New World Screwworm: What Has Changed and What California Dairy Producers Should Know
By: Nick Kimutis, MPH, Livestock Programs, & Dr. Nicki Humphrey, Cattle & Swine Specialist, Animal Health Branch, CDFA
New World Screwworm (NWS) has been the subject of recent CDQAP newsletter updates as detections in Mexico move northward. Earlier articles described what NWS is and what producers should look for. This update focuses on what has changed and what California dairy producers need to know now.
NWS is not currently present in California or the United States.
Where Things Stand in Mexico
As of January 29, the most recent confirmed detections of NWS in Mexico have been reported in the state of Tamaulipas, approximately 187 miles from the United States border. None of the animals associated with these cases had any reported history of movement outside of Tamaulipas. The United States Department of Agriculture (USDA) is working closely with Mexico’s National Service for Agro-Alimentary Health, Safety, and Quality (SENASICA) to gather additional information and assess these cases.
In total, there are currently 8 active cases reported in Tamaulipas and approximately 870 active cases reported across Mexico involving animals. Individual case counts change over time. The broader trend remains important and reflects continued northward movement of NWS from areas where it had been successfully controlled for decades. Updated information is available through the USDA’s NWS website.
Government Response and Surveillance Efforts
United States ports remain closed to imports of cattle, bison, and horses from Mexico. These restrictions are intended to reduce the likelihood that NWS could be introduced into the United States through the movement of animals before infestations are detected and treated. Movement controls and sterile fly technique remain the most effective tools for limiting NWS spread. Because NWS most often spreads long distances through the movement of infested animals, these controls remain a cornerstone of containment efforts.
Mexican authorities recently reported that construction of a sterile fly production facility in Metapa, Mexico is 48% complete and on track to come online by mid-summer. The facility will initially produce approximately 60 million sterile flies per week, with plans to gradually increase output to its full capacity of 100 million flies per week. This expansion will complement existing sterile fly production in Panama.
Progress continues on U.S. operations at Moore Airbase in Texas. In the near term, the facility is being prepared to serve as a sterile fly dispersal site. USDA expects to move into the building by the end of the month, which will allow the facility to receive pupae produced in Panama and begin staff training for fly dispersal operations.
Looking ahead, Moore Airbase is also planned to transition into a sterile fly production facility. USDA anticipates awarding a contract for production between early February and late March, with sterile fly production targeted to begin in 2027. Production is expected to start at lower volumes and gradually scale up over time as operations are established.
Additional information on sterile fly production and dispersal facilities is found here.
To enhance early detection, CDFA and the USDA initiated active NWS surveillance along the California-Mexico border in October 2025 with the placement of eight fly traps. Four Swormlure traps were placed in Imperial County and four in San Diego County. Swormlure traps use an attractant that mimics the scent of animal wounds to lure adult screwworm flies. CDFA continues to monitor these traps and submit collected flies for identification twice weekly as conditions permit.
What CDFA Is Doing for California Producers
CDFA continues to coordinate closely with federal partners and other states as the situation evolves. CDFA convened an NWS workgroup to develop a response plan and other draft guidance documents. Current resources include infographics comparing NWS to other flies, protocols on NWS disposal and cleaning for livestock and horses, sample collection procedures, and a NWS fact sheet. Additional materials, including Spanish translations, are under development.
What to Expect
If California confirms a case of NWS, movement controls will require animals leaving infested zones to undergo thorough inspection and, if necessary, treatment. Animals must have proper documentation, which includes inspection certificates for in-state movement and certifications of veterinary inspection for interstate movement. Any animals found to be infested with NWS will not be permitted to move until they have been treated and cleared of NWS infestation.
Recent detections in Mexico have included neonatal calves, highlighting the importance of early wound management. In affected areas, producers would likely be advised to apply prophylactic treatment to newborn calves and use dipping or spray with Catron IV or another topical insecticide labeled for NWS to promote rapid drying of the umbilical area and reduce the risk of infestation.
Treatment Considerations
Treatment options for New World screwworm have expanded, but important limitations remain. As of today, two products currently have conditional approval from the FDA for use against NWS larvae in cattle.
Dectomax CA1, a doramectin injectable solution, is conditionally approved for the prevention and treatment of infestations caused by Cochliomyia hominivorax larvae and for prevention of reinfestation for up to twenty-one days in cattle.
Exzolt Cattle CA1 is conditionally approved for the prevention and treatment of NWS larvae and for treatment and control of cattle fever tick, with use limited to beef cattle two months of age and older and replacement dairy heifers less than twenty months of age.
Treatment options for lactating dairy cattle remain limited due to milk residue and withdrawal considerations, and all applications must follow label directions and veterinary guidance. CDFA recommends producers work with their veterinarian now to develop treatment plans should they end up having an NWS infestation. The most current information on approved and conditionally approved treatments is available from the FDA here.
What Producers Should Be Doing Now
For California dairy producers, the current risk of NWS remains low. However, preparedness and awareness are essential. Producers are encouraged to monitor alerts from the CDFA and USDA, maintain strong fly control and wound management practices, be mindful of animal movement especially from affected regions, and report any unusual wounds or maggots promptly to their herd veterinarian or CDFA Animal Health Branch District Office.
Learn More
For the latest updates, guidance, and producer resources related to NWS, please visit the CDFA NWS website.
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Watch for Nesting Tricolored Blackbirds
Nesting season for the threatened tricolored blackbird occurs in late February through early June. As wetlands have continued to disappear, dairy forage fields have become a favorite nesting place for the species. Typically, large colonies of thousands of birds will make their nests on a dozen or more dairies across the Central Valley. In more recent years, the birds have been nesting in smaller colonies, across more dairies. The birds’ nesting in forage fields coincides with harvesting season, forcing delay or cancellation of harvests. A nesting colony may occupy a field for 45-90 days, depending on how synchronized the birds are.
Think You Might Have the Birds? Since the species is listed as threatened, it is unlawful to harm or harass tricolored blackbirds. If you believe you might have tricoloreds nesting in your forage fields, you can contact NRCS or partners at Audubon California to have a confidential assessment. Since 2015—through partnership with dairy and conservation groups—NRCS has provided funding to help recoup some of the financial losses to dairies.
Feel free to contact Ian Souza-Cole with any questions: ian.souzacole@audubon.org. You can also contact your trade association or cooperative field representative.
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Register Today for the March 24th California Dairy Sustainability Summit
The 2026 California Dairy Sustainability Summit will take place on Tuesday, March 24 at the Visalia Convention Center. Industry leaders, policymakers, dairy farmers, and other leading experts will take the stage to talk about the future of California dairy, efforts to address water scarcity, changing regulations, and the latest strategies for improving economic and environmental sustainability.
Registration for dairy farmers is $50 and includes full access to sessions, breakouts, the exhibit hall, breakfast, lunch, and the closing reception. This one-day event is designed to equip dairy farmers with helpful information, while promoting collaboration. Speakers will share insights about critical issues, especially the agriculture industry’s ongoing strategy to secure a better water future. Attendees will learn about innovative technologies and on-farm strategies for efficiency, nitrogen management, methane reduction, animal health, and more. Experts will provide both local and global perspective on dairy sustainability, as well as the changing food system landscape, human health and wellness priorities, consumer behavior, and where dairy fits in.
The Summit is co-hosted by Dairy Cares, California Dairy Research Foundation, California Milk Advisory Board, California Dairy Quality Assurance Program, and the Dairy Council of California.
Learn more and Register Today: CADairySummit.com
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