The New World screwworm (Cochliomyia hominivorax) is a housefly-sized parasite that lays eggs in the wounds or mucous membranes of warm-blooded vertebrates (e.g., mammals, birds). The U.S. Department of Agriculture (USDA) eradicated screwworm from the United States in 1966, but recent cases in Mexico have raised concerns about possible new domestic cases. USDA considers screwworm a serious threat to livestock, estimating losses from a 1976 outbreak in Texas at $732 million and $1.8 billion to the state's livestock producers and economy, respectively, in 2024 dollars.
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Source: U.S. Department of Agriculture (USDA), National Agricultural Library, "Slide of Larvae Living in Wound." Note: "Screwworm" describes the larvae's appearance as it burrows into the host. |
Screwworm larvae feed on healthy, living tissue (see Figure 1), causing a painful condition known as "myiasis." Treatment may include wound care, surgical removal, and topical pesticide application. Left untreated, the affected host may die within 1-2 weeks.
Eradication
Previously limited to the U.S. Southwest, screwworm migrated to the Southeast via a shipment of infested animals in 1933. Entomologists at USDA's Agricultural Research Service spent decades developing and deploying the Sterile Insect Technique, which consists of releasing sterile male flies into affected areas to reduce screwworm reproductive success until the population dies out. The United States declared itself free of indigenous screwworm by 1966.
International Cooperation
In 1972, the United States experienced a widespread screwworm outbreak, spurring the establishment of a joint Mexico-United States Screwworm Eradication Commission. Both countries agreed to move the screwworm barrier zone from the southern U.S. border to the narrowest point in Mexico, the Isthmus of Tehuantepec.
The U.S.-Mexico commission continued its eradication efforts south of Mexico, ultimately setting a barrier zone between Central and South America at Panama's Darien Gap. In 1994, the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG) initiated plans for a facility capable of producing up to 100 million sterile flies weekly to maintain the barrier zone. Completed in 2006, the facility is managed jointly by Panama's Agriculture Ministry and USDA's Animal and Plant Health Inspection Service (APHIS). APHIS has funded 90% of COPEG's activities and estimated in 2025 that its investment has saved the U.S. cattle industry $2.3 billion annually.
Current Issues
In 2023, screwworm annual detections increased in Panama––from around 25 cases to more than 6,500 cases. Screwworm spread northward from Panama through Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador.
In November 2024, a screwworm case was reported in southern Mexico. In response, the United States suspended live animal imports from Mexico. After both countries agreed to enhanced preclearance inspection and treatment protocols, the southern border reopened to Mexico-origin cattle and bison in February 2025.
In April 2025, Secretary of Agriculture Brooke Rollins wrote to her counterpart in Mexico with concerns that Mexico was impeding eradication efforts. "Despite the fact that all materials and operations are being funded entirely by the [United States]," Secretary Rollins stated, Mexico's aviation authorities were imposing restrictions on "USDA APHIS-contracted" aircraft fleets, and its customs authorities were imposing import duties on aviation equipment and "sterile fly shipments."
USDA announced on May 11, 2025, that it would again suspend imports of live animals on a "month-by-month basis, until a significant window of containment is achieved." Mexico's President Claudia Sheinbaum reportedly called the action "unfair."
Mexico typically accounts for more than 60% of imported cattle, approximately 1.25 million head annually in 2023 and 2024 (Figure 2). The U.S. cattle herd, at 87 million head, is at its lowest level since 1951. Fewer cattle imports may further constrain supply, which could increase beef prices for U.S. consumers.
U.S. livestock stakeholder groups generally supported USDA's import suspension. One group acknowledged that although the border closure may "cause economic harm for U.S. farmers and ranchers" and "create supply chain disruptions," the costs would be less than responding to a U.S. screwworm outbreak.
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Figure 2. Cattle Imports from Mexico and Mexico's Share of Total Cattle Imports, March 2024-March 2025 |
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Source: USDA, Economic Research Service, "Livestock and Meat International Trade Data," updated May 7, 2025. Note: Shading represents U.S. suspension of live animal imports from Mexico from November 2024 to February 2025. |
Congressional Considerations
The Animal Health Protection Act (7 U.S.C. §§8301 et seq.) authorizes the Secretary of Agriculture to act to eradicate diseases or pests that threaten animal and public health, which includes restricting animal movement, destroying animals, and implementing preventive disease controls. As Congress monitors the northward spread of screwworm, it may consider various options, including maintaining the current approach, evaluating existing eradication efforts, determining program funding levels and staffing, and considering proposals for a domestic sterile fly-rearing facility.
APHIS received emergency funding of $109.8 million in 2023 and $165 million in 2024 from the Commodity Credit Corporation (CCC) for screwworm response activities. In May 2025, USDA announced an additional $21 million transfer of CCC funds to convert an existing fruit fly-rearing facility in Metapa, Mexico, into a sterile fly-rearing facility. For FY2025, APHIS requested an increase of $3.6 million to address aging infrastructure and higher operational costs at its Panama fly-rearing facility. A 2023 APHIS improvement plan included the Panama facility on a list of its three "highest priority facilities in need of repair."
Congress may assess domestic or international capacity needs for sterile fly production and dispersal. Some Members of Congress wrote to Secretary Rollins in March 2025 requesting USDA consider the feasibility of building a domestic sterile fly production facility. The United States operated a sterile fly-rearing facility in Texas from 1962 to 1982. Similar facilities exist today for research purposes, though none are configured for the current requested scale of production. Bills introduced in the 119th Congress (H.R. 3392, S. 1751) would provide a $300 million appropriations authorization for the construction of a screwworm fly-rearing facility in an at-risk state.