A calf at a ranch in La Pryor, Texas, was recently found with a gaping wound near its umbilical cord — and what investigators discovered inside was not a bacterial infection, not a fungal rot, not a tick bite gone wrong. It was alive. Moving. Burrowing deeper. On June 4, 2026, the U.S. Department of Agriculture confirmed what ranchers and entomologists had dreaded since the Mexican outbreak of 2023: Cochliomyia hominivorax — the New World Screwworm — had crossed into the United States for the first time since 1966.

Within 24 hours, Canada moved. The Canadian Food Inspection Agency (CFIA) announced on June 5 that any animal originating from or present in Texas within the prior 21 days would be barred from crossing the Canadian border. Horses, cattle, livestock of all kinds — turned away. It was a swift act of biosecurity that nonetheless sent shockwaves through the beef industry on both sides of the 49th parallel.

What Is the New World Screwworm?

Most maggots are nature’s undertakers — they eat what is already dead. The New World Screwworm has a different philosophy. Cochliomyia hominivorax — whose Latin name translates, without any poetic license required, to “man-eater” — is an obligate parasite. Its larvae consume only living flesh. They will not touch a cadaver.

The adult fly is deceptively beautiful: a metallic blue-green body, vivid reddish-orange eyes, three dark stripes along the thorax. A female mates only once in her 10-to-30-day lifespan. But in that window, she can deposit between 200 and 3,000 eggs — always targeted at a warm-blooded host. Any wound will do. A tick bite. A surgical incision. The soft mucous membranes of the nose, eyes, mouth, or genitals of a newborn animal. Even the umbilicus of a calf still wet from birth.

Once the eggs hatch, the larvae use tiny, rasping tusk-like mandibles and corkscrew-shaped body segments — hence the name “screwworm” — to rotate and drill deeper into living tissue, perpendicular to the skin surface, in precise imitation of a hardware screw. They do not retreat. They do not emerge voluntarily. A single wound can harbor hundreds of larvae. An untreated animal can be killed within ten days.

The Sixty-Year Eradication That Just Crumbled

The story of how the screwworm was driven out of North America is one of the most audacious — and least celebrated — scientific achievements of the twentieth century. It also involves radiation, Nobel Prize-linked research, tens of millions of sterilized flies dropped from aircraft, and a cowboy with an X-ray machine borrowed from the Army.

In the 1930s, an entomologist named Edward F. Knipling was working at the USDA’s Menard, Texas, laboratory when he developed what he called autocidal control: the idea that if you could flood a pest population with sterile males, the females — who typically mate only once — would produce no viable offspring. The population would collapse from within. It was theoretically elegant and practically insane. Nobody had ever done it.

His colleague Raymond C. Bushland became the hands to Knipling’s theory. In 1950, a colleague brought Knipling’s attention to research by Nobel laureate Hermann Joseph Muller, who had demonstrated that radiation could sterilize fruit flies. Bushland borrowed X-ray equipment from a nearby Army hospital and began irradiating screwworm flies in a Kerrville, Texas, lab. The key was finding the dose that rendered males sterile without destroying their competitive drive — their willingness to mate. They found it.

By 1954, the technique — now called the Sterile Insect Technique (SIT) — was tested on the 176-square-mile Dutch island of Curaçao, off Venezuela’s coast. Sterile males were released by aircraft. Within six months, screwworms were gone from the island. The goat herds survived. The experiment had worked at scale.

By 1959, with help from local cattlemen’s associations and a run of unusually cold winters, SIT had eliminated the screwworm east of the Mississippi. By 1964, a USDA plant in Mission, Texas, was cranking out 200 million sterile flies per week. By 1966, the screwworm was declared gone from the continental United States. By 2006, the eradication front had pushed all the way through Central America to the Darién Gap — Panama’s jungle border with Colombia, which now serves as a biological buffer maintained by a permanent sterile-fly barrier program.

For sixty years, it held. Then in 2023, outbreaks appeared in Panama. By 2024, the fly had spread through Mexico. In November 2024, the USDA suspended all live cattle, horse, and bison imports from Mexico. In May 2025, after a brief reopening, restrictions were imposed again due to “unacceptable northward advancement.” And in June 2026, the barrier dissolved entirely — a Texas calf with a wound and a fly that had finally run out of continent to cross.

Canada Closes the Gate

Canada’s response was rapid and blunt. The CFIA’s June 5 directive stated clearly: animals originating from or present in Texas within 21 days of a border crossing will not be accepted into Canada. Several U.S. states — Florida, Georgia, Tennessee, and Montana among them — simultaneously issued emergency regulations tightening veterinary requirements for Texas livestock imports.

