the United States after travelling to El Salvador was confirmed with an infestation. The fly does not discriminate by species. It was named after man-eating because it will, given the chance, eat man.
What $100 Billion Looks Like in Danger
The USDA describes the economic stakes plainly: New World screwworm threatens over $ 100 billion in American economic activity tied to the cattle and livestock industry alone. US cattle herd sizes are already near historic lows. The average American consumes60pounds of beef per year. A prolonged disruption to the cattle supply chain is not an agricultural issue that stays in agriculture.
Mexico estimates its own losses from re-emergence at over $470 million in agricultural income. Research published in Scientific Reports projects that if the screwwormre-establishes itself across North America, economic damage would dwarf the cost of eradication many times over. None of these numbers conveys what it actually looks like for an animal to die from this. But they convey what it costs when the invisible infrastructure keeping it away from us breaks down.
What We Are Now Building
The USDA, working with the Army Corps of Engineers, broke ground in April 2026onasterile fly production facility at Moore Air Force Base in Edinburg, Texas, designed to produce up to 300 million sterile flies per week when fully operational. The Panama facility is already releasing 100 million sterile flies per week. A facility in Mexico is under renovation to produce tens of millions more.
Whether it will work as quickly as it once did is the question nobody can fully answer. The fly has spent years establishing itself through Central America, spreading partly through migration routes and partly through the movement of dogs and companion animals that cross borders with far less oversight than livestock. Some experts working in Latin America are now candid: the screwworm may not be fully eradicable from South America. It is native there. The barrier, not eradication, may be the permanent goal.
The Sixty-Year Lesson
For sixty years, a barrier of sterile flies maintained by an international coalition at enormous ongoing cost held a flesh-eating parasite away from North America. The ranchers who benefited mostly never knew the barrier existed. The California-Texas border in any summer from 1966 to 2025 was safe from screwworm not because of luck or nature but because of a deliberate, expensive, scientifically complex intervention that ran continuously in the background.
The moment the barrier weakens, the moment the sterile fly programme loses ground to an outbreak coming north through the jungle, the parasite is back. Moving.Finding wounds. Depositing eggs. Within hours, hatching. The three-week-old calf in Zavala County, Texas, its umbilical cord still raw, didn't know any of this. It only knew the pain of something it couldn't see beginning to eat it from the inside. That's what sixty years of invisible infrastructure was keeping away. And nowit's back.
References:
- USDA APHIS (June 3, 2026). USDA Confirms Presence of New World Screwworm in the United States. https://www.aphis.usda.gov
- CDC (2026). New World Screwworm Outbreak — Situation Summary. https://www.cdc.gov
- Healthbeat (January 2026). How the U.S. once stopped screwworm threat to cattle and why it's re- emerging. https://www.healthbeat.org
- CNN (June 2026). Flesh-eating screwworm has reached the US — a comeback driven by organised crime. https://www.cnn.com
- USDA (August 2025). USDA Announces Sweeping Plans to Protect the United States from New World Screwworm. https://www.usda.gov
- Scientific Reports (July 2025). The reemergence of the New World screwworm and its potential distribution in North America. https://www.nature.com/articles/s41598-025-04804-9
- AVMA (May 2026). New World Screwworm Detected Within 60 Miles of US Border. https://www.avma.org
- DTN / Progressive Farmer (April 2026). USDA Groundbreaking Marks Step Toward Eradication of
- New World Screwworm Threat. https://www.dtnpf.com
- Oklahoma State University Extension (June 2026). New World Screwworm Information. https://extension.okstate.edu