The Screwworm Returns: A Flesh-Eating Threat to Cattle Country, in Perspective
The Screwworm Returns: A Flesh-Eating Threat to Cattle Country, in Perspective
LodiEye — June 2026
On June 3, 2026, the U.S. Department of Agriculture confirmed a parasite in a three-week-old calf in South Texas that the country had not seen in its livestock since 1966. The New World screwworm — a fly whose larvae eat the living flesh of warm-blooded animals — was back. Within a week, the count had grown to six animals across two states, and a problem that an earlier generation of scientists had declared solved was suddenly a national-security talking point in Washington.
For all the alarm, the situation rewards a clear head. This is a serious threat to a $113 billion cattle industry, but it is not a threat to the food on anyone's plate. It is a story with a remarkable history of American scientific success, a present-tense scramble to repeat that success, and a set of hard questions about whether the agencies tasked with the job are staffed and funded to do it. Here is the screwworm in perspective — and what it could mean for California cattle country, including San Joaquin County.
Photo: Judy Gallagher, via Wikimedia Commons (CC BY 2.0).
What the screwworm actually is
The New World screwworm (Cochliomyia hominivorax) is a type of blowfly, but an unusual one. Most fly larvae feed on dead or decaying material. Screwworm larvae feed on living tissue. A female lays her eggs in an open wound or a body opening — a tick bite, a fresh navel on a newborn calf, a castration or branding site — and when the eggs hatch, the maggots burrow inward with sharp mouth hooks, screwing themselves deeper into the flesh. The name describes the motion. Left untreated, the resulting wounds enlarge, attract more flies, and can kill an animal within roughly a week to ten days.
Photo: John Kucharski, USDA Agricultural Research Service. Public domain, via Wikimedia Commons.
Two facts about the fly's biology drive nearly everything about how it is fought. First, the female mates only once in her life. Second, the parasite does not spread directly from animal to animal the way a contagious disease does — long-distance spread happens almost entirely when an already-infested animal is moved. The single-mating biology is the fly's great weakness, and the movement-driven spread is why border closures and animal-movement controls sit at the center of the response.
A problem America already solved — once
From at least the 1930s through the 1960s, the screwworm was an annual warm-weather scourge for ranchers across the South and Southwest. The solution came from two USDA scientists, Edward Knipling and Raymond Bushland, who developed what is now called the Sterile Insect Technique (SIT). The idea is elegant: mass-rear millions of flies, sterilize the males with radiation, and release them into the wild in numbers large enough to swamp the wild males. Because females mate only once, a female that mates with a sterile male lays eggs that never hatch. Do that generation after generation, and the population collapses.
The rollout reads like a slow march down the map. A 1954 campaign wiped the fly off the island of Curaçao in about seven weeks. Eradication work began in Florida in 1957 and in Texas in 1962, and by 1966 USDA declared the United States free of native screwworm. The effort then pushed south: northern Mexico was essentially cleared by 1982 — the last year Texas reported a case — and through the 1990s the program advanced all the way to Panama, where a permanent biological barrier at the Darien Gap has kept the fly penned in South America ever since. That barrier is maintained by COPEG, a joint U.S.–Panama commission; the Congressional Research Service notes USDA has financed about 90 percent of its work and has estimated the investment saves the U.S. cattle industry roughly $2.3 billion a year.
The clean "eradicated in 1966" line deserves an asterisk. Veterans of the program and later retrospective analyses describe eradication as a decades-long grind with several flare-ups along the way, not a single victory date. But the broad achievement is real, and it became the template for the response now unfolding in Texas. The one prior re-incursion — a 2016–2017 outbreak in the endangered Key deer of the Florida Keys — was stamped out with the same playbook: treat infested animals and blanket the area with sterile flies released from the ground.
How it got back
The current outbreak is the failure of that southern barrier playing out in slow motion. The trigger was a sharp escalation in Panama: annual detections there jumped from roughly two dozen cases to more than 6,500 in 2023, overwhelming the containment zone. From there the fly moved north through Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador. In November 2024, Mexico reported its first case, near the Guatemalan border, and the U.S. promptly suspended live-animal imports from Mexico.
A major driver, according to conservation researchers tracking the spread, is illegal cattle trafficking through Central America, which moves infested animals long distances and reseeds the fly far ahead of where it could crawl on its own. Warmer conditions that expand the fly's viable range are part of the backdrop as well. The combination carried the parasite to within striking distance of Texas over the course of 2025 and, on June 3, 2026, across the line.
