The Swine Health Information Center in collaboration with the American Association of Swine Veterinarians hosted a webinar on June 13, 2025, entitled “Emerging Risk of New World Screwworm (NWS) and Efforts to Prevent Re-introduction into the US.” The webinar had 727 registrants from around the globe, 529 attended live from 18 countries, and it is now available for on demand viewing. The webinar provided the latest information on NWS, including an overview of the parasite, the fly lifecycle, clinical signs of infestation, impact to producers, Texas prevention and preparedness activities, and a status update for NWS in Mexico. To date, NWS has not been identified in the US since it was eradicated from the Florida Keys in 2017.
The identification of NWS in Oaxaca and Veracruz, within 700 miles from the US/Mexico border, raises concern about this potential emerging threat to swine health. Increasing awareness and understanding of this parasitic fly as well as prevention and mitigation steps for NWS infestation help US pork producers protect their herd health.
Presenters offering their expertise included:
Cody Egnor, DVM, veterinary medical officer, United States Department of Agriculture
Nancy Adams, DVM, Texas Animal Health Commission Epidemiology Department
Kathy Simmons, DVM, chief veterinarian, National Cattlemen’s Beef Association
Moisés Vargas-Terán, DVM, international animal health expert consultant for OMSA, FAO, IAEA, WHO, and the IDB
The myiasis caused by New World Screwworm, Cochliomyia hominivorax, is a parasitic fly whose larvae infest and feed on the living tissue of warm-blooded animals, including humans. In his presentation, Dr. Egnor emphasized that, unlike other flies, NWS maggots feed exclusively on living tissue. Female flies lay 200-300 eggs around wounds or natural orifices like the eyes, ears, nose, anus, umbilicus, and genitalia. These eggs hatch within 12 to 24 hours, and the emerging larvae burrow deep into the flesh, tearing at tissue with hook-like mouth parts, leading to severe inflammation and secondary infections. The entire life cycle can be as short as 21 days in tropical climates.
Dr. Egnor highlighted clinical signs of NWS infestations characterized by visible larvae in the wound, often numbering in the hundreds, accompanied by a bloody discharge and foul odor. Affected animals can exhibit depression, reduce feed intake, self-isolate, and show agitated behavior like head shaking. Untreated infestations can be fatal within seven to 14 days, with nearly 100% mortality in newborns with umbilical infestations. NWS is not contagious from animal to animal, but multiple animals in a group can become infested. Multiple infestations in a single wound are common and can attract other blowfly species. While roughly 84% of cases are in cattle, 4% of NWS infestation cases have been identified in swine. Humans can also be infested by NWS. A review of animal husbandry practices such as branding, ear tagging, tick control, wound care, castration, births, and performance of practices seasonally can help prevent infestation.
USDA has actively focused on the use of Sterile Insect Technique (SIT) to reduce and eliminate the flies in affected locations. This method involves the release of sterile flies into the wild population to reduce NWS wild flies’ reproduction and eventually eliminate wild flies from the environment. Prevention activities include robust regulatory controls, active field surveillance for myiasis, sterile flies, and stakeholder engagement. Dr. Egnor stated that NWS is a reportable disease in the US and USDA has resources available for producers to assist in the identification of the fly, how to report a suspect case, and information on USDA’s preparedness activities in the NWS Redbook.
Dr. Simmons provided a historical perspective of NWS in the US and its impact on the cattle industry, stating that NWS caused significant economic losses from the 1930s – 1970s. The US successfully eradicated NWS in 1966, though sporadic outbreaks occurred until the 1970s, with the last US outbreak in 2016 in the Florida Keys. Freedom from NWS provides an estimated $1 billion in economic benefits to US livestock producers and $3.7 billion to the general economy annually. If NWS was identified in the US, eradication would be a significant challenge due to the size of the US cattle herd, speed and distance of cattle movements, and the increased wildlife populations that interface with livestock. Mexico exports, on average, 1.2 million head of cattle to the US each year. The closure of the southern border to cattle, bison, and horses due to NWS in Mexico has affected the livelihood of many US cattle feeders who depend on Mexican cattle to fill their feedyards. This is especially significant due to the lowest numbers since 1951 for the US cattle herd.
