Beef cattle and dairy producers across the western United States are growing increasingly concerned as the New World screwworm—Cochliomyia hominivorax, a flesh‑burrowing blowfly larva—continues advancing northward through Mexico. Earlier this fall, the parasite was detected in cattle just 70 miles from the U.S. border, with another confirmed case in late November in Nuevo León, directly adjacent to Texas.

Although the screwworm has not yet entered the United States, Mexico and Central America have reported more than 140,000 animal cases and over 1,000 human cases since the outbreak began in 2023. The steady northward movement has raised alarms among livestock producers, veterinarians and state agencies.

According to USDA economic models, a screwworm incursion in Texas alone could cost livestock producers $732 million annually, with statewide economic losses reaching $1.8 billion. California producers—responsible for the nation’s top‑valued dairy sector ($8.61 billion) and a $4.98‑billion cattle and calf industry—are watching developments closely.

“Controlling and preventing the impact of the New World screwworm are the majority of the questions I’m receiving now,” said Brooke Latack, University of California Cooperative Extension (UCCE) livestock advisor for Imperial, Riverside and San Bernardino counties. Imperial County, which borders Mexico, is home to roughly 329,000 head of cattle. “There is definitely concern among producers, industry members and veterinarians.”

For the dairy sector, the stakes are even higher. “Dairy cows produce milk every day, and it must be processed immediately,” said Daniela Bruno, UCCE dairy advisor for Fresno, Madera and Kings counties. “If a farm is quarantined or a plant shuts down, milk spoils quickly and must be dumped.” Bruno urges producers to diversify processing options, review insurance coverage and strengthen biosecurity protocols—especially as avian flu has also reemerged in California dairies.

A Parasite With Devastating Impact
The New World screwworm poses severe animal‑welfare and economic risks. Mortality can reach 100% in newborn calves, and the parasite can infect any warm‑blooded animal, including wildlife, pets and humans.

“It can affect and kill any warm‑blooded animal, but we are particularly concerned about our cattle here,” said Gaby Maier, Cooperative Extension specialist in beef cattle herd health at the UC Davis School of Veterinary Medicine. Data from Panama’s recent outbreak show that 83% of cases occurred in bovines.

Female screwworm flies lay hundreds of eggs in even the smallest wounds—such as tick bites or natural body openings. The larvae then burrow head‑first into living tissue using sharp mouth hooks, enlarging the wound as they feed. Secondary infections, sepsis and death can occur within seven to 14 days.
Producers who witnessed infestations before the parasite’s eradication in the U.S. describe the smell of decomposing flesh in live animals and, in some wildlife cases, larvae consuming entire organs. “It’s really out of a horror movie,” Maier said.

Renewed Federal and State Response
The New World screwworm was eradicated from the U.S. in the 1960s through the release of sterile male flies. A USDA‑Panama facility (COPEG) continues producing 110 million sterilized flies weekly to maintain a protective barrier in Central America.

Experts cite several possible reasons for the parasite’s resurgence north of the eradication zone, including illegal livestock movement, reduced border inspections during COVID‑19 and deforestation in the Darién Gap.

In response to the current threat:
• The U.S. southern land border is closed to cattle and horse movement
• Surveillance and rapid‑response teams have been expanded
• Sterile fly releases are deployed immediately around confirmed cases
• New preventive and treatment drugs have been conditionally authorized
• USDA has launched screwworm.gov to provide real‑time updates for producers and the public

State and federal agencies emphasize that early detection and strict biosecurity remain the most effective tools for preventing an incursion.