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New World Screwworm in Mexico
Rosslyn Biggs, DVM, OSU College of Veterinary Medicine, Extension Beef Cattle Specialist
Cattle producers face an ever-evolving list of challenges in maintaining the health and safety of their herds. Now, a threat has reemerged in Mexico that could impact the livelihood of many ranchers—New World Screwworm (NWS). This parasitic fly is known for infesting open wounds on livestock, especially cattle. NWS also poses a significant threat to other mammals, including humans, and occasionally, birds.
In November 2024, reports confirmed that the pest had resurfaced in the southern Mexico state of Chiapas. Prior to this time NWS had effectively been eradicated from the continental United States (US) since the 1970s with partnerships between the US and Mexico pushing the pest to Mexico’s southern border by 1986. Although due to the pest’s ability to be unknowingly transported there have been isolated incidents of NWS in the US such as the 2016 identification in the Florida Keys in Key deer, pets, and swine.
NWS, Cochliomyia hominivorax, is a parasitic fly native to parts of Central and South America. Its larvae infest open wounds on mammals and feed on live tissue, unlike other maggots that feed on dead tissue. Infected animals experience significant pain, swelling, foul odor, and infection. If left untreated, the infestation can lead to severe tissue damage and even death within two weeks.
A female fly typically lays eggs near open wounds, mucous membranes, or body orifices. In cattle, the primary risk of screwworm infestation comes from exposed tissue such as areas created during branding, tagging, dehorning, or castration. Even minor cuts and the umbilicus of newborns are vulnerable.
The NWS female fly only mates once in its lifetime. With this understanding, the sterile insect technique has been utilized for eradication of the pest. The US and Panama operate the sterile male fly production facility in Panama through the US Commission for Eradication and Prevention of Screwworm. This facility has historically produced 100 million sterile flies per week. Male flies are irradiated at the facility and then released to mate with wild females. Over time, along with stringent treatment of infected animals and movement restrictions, the fly was maintained at the biological barrier in Panama. Multiple factors played a role in the reemergence of the pest into Costa Rica, Nicaragua, Honduras, Guatemala, El Salvador, Belize and now Mexico.
To protect the US, ruminant movement from Mexico was halted after the identification of NWS in the country. In February 2025, the USDA announced a protocol to resume imports. The comprehensive protocol involves significant inspection and treatment procedures. Additionally, the USDA is working to release sterile flies at strategic locations in Mexico and Central America.
NWS is a state and federally reportable Foreign Animal Disease in the US. If producers suspect NWS, they should contact their veterinarian and animal health authorities for instructions on how to submit samples for confirmation.
The resurgence of the New World Screwworm is a real threat to the cattle industry. By taking proactive steps to protect livestock, cattlemen and animal health officials can help prevent the spread of this destructive pest.
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