The New World screwworm, a flesh-eating parasite known for its devastating impact on livestock, has been detected in cattle in Mexico close to the U.S. border. In late November, another case was confirmed in Nuevo León, a Mexican state adjacent to Texas. While the parasite is not currently present in the United States, its northward spread has raised concerns among beef and dairy producers.
Since 2023, more than 140,000 animal cases and over 1,000 human cases have been reported in Mexico and Central America as the outbreak continues to move north. The U.S. Department of Agriculture estimates that an outbreak in Texas could cost livestock producers $732 million annually and result in up to $1.8 billion in losses for the state’s economy.
California’s agriculture sector is particularly vulnerable due to its large dairy and cattle industries. Dairy products/milk are California’s top commodity at $8.61 billion in gross value for 2024, while cattle and calves rank fourth at $4.98 billion.
“Controlling for and preventing impact from 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. She noted that Imperial County alone houses about 329,000 head of cattle.
“There is definitely concern about the screwworm down here for producers, industry members and the veterinarians that serve the area,” Latack said.
Daniela Bruno, UCCE dairy advisor for Fresno, Madera and Kings counties, emphasized the particular risks faced by dairy operations: “Unlike beef cattle, which can be held back from market during a quarantine, dairy cows produce milk every day that must be processed immediately – if a farm is quarantined or a plant shuts down, milk spoils quickly and has to be dumped.”
Bruno recommended that producers diversify processing options, review insurance coverage and strengthen biosecurity measures against threats such as screwworm and avian flu.
The New World screwworm affects all warm-blooded animals but appears to prefer bovines. Gaby Maier, Cooperative Extension specialist at UC Davis School of Veterinary Medicine, explained: “It can affect and kill any warm-blooded animal – any bird, any mammal, including humans… But we are particularly concerned about our cattle here.”
Maier cited data from Panama showing that 83% of recent screwworm cases were found in bovines. The larvae enter through wounds or body openings before burrowing into flesh with strong mouth hooks. Infestation leads to tissue destruction followed by secondary infection; death can occur within seven to fourteen days.
“I have talked to people who have seen these infestations in person… They say it was horrible – the smell of decomposing flesh in a live animal,” Maier said.
Although eradicated from California and southern U.S. states since the 1960s through release of sterile male flies—a method still maintained by a USDA facility in Panama—recent outbreaks may be linked to factors such as illegal cattle trade across borders or decreased inspections during COVID-19.
In response to renewed risk near U.S. borders, authorities have closed southern land borders to movement of cattle and horses while increasing surveillance efforts. Confirmed cases trigger releases of sterile flies; new drugs have also received conditional authorization for prevention and treatment.
A new website (screwworm.gov) has been launched with updates for both producers and the public.
Maier advised industry professionals: “Monitor the situation and stay up-to-date – if the screwworm is detected, it will be widely publicized so just watch out for those announcements.” She also cautioned against unnecessary use of dewormers or medications before confirmation of detection to prevent resistance development.
More than 100 people attended a recent webinar on screwworm hosted by UCCE advisors alongside partners like the California Cattlemen’s Association; monitoring projects are underway at UC Riverside for early detection efforts.
Producers are urged to closely observe their herds for signs such as distress or rotting odor—another indicator of infestation—and report suspected cases immediately to veterinarians or state officials. Preventing wounds on animals is key; production practices like dehorning or ear tagging should be managed carefully as they create entry points for larvae.
“This is definitely something we want to keep out at all cost,” Maier emphasized. “It’s an animal welfare problem, it’s an animal movement problem, and it will probably be a trade problem – overall, there is nothing good about it.”
