California livestock producers are being urged to remain vigilant as the New World screwworm, a parasitic insect known for causing severe losses in cattle, has been detected near the U.S.-Mexico border. Scientists from UC Agriculture and Natural Resources (UC ANR) have provided resources and guidance to help prevent an outbreak.
“This is something we definitely want to keep away at all costs,” said Gaby Maier, Cooperative Extension specialist in beef cattle health and production at UC Davis School of Veterinary Medicine. She referred to the New World screwworm, which has resulted in thousands of cattle deaths and significant financial losses for livestock industries south of the border.
“It’s an animal welfare issue, it’s a problem related to animal transport and probably also will be in the commercial realm; overall, there’s nothing good about it,” Maier added during a recent webinar attended by over 100 livestock producers and stakeholders.
While the pest has not yet been found in any U.S. state, concerns are high among California’s agricultural community. In Texas, a potential outbreak could cost ranchers $732 million annually and lead to $1,800 in losses per affected operation, according to estimates from the United States Department of Agriculture.
The New World screwworm is a larva of a blue fly that invades animal tissue. It was recently detected just 70 miles from the U.S. border in Mexican cattle and confirmed again later that year in Nuevo León, adjacent to Texas. The parasite has caused more than 144,000 cases in animals and over 1,100 cases in humans since its resurgence began in 2023 across Mexico and Central America.
In California, dairy products led agricultural output in 2024 with $8.61 billion, while cattle ranked fourth at $4.98 billion—both sectors potentially threatened by the pest.
“Most questions I’m receiving are about control and prevention of the impact of the New World screwworm,” said Brooke Latack, livestock advisor for UC Cooperative Extension serving Imperial, Riverside and San Bernardino counties. Imperial County alone has about 329,000 head of cattle.
“There is definitely concern among producers, industry members and veterinarians who serve this area,” Latack said.
For dairy operations, even small disruptions can cause substantial financial harm due to daily milk production requirements. “Unlike beef cattle that can be kept off market during quarantine, dairy cows produce milk every day that must be processed immediately—if a farm is quarantined or a plant closes, milk spoils quickly and must be discarded,” explained Daniela Bruno, UCCE dairy advisor for Fresno, Madera and Kings counties.
Bruno recommended producers diversify processing options, review insurance coverage and strengthen biosecurity measures against threats like screwworms or avian influenza.
Screwworm larvae can kill newborn calves with mortality rates reaching up to 100%. “It can affect and kill any warm-blooded animal—any bird or mammal including humans,” Maier stated. “But we’re particularly concerned about our cattle here.”
Maier cited data from Panama showing that 83% of recent screwworm cases were found in cattle; human cases remain rare but possible.
Infestation begins when flies lay hundreds of eggs on an animal’s wound or natural opening; larvae then burrow into flesh using sharp hooks. As they feed on tissue, wounds expand rapidly leading to secondary infections that can kill within two weeks.
“I’ve spoken with people who have seen these infestations firsthand from their youth,” Maier said. “They say it’s horrible—the smell of rotting flesh on a living animal.” She added: “They just keep digging; they keep eating tissue—it’s something out of a horror movie.”
Though once endemic across California and southern states until eradication campaigns succeeded decades ago using sterile male fly releases—a method still employed by USDA facilities in Panama—new outbreaks are attributed to factors such as illegal livestock trade across borders or reduced inspections during COVID-19 disruptions.
In response to recent detections near the border region—including closures for livestock movement—USDA agencies have intensified surveillance efforts while releasing sterile flies as containment strategy. Conditional approval was also granted for new preventive drugs. Updates are available at screwworm.gov.
Experts advise meat and dairy producers—as well as those raising sheep or goats—to stay informed through official channels like USDA websites or state agriculture department pages. Producers should monitor animals closely for signs such as drooping ears or lagging behind herds; foul odors may signal infestation before visible symptoms appear.
“Monitor the situation and stay updated—if screwworm is detected it will be widely publicized so watch for those announcements,” Maier advised.
Producers are encouraged not to use antiparasitic drugs unless detection is confirmed locally to avoid resistance development among parasites.
Preventing wounds remains key: procedures like dehorning or castration increase risk periods for infestation; shifting calving seasons outside peak fly activity may reduce exposure since screwworms do not thrive under extreme temperatures.
If infestation is suspected producers should contact veterinarians promptly for treatment options and notify state authorities such as CDFA.
UC Agriculture and Natural Resources shares science-based practices statewide through research centers located across diverse ecosystems (https://ucanr.edu/). The organization maintains offices in all California counties along with nine Research and Extension Centers serving as living laboratories (https://ucanr.edu/). Affiliated with the University of California (https://ucanr.edu/), UC ANR applies university expertise via outreach initiatives supporting communities throughout California (https://ucanr.edu/).
Glenda Humiston serves as vice president of UC Agriculture and Natural Resources (https://ucanr.edu/).
For further information on resources related to nutrition networks or youth education developed over more than a century by UC ANR visit their official website.