The threat of New World Screwworm (NWS), Cochliomyia hominivorax, continues to plague cattle producers in Mexico. Eradicated from the continental U.S., this parasitic fly poses growing risks to livestock and wildlife as its range has expanded moving northward in Mexico. Additionally, as of June 2025, the Mexican Health Ministry has confirmed 13 human cases of screwworm infestation.
In November 2024, NWS reappeared in Mexico, marking its first confirmed sighting since U.S. eradication efforts pushed it south in the 1970s. To protect the U.S., ruminant movement from Mexico was halted. In February 2025, live ruminant imports briefly resumed under a strict pre-clearance protocol involving ivermectin injections, dip treatment, inspections, and a mandatory holding period on the Mexican side of the border. However, the fly has now spread closer, approximately 700 miles from the U.S. border, effectively a trailer ride from southern border ports. On May 11, 2025, U.S. Secretary of Agriculture Brooke Rollins announced the suspension of live cattle, horse, and bison imports from southern border ports into the U.S.
The NWS 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 injuries, such as a tick bite, and the umbilicus of newborns are vulnerable.
The NWS female fly only mates once in its lifetime. Control measures have historically focused on the sterile insect technique by releasing sterile male flies. The USDA is evaluating and supporting control measures in collaboration with Mexico as the fly continues to move north. In the U.S., the USDA, state animal health officials, and industry stakeholders are collaborating to develop educational materials and response plans, including treatment protocols, in preparation for potential NWS outbreaks.
So, what can cattlemen and other animal owners do to prepare?
· Examine animals daily, especially after processing.
· Build a relationship with a veterinarian and have a plan in place for communication and treatment if an animal shows signs of illness or disease.
· Utilize biosecurity protocols to limit animal movement and unnecessary exposure to disease.
· Stay alert of disease announcements in the operation’s area.
· Report clinical signs consistent with NWS to a veterinarian or the State Veterinarian’s office.
The resurgence of NWS in Mexico presents a serious threat to North American livestock, wildlife, and humans. However, coordinated, science-backed responses—sterile fly releases, border controls, investment in facilities, and sustained surveillance—offer a path to pushing this dreaded pest back to its endemic zones. Oklahoma Department of Agriculture, Food & Forestry, OSU Extension and our industry partners are working through Oklahoma specific response plans and
developing resources to ensure livestock producers are prepared in the event NWS is found in the state.