The CFIA was candid about the biology: Canada’s cold climate cannot sustain the fly year-round. The screwworm prefers temperatures between 25 and 30 degrees Celsius and cannot overwinter in most of the country. But the agency acknowledged that summer months provide a window, however brief, during which imported animals could seed a local outbreak. “While our colder climate is not hospitable for the long-term establishment of the fly in Canada, they can survive shorter periods of time in the summer months,” the CFIA stated.

Derrell Peel, Oklahoma State University’s extension livestock marketing specialist, described the Canadian ban as largely symbolic in terms of trade volume — Texas exports relatively few animals directly to Canada. But symbolism, in biosecurity, is sometimes the whole point. The optics of the world’s second-largest country closing its border over a cattle parasite communicate something the trade statistics cannot: the screwworm is a threat that serious nations take seriously.

Can Humans Get It? Yes — And It’s Worse Than You Think

The screwworm is primarily a livestock crisis, but its medical record with humans is disturbing and under-reported. The fly does not discriminate. Any open wound on any warm-blooded body will attract the female fly. What follows is classified as myiasis — infestation of living human or animal tissue by fly larvae.

Human cases have most commonly involved travelers returning from Central America, the Caribbean, or South America — particularly the Dominican Republic, Costa Rica, and Mexico. In April 2025, Canadian infectious disease specialist Dr. Isaac Bogoch took to social media to warn Canadians traveling to these destinations, noting that the infection had been documented in an 80-year-old Canadian man who visited a Toronto hospital after contracting it in Costa Rica. In August 2025, the first U.S. travel-associated human case was confirmed in a Maryland man — the first such documented U.S. case on record.

Medical literature holds considerably darker examples. A 2011 case study published in an entomological journal documented a hospitalized cancer patient with a tracheostomy who was found to have larvae of Cochliomyia hominivorax living inside his trachea — his breathing tube — while undergoing chemotherapy in a Brazilian public hospital. A separate 2015 study in the CDC’s journal Emerging Infectious Diseases documented pin-site myiasis — larvae colonizing the metal pin insertion points of an external fracture fixator in a Colombian patient, a complication type only recognized in medical literature within the past two decades.

On “Screwworm” as Language: What Rhymes Are Actually Fine

Let’s address this directly, because it is a perfectly reasonable question and because language policing in journalism helps nobody. The word “screwworm” is a legitimate, longstanding entomological and agricultural term. It appears in USDA publications, CDC clinical guides, peer-reviewed journals in parasitology and veterinary medicine, Canadian federal government communications, and international food security policy documents. There is nothing euphemistic about it. The “screw” refers to the spiral, screw-shaped burrowing motion of the larvae. The “worm” refers to the larval maggot form. The name was formalized in the nineteenth century.

Rhymes that are technically fine in this context include words like brew, crew, flew, new, true, through, view, woo — and their compound cousins like corkscrew (highly apt), hairscrew, dew, hewn. These are phonetic observations, not innuendo. The term “screw” carries industrial, mechanical, and biological meanings that predate — and in this context wholly dominate — any colloquial slang associations. Anyone who snickers at a USDA press release about screwworms is experiencing a vocabulary failure, not a dirty joke. Call the thing by its name. The livestock ranchers who lose cattle to it certainly do.

The Bigger Picture: Food Security, Climate, and the Next Eradication

The World Organisation for Animal Health (WOAH) lists New World Screwworm as one of only six priority transboundary animal diseases (TADs) targeted for elimination across the Americas. The economic stakes are not abstract. In South America, where the fly remains endemic, annual losses have been estimated at approximately $3.6 billion. In the 1950s, before eradication, screwworm caused losses of over $200 million annually to U.S. meat and dairy alone — roughly $1.8 billion in today’s dollars.

Climate is not a neutral factor here. Bioclimatic modeling published in Scientific Reports in 2025 assessed the fly’s potential spread and identified Texas and Florida as the highest-risk U.S. states — findings that, with the 2026 Texas confirmation, now look prophetic. The same models flagged Mexico’s Pacific and Atlantic coasts and the Yucatan Peninsula as high-invasion corridors. The sterile insect technique that eradicated the fly the first time remains viable, and the USDA has stated it is deploying more than 100 million sterile flies weekly in the response zone. Whether that will be enough — and how far north the fly can establish before winter cuts it off — is the question ranchers, veterinarians, and biosecurity officials are watching in real time.

The fly was beaten once. It took decades, X-ray machines, borrowed Army equipment, hundreds of millions of deliberately infertile insects, and two scientists who eventually won the World Food Prize. The second eradication effort has begun. This time, the whole continent is watching.