Hundreds of larvae burrow through living flesh with sharp mouths
A parasite absent from American soil for sixty years has returned to Texas, forcing Canada to close its borders to livestock from the state and prompting a disaster declaration from the nation's largest beef producer. The New World Screwworm, a fly whose larvae consume living flesh, has been moving northward through Central America and Mexico for years, and its arrival in two calves near the Rio Grande marks a threshold moment for North American agriculture. What unfolds now is a test of whether modern biosecurity — sterile flies, sniffer dogs, quarantine zones — can hold a line that biology and commerce have already begun to blur.
- A flesh-eating parasite unseen in Texas since 1966 has appeared in two calves within miles of the Mexican border, triggering a state of disaster declaration from Governor Abbott.
- Canada moved swiftly to ban all cattle and horses that passed through Texas in the previous three weeks, sending a shockwave through a bilateral livestock trade that moves hundreds of thousands of animals each year.
- The screwworm's brutal life cycle — larvae hatching in open wounds and tunneling through living tissue until the host dies — makes every untreated animal a potential vector as summer warmth approaches.
- U.S. officials have drawn a twenty-kilometer quarantine zone and are preparing to release hundreds of millions of sterile flies, but experts are openly questioning whether these measures can outpace the parasite's northward momentum.
- Canada's ban is precautionary rather than alarmed — colder climates disfavor the screwworm — yet the disruption to trade and the psychological weight of a sixty-year barrier breaking are already being felt across the continent.
Canada has closed its border to cattle and horses from Texas after a parasite absent for sixty years reappeared in two calves near the Rio Grande. The New World Screwworm — a fly whose larvae hatch in open wounds and burrow through living flesh until the host dies — was last documented in Texas in 1966. Its return has unsettled one of North America's most consequential agricultural relationships.
The first infected animal, a three-week-old calf, was found in La Pryor, thirty miles from Mexico, with larvae burrowed into its umbilical wound. A second case emerged days later in Zavala County, barely six miles away. The U.S. Department of Agriculture confirmed both, established a twenty-kilometer control zone, and imposed quarantines across the region. Texas Governor Greg Abbott declared a state of disaster, warning that approaching summer heat would favor the parasite's spread.
Canada's food inspection agency responded by banning any livestock that had been in Texas within the previous three weeks. Officials were candid that the colder Canadian climate makes a sustained outbreak unlikely, but with 550,000 American cattle imported in 2025 alone, the precaution carries real economic weight. Farmers were advised to watch for wounds with discharge or foul odor, and travelers returning from Texas were told to check their pets.
The outbreak is the northern edge of a crisis that has been building for years in Central America and Mexico, where the screwworm has circulated largely unchecked. The parasite spreads most efficiently when humans move infected animals, though the flies travel short distances on their own. To push back, U.S. officials plan to release hundreds of millions of sterile flies — a biological strategy designed to collapse the breeding population — alongside sniffer dogs trained to detect infection in cattle. Whether these tools, combined with border controls and quarantines, can contain an outbreak that has already crossed a barrier many considered permanent remains the urgent question of the coming weeks.
Canada has closed its border to cattle and horses from Texas, a decision that rippled across North America's livestock trade this week after a parasite thought dead for six decades suddenly reappeared in two calves. The New World Screwworm—a flesh-eating fly whose larvae tunnel through living tissue—had not been documented in Texas since 1966. Now it is back, and spreading.
Canada's food inspection agency announced the ban on livestock that spent any time in Texas within the previous three weeks. The move came swiftly after the U.S. Department of Agriculture confirmed a second infected calf on Friday, just days after the first discovery. That first animal, a three-week-old calf, was found in La Pryor, a town thirty miles from the Mexican border, with the parasites burrowed into its umbilical wound. The second case emerged in Zavala County, less than six miles away, in a one-month-old calf discovered during routine testing of suspected animals. Texas Governor Greg Abbott declared a state of disaster that same Friday, characterizing the outbreak as an imminent threat to the state's agricultural industry—a significant statement from the leader of the nation's largest beef-producing state.
