Screwworm Eradication Push Reignites as USDA Signals Sustained Campaign in Cattle Country

On a humid afternoon in late June, a USDA statement landed on agricultural trade desks like a weather advisory nobody had ordered: the screw worm eradication campaign would be sustained, not wound down. The phrasing was small. The stakes are not.

For more than a decade the United States has flirted with the southern frontier of a parasite that does not respect the border, does not respect the paperwork at a port of entry, and does not respect the patience of a cattle buyer in Texas, Florida, or Chihuahua. The New World screwworm, a fly whose larvae eat living tissue in warm-blooded mammals, was declared eradicated from the United States in 1966. It was declared eradicated from Mexico in 1991. It was declared eradicated from Central America in 2001. Then, slowly, then suddenly, it came back.

The USDA's decision to keep the pedal down on sterile-fly production and dispersal is less a victory lap than an admission. The campaign that beat the screwworm the first time, a decades-long project run out of a sterile-fly factory in Chiapas and operated jointly with Mexico, has been running hot for two years without an off-ramp in sight. Sustained, in this context, means funded, means staffed, and almost certainly means another congressional line item the agriculture committees will have to defend in a budget cycle that has so far been unkind to anything that smells of recurring obligations south of the Rio Grande.

The fly that won't stay dead

The biology is brutal and straightforward. A female screwworm lays eggs in any open wound on a mammal, from a deer's scrape to a calf's navel. The eggs hatch, the larvae burrow, and the animal dies slowly of infection or starvation. A single infested animal can seed a county.

The eradication method, pioneered by the US Department of Agriculture in the 1950s and scaled across the hemisphere through the 1970s, is a piece of applied biology that has aged remarkably well. Mass-reared male flies are sterilised with low-dose radiation and released by the billion over infested zones. Mating with a sterilised male produces no offspring. The local population collapses within a few generations. There is no chemical spray. There is no fence. There is just a release rate dense enough to swamp the wild males.

The method worked. By the turn of the century, screwworm had been pushed from the United States through Mexico and into Panama, where a permanent barrier of sterile-fly release kept it pinned to the Darien Gap. That barrier, operated out of the Panama–United States Commission for the Eradication and Prevention of Screwworm (COPEG), was the load-bearing wall of North American livestock health for a generation. It was also, by every account from veterinarians who have worked the line, chronically under-funded relative to the cost of letting it slip.

The barrier that thinned

Sometime in the early 2020s, screwworm reappeared in numbers that the Darien barrier was no longer fully suppressing. The exact sequence of breakdowns is contested and politically inconvenient. What is not contested is the trajectory: detections moved north through Panama, into Costa Rica, into Nicaragua, Honduras, and Guatemala, and into southern Mexico. By late 2025 the parasite was again turning up in cattle in Chiapas and Tabasco, and a Mexican federal declaration of emergency had activated bilateral response protocols with the United States.

The Panama factory, designed to produce roughly 100 million sterile flies a week, has been running above its design capacity for the better part of two years. Production has been expanded at a sister facility in Metapa, Chiapas. A second production site, long discussed and repeatedly deferred, has moved from concept to construction on US soil, with a site in southern Texas announced in the past year as part of the push to shorten the sterile-fly supply chain. None of these moves, on their own, are signs of a campaign being wound down. They are signs of a campaign being rebuilt on the assumption that the old footprint is no longer enough.

The June USDA signal that the programme would be sustained, rather than tapered, is best read inside that history. Officials speaking on background to trade and livestock outlets in the past month have framed the southern front as a multi-year capital project, not an emergency surge. The distinction matters in Washington. Emergencies end. Capital projects get line items.

What cattle country already paid

For US producers the cost of the past two years has not been abstract. Between November 2025 and May 2026 the USDA suspended or restricted the southern cattle import pipeline multiple times, most recently a partial closure that covered several Mexican states where live cattle movements had been tied to confirmed detections. Suspension is the policy instrument the agency reaches for when surveillance is not yet confident; it is also the instrument that prices into feeder markets within a week.

The numbers that livestock economists watch most closely are not the infection tallies. They are the import volumes and the feeder-cattle basis in Texas and Oklahoma. Both moved sharply when border protocols tightened. The Mexican cattle trade had been one of the more quietly important flows in North American beef, supplying a non-trivial share of US feedlot placements in tight supply years. Disrupting that flow in a tight-supply year is the kind of decision that ends up in a CBO score.

None of which is to say the closures were wrong. They were, by the standard practice of veterinary epidemiology, the correct reflex. The point is that the cost of the policy is borne first by the buyers and sellers on the southern tier, and only later, if at all, by the budget in Washington. Sustained funding is the mechanism that lets the agency buy the sterile flies fast enough to make those closures short rather than permanent.

The wider biosecurity ledger

The screwworm story is one line on a much larger ledger. Across the past three years US animal-health authorities have run simultaneous campaigns on avian influenza in dairy herds, on African swine fever preparedness in the Caribbean basin, and on the chronic surveillance gaps around feral swine in the Gulf states. Each of those campaigns is technically distinct. Each of them draws on the same bench of entomologists, the same regional laboratory capacity, and the same diplomatic channels that keep a sterile-fly factory running in another country's territory.

The recurring complaint from people who work the programmes, voiced in congressional testimony and in trade-press coverage that rarely makes the front page, is that US biosecurity funding cycles between panic and amnesia. Money arrives after an outbreak; it leaves after the outbreak is contained. The infrastructure that prevents the next outbreak is what gets squeezed in the between-years. The sterile-fly barrier at Darien was the textbook case for two decades. The current sustained-campaign framing is, quietly, an attempt to break that pattern in the one corner of the ledger where the biology leaves room to plan ahead.

What to watch in the next six months

Two dates on the calendar will tell the story. The first is the FY2027 budget request, due in the early autumn, which will show whether the sustained-campaign language translated into a multi-year line for sterile-fly production or remained a verbal commitment. The second is the southern hemisphere spring, when warmer temperatures push fly activity back up and detections in Central America typically rise before the calendar turns.

Between those two dates, the more useful indicator for cattle-country buyers is less the press release than the release rate. If the weekly sterile-fly output from the Panama and Chiapas facilities continues to climb, and if the Texas plant comes online within the announced timeline, the barrier gets thicker. If output plateaus or slips while detections push north, the closures that producers absorbed in the spring will not be the last ones.

The screwworm campaign is a long, unglamorous piece of biological infrastructure, and it is the kind of story that only earns attention when it fails. The June signal from USDA is the kind of language that suggests Washington is finally preparing to fund it like the load-bearing structure it has always been.