Dr. Simmons highlighted the USDA’s efforts to prevent NWS introduction with increased measures initiated on February 1, 2025, to require a comprehensive treatment and inspection process for all cattle entering the US from Mexico to mitigate the risk of NWS. Further protection efforts occurred on May 11, 2025, when US Secretary of Agriculture Brooke Rollins announced a temporary border closure for live cattle, bison, and horses from Mexico. Additional efforts have been focused on the use of SIT which involves the propagation and release of sterile male flies into NWS activity areas, reducing the reproduction rate and causing eventual die-out of the flies. Currently, Panama has the only operating sterile NWS fly production site, producing flies at the rate of 100 million to 117 million sterile flies per week, but many more flies are needed to cover the expanded affected areas. Additional facilities to produce sterile NWS flies and approval of products to combat NWS are urgently needed, per Dr. Simmons.
Dr. Adams stated that the potential impact of NWS in the US would be substantial, with estimated costs to eradicate reaching $1.27 billion, and annual producer and consumer impact near $540 million. Losses attributed to NWS could include increased production costs due to animal deaths, decreased livestock production, and higher needs for veterinary services, medication, insecticides, and labor. The US cattle industry is particularly vulnerable, as it relies on Mexican cattle imports, which have been significantly reduced due to NWS mitigation protocols and border closures affecting cattle.
The Texas Animal Health Commission (TAHC) has developed resources for producers and veterinarians to assist in surveillance and rapid detection of NWS suspect cases. Protocols include detection of maggots deep in living tissues and wounds. Kits have been developed and can be provided for the collection of samples if NWS is suspected. NWS is a reportable disease in the US and veterinarians and producers are instructed to report any suspect cases to the appropriate state or federal animal health officials.
TAHC is focusing on preparedness efforts in collaboration with USDA APHIS and other agencies to refine response plans, develop strike teams, identify potential US sterile fly production facilities, and monitor SIT effectiveness. Outreach materials and training for veterinarians and producers are being developed to enhance awareness, reporting, and management of NWS. Producers are advised to regularly monitor animals, inspect pets and vehicles when traveling from infested areas, keep wounds clean and covered, and use repellents. It is critical not to move suspected infested animals.
Dr. Vargas-Teran reviewed the history of NWS control using SIT between Colombia and Panama, creating a barrier using sterile insects for prevent northern NWS flies’ incursion. A significant change occurred in Panama in April 2022 north of the control barrier, with the significant increase in NWS cases that reinfected the entire country, in 2023. NWS has since spread rapidly through Central America, reaching Costa Rica, Nicaragua, Honduras, Guatemala, El Salvador, and Belize. In July 2024, Mexico implemented an emergency prevention plan to prevent the disease introduction into the country.
However, on November 1, 2024, the first case was detected and confirmed in a quarantine station in Mexico in the Chiapas state, near the Guatemalan border. By May 2025, NWS had spread to seven Mexican states (Chiapas, Tabasco, Campeche, Quintana Roo, Yucatan, Oaxaca, and Veracruz). This prompted the suspension of live cattle, bison, and horse imports through US southern border ports due to the presence of NWS cases. Chiapas has the highest number of active cases with cattle being the primary species impacted.
Dr. Vargas-Teran noted that the response of veterinarians and producers in Mexico focus on modified timing of production practices, such as branding, to reduce the potential for NWS infestation. Affected animals are either treated with topical and/or injectable medications to kill the flies with additional wound management steps as needed. An increased veterinary workforce has been deployed to perform active surveillance in the field for NWS and collection kits for larvae are provided in the number of 150,000. Producer education and outreach are ongoing in the field, especially in areas with active cases. He noted that animals are not the only species affected as there have been at least eight human cases in Mexico and producers should be aware of this potential human health threat.
Presently, the goal of SIT in Mexico is to stop the northward progression of the disease with aggressive fly releases in affected areas and the establishment of a sterile fly barrier at the Tehuantepec Isthmus, to push back the disease to the international border between Panama and Colombia, where the previous sterile fly barrier was operating. It would take approximately eight to 12 months to have additional fly production available for use in Mexico to strengthen the strategy. Other control efforts are focused on animal quarantine and halting illegal animal movements.
For more information, resources are available from the USDA APHIS website on New World Screwworm, the TAHC NWS Fact Sheet, and the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG).
The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at [email protected] or Dr. Lisa Becton at [email protected].
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