The screwworm is a parasite with a brutal life cycle. Female flies lay eggs in open wounds or mucous membranes of warm-blooded animals. When the eggs hatch, hundreds of larvae emerge with sharp mouths and begin burrowing through living flesh. Left untreated, the infection kills its host. Abbott told reporters he expected the outbreak to spread as summer arrived, bringing the warm, humid conditions the parasite thrives in. The U.S. Department of Agriculture responded by establishing a twenty-kilometer control zone around the initial discovery site, implementing quarantines and movement restrictions while officials conducted surveillance across the region.
This outbreak is the leading edge of a larger crisis moving north from Central America and Mexico, where the screwworm has been circulating for years. The parasite travels long distances primarily when humans transport infected animals, though the flies themselves can cover short distances on their own. For nearly six decades, U.S. and Latin American officials have fought screwworm infestations with limited success. The parasite was declared eradicated in the United States in 1966, but cases have surfaced periodically since, including a notable outbreak in the 1970s.
Canada's ban reflects caution rather than panic. Officials acknowledged that the parasite is unlikely to establish itself in Canada because the colder climate does not favor its survival—screwworms historically spread in warm, humid regions. Still, Canadian authorities urged farmers to watch their livestock for wounds accompanied by discharge or foul odor, and advised residents traveling to Texas to inspect their pets. The stakes for both countries are substantial. The U.S. and Canada maintain a two-way cattle trade worth hundreds of thousands of animals annually. In 2025 alone, Canada imported 550,000 cattle from the United States for slaughter, breeding, and dairy and wool production.
The U.S. response involves tactics both conventional and experimental. Officials plan to release hundreds of millions of genetically altered sterile flies into affected areas, a strategy designed to reduce the breeding population. They will also deploy sniffer dogs trained to detect the parasite in cattle. Yet some experts remain skeptical that these measures will be sufficient to halt the spread, particularly as summer weather approaches and conditions become more favorable for the screwworm. The coming weeks will test whether the combination of border controls, quarantines, and biological intervention can contain an outbreak that has already breached a barrier many believed impenetrable.
Citações Notáveis
This is likely to spread over the course of the summer— Texas Governor Greg Abbott
An imminent threat or occurrence of widespread danger and injury to the Texas agricultural industry— Abbott's state of disaster proclamation
A Conversa do Hearth Outra perspectiva sobre a história
Why does a parasite that was supposedly eradicated sixty years ago suddenly reappear now?
The screwworm was never truly gone from the Western Hemisphere—it persisted in Central America and Mexico the whole time. What changed is that it's moving north again, likely carried by infected animals crossing the border. The conditions in Texas right now are perfect for it: warm weather, cattle in close proximity, and a population with no immunity because no one has seen this threat in living memory.
If Canada's climate is too cold for the screwworm, why is the country banning Texas cattle at all?
Because you can't be certain. A single infected animal could arrive, infect local livestock before anyone notices, and create a foothold. The ban is cheap insurance compared to the cost of an outbreak. It also sends a signal to farmers to stay vigilant.
What does "genetically altered sterile flies" actually mean in practice?
They're breeding millions of male screwworms that cannot reproduce. Release them into the wild, and they mate with wild females, producing no offspring. It's elegant in theory, but it requires releasing enough sterile males to overwhelm the wild population—we're talking hundreds of millions of insects. And it only works if you can keep doing it faster than the wild population reproduces.
Is there any chance this spreads beyond Texas?
Yes. The parasite moves with livestock, and livestock move constantly across state lines for slaughter and breeding. The control zone is only twenty kilometers wide. If even one infected animal escapes that zone before anyone detects it, the outbreak expands. That's why officials are so concerned about summer—warmer weather accelerates everything.
What happens to the cattle trade if this gets worse?
It freezes. Other states and countries will do what Canada just did. A major outbreak could cost the industry billions and take years to recover from. That's why Abbott called it a disaster—because for Texas agriculture, it genuinely is.