Where things stand now
As of June 9, 2026, USDA-APHIS had confirmed six animal cases in the United States: four cattle, one goat, and one dog. Five are in Texas — in Zavala (the index calf in La Pryor), La Salle, Gillespie, and Andrews counties — and one, a dog, is in Lea County, New Mexico, the first companion-animal case since the fly's return. No locally acquired human cases have been reported.
U.S. confirmed screwworm cases by animal type
Source: USDA Animal and Plant Health Inspection Service, as of June 9, 2026. Counts have shifted day to day; one case was reclassified from Texas to New Mexico.
The federal-state response is a near-replay of the historic campaign, scaled up. USDA reactivated the sterile-fly dispersal facility at Moore Air Base in Edinburg, Texas, and began aerial releases on top of the roughly four million sterile flies already being dispersed in the region each week, while also expediting targeted ground releases of the kind that ended the Florida Keys outbreak. The longer game is production capacity. The single domestic plant that once anchored this fight closed in 1982; the U.S. has depended on facilities abroad ever since.
Sterile-fly production and release capacity, by facility
Sources: USDA-APHIS, Congressional Research Service, ABC News. Figures are weekly capacity. The Mexico (Metapa) figure is the upper end of an announced 60–100 million range; the Mexico and Texas plants are planned or under construction, with Metapa targeted for summer 2026.
Who is responsible — the agency landscape
Several federal bodies share the work of tracking, reporting, managing, and preventing cattle diseases and pests like this one:
- USDA Animal and Plant Health Inspection Service (APHIS) is the lead agency — surveillance, eradication, the sterile-fly program, and the authority to restrict animal movement and imports under the Animal Health Protection Act.
- APHIS National Veterinary Services Laboratories (NVSL) provides diagnostic confirmation; every suspect sample, including those collected in California, is confirmed there.
- USDA Agricultural Research Service (ARS) handles the underlying science, centered at the Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas, named for the SIT pioneers.
- USDA Food Safety and Inspection Service (FSIS) is the food-safety backstop at the processing stage.
- The Centers for Disease Control and Prevention (CDC) covers the human-health dimension.
- State animal-health agencies — the Texas Animal Health Commission and, in our case, the California Department of Food and Agriculture (CDFA) Animal Health Branch — run on-the-ground surveillance and response.
- Internationally, COPEG maintains the Panama barrier and APHIS coordinates with Mexico's animal-health service, SENASICA.
The money and the people
This is where the perspective gets complicated, because the funding picture cuts in two directions at once.
On one side, the screwworm fight specifically has drawn large, fast infusions of money. APHIS received emergency funding of $109.8 million in 2023 and $165 million in late 2024 from the Commodity Credit Corporation. In May 2025 it committed $21 million to convert a fruit-fly facility in Metapa, Mexico into a sterile-fly plant, and in August 2025 USDA and the State of Texas announced a $750 million domestic sterile-fly production facility — the first U.S.-based plant since 1982, projected to produce up to 300 million flies a week.
Announced federal screwworm funding commitments
Sources: Congressional Research Service; USDA-APHIS; Texas Governor's Office. Mixes operational emergency funding (2023, 2024) with facility investments (2025). The $750 million domestic-facility figure is a capital project, not annual operating money.
On the other side is the standing workforce that actually detects and responds to incursions. According to USDA's Office of Inspector General, more than 2,100 employees left APHIS between January and June 2025 — roughly 25 percent attrition. Administration budget documents reviewed by trade press show proposed APHIS staffing down about 17 percent from 2025 to 2026, with related project funding lower as well. Separately, USDA in March 2025 cut funding for animal-disease prevention work — including screwworm-related contributions routed through international channels — that had supported outbreak investigations abroad. This week, a group of Democratic senators led by Oregon's Jeff Merkley wrote to Agriculture Secretary Brooke Rollins warning that staff reductions could hamper the screwworm response.
The two trends are not strictly contradictory: a structural drawdown of the broad animal-health workforce is running alongside a targeted, crisis-driven capital surge into screwworm-specific infrastructure. But they raise a real question that reporters and lawmakers are now pressing — whether emergency money for fly factories can substitute for the field veterinarians and inspectors who find an infestation in the first place. A widely circulated claim that budget cuts had gutted the sterile-fly program turns out to be incomplete in both directions: there were genuine cuts to animal-disease prevention in early 2025, and there were genuinely large investments announced in the same period.
