US steps up response to screwworm threat

On August 19, the U.S. Department of Health and Human Services (HHS) issued a declaration empowering the Food and Drug Administration (FDA) to grant emergency use authorizations (EUAs) for animal drugs to treat or prevent infestations caused by the New World screwworm (NWS).

There are currently no FDA-approved drugs for NWS in the United States, FDA Commissioner Marty Makary said.

The declaration marks the most recent federal effort to head off the northward advance of the parasite. To date, NWS has not been reported or detected in the U.S. in animals, according to the U.S. Department of Agriculture (USDA). Though generally eradicated from North America decades ago, the pest has reemerged this past year in Mexico and continues to move northward.

On July 9, the USDA announced that Mexico’s National Service of Agro-Alimentary Health, Safety, and Quality reported a new case of NWS in the city of Ixhuatlan de Madero, approximately 160 miles northward of the current sterile fly dispersal grid and 370 miles south of the U.S.-Mexico border. This new northward detection comes nearly two months after northern detections were reported in Oaxaca and Veracruz, less than 700 miles away from the U.S. border.

“Today we are taking decisive action to safeguard the nation’s food supply from this emerging threat,” HHS Secretary Robert F. Kennedy Jr. said in a statement. “This authorization equips FDA to act quickly, limit the spread of New World Screwworm, and protect America’s livestock.”

The EUA process allows FDA to authorize the use of certain animal drug products to prevent, control, or treat NWS that are approved for other uses or available abroad but not officially approved for use against NWS in the United States. Veterinarians and producers will receive future guidance on the appropriate use of any authorized products, according to the agency.

Response efforts

Just days before, on August 15, U.S. Secretary of Agriculture Brooke Rollins and Texas Gov. Greg Abbott announced enhanced initiatives and investments to monitor and prevent the spread of screwworm that build on a five-pronged plan in July.

Part of the enhanced plan includes building the nation’s first sterile fly production facility in Edinburg, Texas, at Moore Air Force Base. It will accompany an $8.5 million sterile fly dispersal facility currently under construction that should be up and running by year’s end. That facility will take larvae produced in Panama or Mexico, continue growing them, and distribute the hatched sterile flies to sites of infestation.

The production and dispersal sites are part of the Sterile Insect Technique (SIT) used to prevent the spread of the NWS fly that lays the screwworm larvae. Sterile male flies are produced then released into the wild to mate with NWS fly females, which reproduce only once.

The $750 million production facility, to be built with the Army Corps of Engineers, is expected to produce up to 300 million sterile flies per week. It will join existing and planned facilities in Panama and Mexico as part of an international network designed to suppress NWS populations.

“We have assessed the information on the ground in Mexico and have determined we must construct an additional sterile fly production facility in the United States to stop the northward advancement of this terrible pest that is threatening American cattle production,” Rollins said.

Construction of the production facility is expected to take at least a year. Until it gets up and running, the USDA will spend up to $100 million on technology to accelerate the U.S. response.

This summer, APHIS held four public listening sessions to learn from companies, industry, academia, and the public about e-beam and X-ray technologies, potential sterile NWS strain improvements, genetically engineered flies, modular and other facilities for sterilization, and fast-tracking animal drug and treatment approvals, among other proposals.

“APHIS is further evaluating and validating the most promising options to help inform ongoing decision-making,” according to the announcement.

Producer pressure

Abbott stressed the economic stakes. “Texas agricultural producers feed the world, with our state’s food and agriculture sector supporting over two million well-paying jobs and creating over $867 billion in total economic impact,” the governor said. “All of this is at risk because of the New World screwworm.”

Additional components of USDA’s expanded plan include enhanced border surveillance with mounted patrol Tick Riders, training detector dogs to detect NWS in livestock and other animals along the border and at ports of entry, and expanded aerial release of sterile flies in Mexico.

Members of Congress from both parties praised the announcement. House Agriculture Committee Chairman Glenn “GT” Thompson called the Texas facility “exactly the kind of forward-looking investment we need,” while Senate Agriculture Committee Chairman John Boozman said the move represented “bold, decisive action to protect American livestock.”

The USDA announcement followed weeks of mounting pressure from agricultural groups for the government to do more to contain the NWS threat.

On August 1, a coalition of 175 organizations, including the American Farm Bureau, the National Cattlemen’s Beef Association, and the National Milk Producers Federation, sent a letter to Rollins and congressional leaders urging immediate federal funding and construction of a U.S.-based sterile fly production facility.

The letter emphasized the potential economic impact, citing USDA estimates that a contemporary outbreak could cost producers $4.3 billion annually and cause a total economic loss of more than $10.6 billion across the historic NWS range in the U.S.

“Our producers cannot afford a NWS outbreak that decimates herds, causes financial losses, increases grocery prices, and threatens overall food security,” the groups wrote.

‘Aggressive parasite’

During a presentation at AVMA Convention 2025 in Washington, D.C., this July, Dr. Brianna Schur, deputy administrator for USDA’s Animal and Plant Health Inspection’s (APHIS) Veterinary Services, provided a clinical overview of NWS biology and the challenges of containing it.

“New World screwworm is an aggressive parasite. Larvae don’t wait for dead flesh, they create it,” Dr. Schur said.

A single female fly can lay up to 500 eggs in wounds or natural orifices. Within 12 to 24 hours, larvae hatch and burrow into living tissue, causing severe necrosis. Untreated hosts typically die within one to two weeks, often from secondary bacterial infections.

Dr. Schur stressed that veterinarians are the front line of surveillance. “Private practitioners are our early-warning system,” she said. Foul-smelling wounds, visible maggots, depressed behavior, self-isolation, and umbilical infections in newborn livestock are among the key clinical signs. Tick bites, ear tags, and even minor abrasions can serve as entry points.

Because other fly species can co-infest wounds, she said APHIS urges practitioners not to attempt field identification but to preserve larvae in alcohol and report cases immediately to state or federal animal health officials.

Dr. Schur also cited the potential significant economic consequences of a screwworm incursion. She cited a 2024 APHIS report estimating that an outbreak infecting 7.2 million Texas cattle could cause $732 million in producer losses and $1.8 billion in economic damage to the Texas economy alone. Dr. Schur likened the potential disruption to the fallout from a single U.S. case of bovine spongiform encephalopathy in 2003, which temporarily shut down global beef trade.

For veterinarians, vigilance remains critical. “Detecting the signs early gives us the best chance of containment,” Dr. Schur said.

Cross-border cooperation

As Dr. Schur explained, NWS eradication relies on sustained international cooperation. Through the Commission for the Eradication and Prevention of Screwworm (COPEG), USDA and Panama’s agriculture ministry currently operate a facility producing 100 million sterile flies weekly for aerial release.

With detections spreading north in Mexico, including more than 3,000 animal cases and 26 human cases reported as of July, the country has expanded checkpoints and aerial release zones with U.S. support.

The USDA has invested $21 million to retrofit a fruit fly facility in Metapa, Mexico, that could add 60 million to 100 million sterile flies per week. Meanwhile, APHIS experts are collaborating with Mexican officials on surveillance, reporting, and training programs.

In addition, the USDA has updated its national response strategy, trained state and tribal veterinarians, and distributed pest alerts, bilingual brochures, and photographic ID cards displaying screwworm infestation to animal health professionals.

Whether Mexico can halt northward spread before the fly reaches the Rio Grande is unclear, but Dr. Schur said that the tools that worked before can work again. “The science is on our side,” she said. “Success depends on rapid reporting, disciplined animal‑movement controls, and sustained sterile‑fly pressure.”

A version of this story appears in the October 2025 print issue of JAVMA