After 60 Years, the Screwworm Is Back — and Texas May Not Be Able to Stop It

On June 3, 2026, USDA Secretary Brooke Rollins confirmed what ranchers and entomologists across the Southwest had been dreading for months: a three-week-old calf in La Pryor, Texas — a small town in Zavala County, roughly 100 miles from the Mexican border — tested positive for New World screwworm (Cochliomyia hominivorax), a parasitic fly whose larvae feed on the living tissue of warm-blooded animals [1]. It was the first confirmed case in the United States since 1966, when the pest was declared eradicated from U.S. soil through what remains one of the most celebrated pest-control campaigns in agricultural history [2].

Two days later, Governor Greg Abbott expanded a statewide disaster declaration, authorizing "all available resources of state government" to respond, including reassigning personnel from university systems to accelerate sterile fly shipments and facility construction [3]. The USDA established a 20-kilometer quarantine zone around the detection site and restricted the movement of all warm-blooded animals — livestock, pets, and wildlife — without prior authorization from the Texas Animal Health Commission [4].

The question now is whether the response will be fast enough.

How the Barrier Broke

The New World screwworm was eliminated from the United States through the sterile insect technique (SIT), a method in which mass-produced sterile male flies are released into the wild to mate with females, producing no offspring. Because female screwworm flies mate only once, repeated sterile releases can collapse a wild population within a few generations [5]. After eradication from the U.S. in 1966, the program pushed the fly progressively southward through Mexico and Central America, eventually establishing a biological barrier at the Darién Gap in Panama by 2000 [6].

That barrier was maintained by the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG), a joint operation between USDA's Animal and Plant Health Inspection Service (APHIS) and Panama's Ministry of Agriculture Development. The COPEG facility in Pacora, Panama — the only screwworm fly-rearing facility in the world until recently — produced approximately 100 million sterile flies per week for continuous aerial dispersal across the barrier zone [7]. APHIS funded roughly 90 percent of COPEG's operations, and the two countries jointly invested approximately $350 million over the program's 25-year history [8].

The system began unraveling in 2022. Multiple factors converged: pandemic-era supply chain disruptions reduced the flow of materials to the facility; monitoring and surveillance were relaxed during COVID-19; and record levels of human and animal movement through the Darién Gap — more than 1.2 million migrants crossed between 2021 and 2024 — created new vectors for infested animals to bypass the barrier [9]. Illegal cattle trafficking in the region compounded the problem [10]. A 2023 APHIS improvement plan flagged the Panama facility as one of its three "highest priority facilities in need of repair," but repairs had not been completed before the barrier was breached [8].

By late 2023, screwworm cases were detected north of the sterile fly dispersal zone in Panama. Cases surfaced in Costa Rica, Nicaragua, Honduras, and Guatemala through 2024 [11]. In November 2024, Mexico confirmed its first case — a cow in the southern state of Chiapas [12]. From there, the spread was rapid.

26,000 Cases and Counting

By mid-2026, over 26,200 screwworm cases had been identified across Mexico, with approximately 2,700 cases under active monitoring at any given time [9]. Cases reached Tamaulipas, Nuevo León, and Coahuila — three Mexican states that share a border with Texas. As of May 19, 2026, USDA reported 156 active cases in Tamaulipas alone, with 48 in Nuevo León and 15 in Coahuila [13].

Source: USDA APHIS / CDC

Data as of Jun 6, 2026CSV

The USDA had taken preparedness measures before the Texas detection. Since early 2025, more than 8,000 screwworm surveillance traps were deployed across the southern border region, producing 58,000 samples and testing 19,000 wildlife specimens — all negative, until the Zavala County calf [1]. In May 2025, Secretary Rollins shut down the U.S. southern border to imports of live cattle, bison, and horses from Mexico [14]. APHIS received emergency funding of $109.8 million in 2023 and $165 million in 2024 from the Commodity Credit Corporation for screwworm response activities [8].

None of it was enough to prevent the fly from crossing.

The Production Gap

The central challenge is arithmetic. The COPEG facility in Panama produces 100 million sterile flies per week — capacity designed to maintain a relatively narrow barrier zone at the Darién Gap, not to fight an active infestation across a country the size of Mexico, let alone one threatening 1,200 miles of open Texas-Mexico border [7].

USDA has moved to expand production. In August 2025, the agency announced $21 million to help Mexico renovate an existing fruit fly facility in Metapa, Chiapas, which is expected to produce an additional 60 to 100 million sterile screwworm flies per week. Mexico anticipated production beginning in summer 2026 [15]. Separately, a new sterile fly dispersal facility was completed at Moore Air Base in Edinburg, Texas, in February 2026, though this facility is designed to distribute — not produce — flies [16]. A larger production facility at the same base, contracted through the U.S. Army Corps of Engineers, targets a maximum capacity of 300 million sterile flies per week, but construction timelines extend well beyond the current emergency [17].

Source: USDA APHIS

Data as of Jun 6, 2026CSV

At full theoretical capacity across all three facilities — Panama, Metapa, and Moore Air Base — the U.S. could deploy up to 500 million sterile flies per week [15]. Whether that capacity will come online in time is another matter. The 2016 Florida Keys outbreak required 188 million sterile flies to eradicate screwworm from six small islands [18]. The geographic scale of the current threat dwarfs that precedent.

Economic Stakes

APHIS has estimated that the screwworm eradication program and the Panama barrier have saved the U.S. cattle industry approximately $2.3 billion annually [7]. The American Farm Bureau Federation describes the pest as having "moved beyond the containment threshold" [6].

