A parasite moving closer to American soil with each passing season
A parasitic fly that consumes living flesh has been detected within 25 miles of the United States border, prompting federal investigators to examine a suspected case in a Texas calf — the closest brush with domestic soil in nearly a year. The New World screwworm, once eradicated from the U.S. through a decades-long campaign, has been advancing northward through Central America and Mexico with quiet persistence, and its arrival now feels less like a distant possibility than an approaching threshold. What unfolds in a federal laboratory in Iowa may determine whether a chapter closed in the 1960s is about to reopen.
- A three-week-old calf in Zavala County, Texas, may carry the first confirmed domestic screwworm infection since the eradication era — tissue samples are now awaiting federal confirmation.
- Mexico is managing over 26,000 confirmed cases with nearly 2,700 still active, and the parasite has been detected just 25 miles from U.S. soil — the nearest incursion in at least nine months.
- The screwworm's northward march has accelerated over three years, moving from South America through Central America and deep into northern Mexico, with detections edging closer to the border each season.
- A 2025 human case in a Maryland traveler proved the parasite can cross borders inside human tissue, shifting the threat from a livestock concern to a public health matter.
- Federal agencies have deployed personnel on the ground, and a local Texas congressman has pledged coordinated action — but the outcome hinges entirely on laboratory results still pending.
The U.S. Department of Agriculture is investigating a possible New World screwworm case in a three-week-old calf in Zavala County, Texas, with tissue samples sent to federal laboratories in Ames, Iowa, for confirmation. The timing is difficult to ignore: just days before the Texas discovery, Mexican authorities found the same parasite in a goat in Coahuila state — only 25 miles from U.S. soil, the closest confirmed sighting in at least nine months.
Mexico is now managing a sprawling infestation of more than 26,000 confirmed cases, with roughly 2,700 still active. The parasite has been moving northward for three years, advancing through Central America and into Mexico's northern states with steady momentum. In April, a detection was recorded just 60 miles from the Texas border. The USDA publishes updates twice weekly on cases within 400 miles of the U.S. and maintains that domestic risk remains very low — but the trajectory suggests otherwise.
The screwworm is a flesh-eating fly that lays eggs in open wounds or body openings; once hatched, larvae feed on living tissue, causing severe and disfiguring infections. Last year, a traveler returning to Maryland from El Salvador became the first person in the U.S. to contract the parasite — a reminder that it can cross borders in human tissue, not just livestock. The person recovered with no further transmission, but the episode marked a shift from agricultural threat to public health concern.
Rep. Don McLaughlin, whose district includes the affected area near La Pryor, confirmed that samples were taken from calves on a local ranch and pledged to work alongside every relevant agency if the case is confirmed. If the Texas calf tests positive, it will be the first domestic case since the eradication program of the 1960s — a milestone that would reopen a chapter many believed permanently closed.
The U.S. Department of Agriculture announced Wednesday that it is investigating a possible case of the New World screwworm in Texas—a parasitic fly that consumes living flesh. The suspected infection was found in a three-week-old calf in Zavala County, in the state's southern reaches, and a tissue sample has been sent to federal laboratories in Ames, Iowa, for confirmation.
The timing is unsettling. Just days earlier, Mexican authorities detected the same parasite in a five-year-old goat in Coahuila state, a region that borders southwestern Texas. That discovery was only 25 miles from U.S. soil—closer than any confirmed sighting in at least nine months. The USDA tracks these incursions carefully, publishing updates twice weekly on any cases found within 400 miles of the American border. The agency maintains that the screwworm "is not currently present" in the United States and that "the current risk to livestock, other animals, and people in the United States remains very low." But the pattern tells a different story.
Mexico is now dealing with a sprawling infestation. Federal data shows 26,216 confirmed screwworm cases across the country, with roughly 2,700 still active. In Coahuila alone, officials have tracked 32 cases, including 19 that remain ongoing. The parasite has been creeping northward for three years, moving through Central America and into Mexico with steady momentum. In April, Texas officials announced a detection just 60 miles from the border. Last year, a traveler returning to Maryland from El Salvador became the first person in the United States to contract the infection—a sign that the parasite can cross borders in human tissue, not just animal.
Rep. Don McLaughlin, whose district includes the affected area near La Pryor, a small community about 50 miles south of the Mexican border, said samples were taken from calves on a local cattle ranch. "If this case is confirmed I will stand lock step with every local, state and federal agency to work together and fight this horror," he wrote on social media. The USDA has already mobilized personnel on the ground and is coordinating with local partners.
The New World screwworm is a flesh-eating parasitic fly that targets warm-blooded animals and humans. The female lays eggs in open wounds or body openings—eyes, ears, nose, mouth—and once the larvae hatch, they feed on living tissue. The infection can be severe and disfiguring. The parasite is native to South America and parts of the Caribbean, but its range has expanded northward with alarming consistency. The Maryland case last year, though it ended in recovery with no onward transmission, demonstrated that the threat is no longer purely agricultural. It is now a matter of public health.
What happens next depends on laboratory confirmation. If the Texas calf tests positive, it will mark the first domestic case since the eradication program of the 1960s. If it does not, the investigation will close, but the underlying risk will remain: a parasite moving closer to American soil with each passing season, and a border region increasingly vulnerable to its arrival.
Citações Notáveis
If this case is confirmed I will stand lock step with every local, state and federal agency to work together and fight this horror— Rep. Don McLaughlin of Texas
The current risk to livestock, other animals, and people in the United States remains very low— USDA
A Conversa do Hearth Outra perspectiva sobre a história
Why does a parasite in Mexico matter so much to American officials right now?
Because it's moving north. Twenty-five miles away is not a border—it's a distance you can drive in half an hour. The screwworm has been confined to South America for decades, but in the last few years it's been spreading through Central America and Mexico. Each detection closer to Texas changes the calculation.
What exactly does the screwworm do to an animal?
It lays eggs in any open wound or opening—a cut, a sore, even the animal's nose or mouth. When the larvae hatch, they eat the living tissue around them. It's not a quick death. It's a slow, painful infestation that can destroy an animal's face or limbs. For livestock, it's catastrophic.
Is there a reason it's moving north now, after all these years?
Climate change is part of it. The parasite thrives in warm, humid conditions. As those conditions expand northward, so does the fly's range. But it's also just the nature of parasites—they spread when they can.
What about that human case in Maryland last year?
A traveler came back from El Salvador infected. The person recovered, and there was no transmission to anyone else. But it proved the screwworm can travel in human tissue, not just livestock. That changes the threat profile.
If it reaches Texas, what happens?
The USDA would activate an eradication program. They did it successfully in the 1960s. But that was decades ago, with different tools and different knowledge. A new outbreak would be expensive, disruptive, and uncertain. Prevention is far easier than cure.