Closer to home
California is the nation's top dairy state and a major beef producer, and San Joaquin County sits squarely in that agricultural economy, so the stakes here are not abstract even though the nearest cases are well over a thousand miles away. State officials' message is measured: CDFA describes the current risk to California producers as low while stressing that preparedness matters now.
The state has been building that preparedness for months. CDFA's Animal Health Branch is coordinating with USDA and other border states; fly traps in Imperial and San Diego counties have collected roughly 1,500 suspect flies, all confirmed negative; and CDFA's pesticide office awarded about $507,000 to a three-year University of California project to build an early-detection surveillance network and integrated pest-management guidelines, working alongside the Department of Fish and Wildlife, the Department of Public Health, and the Governor's Office of Emergency Services. For producers, the practical advice is unchanged and unglamorous: inspect animals closely — especially navels on newborns and any fresh surgical or branding wounds — keep up fly control, be cautious about animal movement from affected regions, and report any unusual wound or maggots to a herd veterinarian or the local CDFA office before treating it.
Who tracks screwworm in California, and who to call
The lead agency in California is the CDFA Animal Health Branch, which runs livestock surveillance, coordinates with USDA, and is the office that takes reports of suspected cases. It works with several partners: CDFA's Office of Pesticide Consultation and Analysis, which funded the early-detection research project; the California Animal Health and Food Safety (CAHFS) laboratory system at UC Davis, which screens specimens before final confirmation at USDA's national lab; the California Department of Public Health on the human-health side; the Department of Fish and Wildlife for wildlife; and the Governor's Office of Emergency Services for broader emergency coordination. County agricultural commissioners and private veterinarians are the local front line.
If you see a suspicious wound, maggots, or a fly you think could be a screwworm — on livestock, a pet, or wildlife — in San Joaquin County, the guidance is consistent across agencies: do not try to remove or destroy the larvae yourself, contact a veterinarian, and report it. The key contacts:
- Your veterinarian first for any animal with a non-healing or maggot-infested wound; a vet can collect and submit specimens correctly.
- CDFA New World screwworm line: (866) 922-2473, or email NWSinfo@cdfa.ca.gov.
- CDFA Animal Health Branch — Modesto District Office, which covers San Joaquin County and its neighbors: (209) 491-9350. Branch headquarters in Sacramento: (916) 900-5002.
- USDA-APHIS Veterinary Services, California: (916) 854-3950.
- Wildlife: report suspected cases in wild animals to the California Department of Fish and Wildlife in addition to CDFA.
- People: human infestation is rare, but anyone with an infested wound — especially after travel to an affected region — should see a physician, who reports to local and state public health.
Reports do not have to come from ranchers. Because the fly turns up on pets and wildlife too, a dog owner or a hunter who notices maggots in a wound is exactly the kind of early warning the surveillance system is built to catch.
The screwworm is a genuinely nasty parasite, and a wider outbreak would cost the livestock economy billions at a moment when beef prices are already at records. But it is also, uniquely, a problem the United States has beaten before with a method that still works. Whether it is beaten quickly this time will depend less on the biology than on the money, the facilities, and the people pointed at the job.
LodiEye is a civic research and analysis project, not a professional news organization, and the author is not a professional journalist. It is produced to complement — not replace — the reporting of local and regional outlets such as the Lodi News-Sentinel, Stocktonia, the Sacramento Bee, and CalMatters, which readers are encouraged to follow and support. This report is offered as sourced, balanced background and analysis.
This report was prepared with AI assistance across five capacities:
- Source Discovery: Perplexity and Claude were used to locate primary and reputable secondary sources, including USDA-APHIS releases, the Congressional Research Service, and CDFA materials.
- Credibility Validation: Claims were checked against original agency documents and multiple independent outlets, with single-source figures flagged as estimates.
- Analysis and Synthesis: Claude assisted in organizing the history, current status, agency roles, and funding picture into a coherent narrative.
- Presentation: Claude assisted in drafting the text and preparing the data visualizations.
- Final Review: The editor reviewed and approved the report before publication.
LodiEye strives for accuracy. If you spot an error or can provide additional context, please write to editor@lodi411.com.