Texas alone holds an estimated 37,000 cattle in Zavala County, where the first case was confirmed [4]. Statewide, the figures are far larger: Texas A&M researchers estimate that a screwworm establishment in the state would cause $2.1 billion in losses to the cattle industry and $9 billion to the hunting and wildlife industry [19]. Nationally, the U.S. cattle sector is valued at over $100 billion.

Cattle futures markets initially dropped on the news — August feeder cattle fell 5.80 cents to 342.625 cents per pound when the suspected case was reported — but then rallied after confirmation, with most-active August feeder cattle rising 10.750 cents to 353.375 cents per pound [20]. Livestock market analysts attributed the counterintuitive rebound to expectations that reduced cattle supplies during the summer grilling season would be price-bullish [20]. But for producers on the ground, the calculus is harsher: cow-calf operators now face additional costs for veterinary care, labor, and surveillance, and some question whether spending $4,500 on a bred heifer makes economic sense amid screwworm uncertainty [20].

The Wildlife Blind Spot

Livestock losses dominate the policy response, but screwworm is an equal-opportunity parasite. The fly's larvae infest any open wound on any warm-blooded animal — a tick bite, a barbed-wire scratch, a newborn's navel. Texas is home to more than 100 mammal species at risk, including over 5 million white-tailed deer [19].

The 2016 Florida Keys outbreak offered a preview. Screwworm killed or forced the euthanasia of 135 endangered Key deer — more than 10 percent of the estimated population of roughly 1,000 animals [18][21]. The subspecies was already under federal protection, and the loss pushed recovery efforts back years.

South Texas harbors its own vulnerable species. The ocelot, a federally endangered wild cat with fewer than 80 individuals remaining in the U.S., ranges through the brush country of the lower Rio Grande Valley [19]. Javelina populations, pronghorn herds, and desert bighorn sheep could all face significant mortality from screwworm infestation. Wildlife biologists at Texas A&M AgriLife Extension have warned that rangeland ecosystems face cascading effects: loss of wildlife diversity, disruptions in predator-prey dynamics, and reduced hunting lease and wildlife tourism revenue [22].

Information on potential impacts to tribal nations with reservation lands in the affected region is limited in current reporting. The Kickapoo Traditional Tribe of Texas holds a reservation in Maverick County, adjacent to the quarantine zone, but no public statements from tribal officials regarding screwworm preparedness have been documented as of this writing.

The Florida Precedent — and Its Limits

The 2016 Florida Keys outbreak is the closest historical comparison, and defenders of the SIT approach point to it as proof the system works. Screwworm was detected in the Keys on September 30, 2016; eradication was declared on March 23, 2017 — roughly six months, with 188 million sterile flies deployed across six affected islands [18]. Only one case was found on the Florida mainland, south of Miami.

But the geographic constraints that made the Keys containable are precisely what makes the Texas situation different. The Keys are an island chain, surrounded by water, with limited connectivity to the mainland. South Texas is an open landscape with a 1,200-mile land border. Animals — both domestic and wild — cross freely. White-tailed deer do not respect quarantine lines.

The American Farm Bureau Federation has noted that the pest has "moved beyond the containment threshold" in Mexico, raising the question of what happens if eradication fails in Texas [6]. No public federal document specifies the criteria that would trigger a declaration that eradication has failed and a new permanent SIT barrier must be established at the U.S.-Mexico border or further north. Entomologists and agricultural policy analysts have raised this as a gap in the current response framework.

Was the 60-Year Absence Real?

The narrative of a clean 60-year absence deserves scrutiny. The USDA's official position is that New World screwworm was eradicated from the U.S. in 1966 and had not been detected on U.S. soil (outside of the 2016 Florida Keys event) until the June 2026 Texas case [2].

Some entomologists and agricultural scientists have questioned whether the surveillance record fully supports this claim. The Wildlife Society published an analysis arguing that "another screwworm incursion [was] inevitable" and noting that maintaining barriers, rearing facilities, and surveillance operations across multiple countries is expensive and vulnerable to funding disruptions [10]. Congressional researchers at the Congressional Research Service documented that APHIS's requested funding increase of $3.6 million for FY2025 to address aging infrastructure and higher operational costs at the Panama facility was modest relative to the scale of the threat [8].

Whether screwworm flies occasionally crossed the border in small numbers during the intervening decades — only to die out without establishing breeding populations or being detected by a surveillance system with inherent gaps — is a question the current outbreak may force into the open. The 8,000 traps deployed since early 2025 represent a significant escalation of monitoring [1], which raises the question of what the detection capacity was in prior decades.

What Happens Next

The immediate race is against the calendar. Summer temperatures accelerate the screwworm life cycle — a generation can complete in as little as three weeks in warm conditions — and expand the geographic range where the fly can survive and reproduce. Every week of delay in scaling sterile fly production reduces the probability of rapid eradication.

The CDC issued a Health Alert Network notice about New World screwworm cases in northern Mexico, noting that human cases have also been documented — Mexico's Ministry of Health reported 141 human infections across eight states, predominantly in Chiapas [23]. While human screwworm myiasis is treatable and rarely fatal, it adds another dimension to the public health response.

USDA is coordinating with Mexican agricultural authorities and has committed to continued aerial dispersal of sterile flies across the border region. The Metapa facility in Mexico and the expanded Texas operations represent a significant investment. But the underlying vulnerability remains: the biological barrier that protected the United States for six decades was a single point of failure, maintained by a single facility, and it broke.

Whether the response now unfolding can put that barrier back together — or whether the U.S. will need to learn to live with the screwworm again — depends on decisions being made in the next few months.