US-Mexico Border Re-Closes to Livestock Amid Screwworm Threat

The intricate dynamics of cross-border agricultural trade between the United States and Mexico have once again faced significant disruption, following the U.S.’s re-closure of its southern border to livestock imports. This recent action, prompted by the detection of the New World screwworm fly (Cochliomyia hominivorax) migrating further north within Mexico, underscores persistent biosecurity challenges and highlights a growing diplomatic tension over the perceived severity of the threat to the American beef industry.

• The U.S. re-closed its southern border to livestock imports due to a New World screwworm fly (NWSF) infestation.
• The NWSF was detected approximately 185 miles northeast of Mexico City, significantly closer to the U.S. border.
• USDA Secretary Brooke Rollins announced a nearly $30 million investment for new sterile fly breeding and dispersal facilities.
• Mexican President Claudia Sheinbaum characterized the U.S. decision as “totally exaggerated,” citing a 19% reduction in infected animals since June 24.
• The USDA plans to open a new fly factory in southern Mexico by July 2026 and establish a sterile fly holding site in southern Texas.

The Resurgence of a Costly Pest

Historically, the screwworm has posed a considerable economic threat to livestock populations. Female flies lay eggs in open wounds of warm-blooded animals, with the hatching larvae uniquely feeding on live flesh and bodily fluids, rather than necrotic tissue. This parasitic activity can lead to severe debilitating conditions and fatalities in livestock, causing substantial economic losses, as evidenced by outbreaks in the U.S. decades ago. The United States successfully eradicated the pest in the 1970s through a sophisticated sterile insect technique, involving the release of sterile male flies to prevent successful reproduction in wild populations. Since then, the fly had largely been confined to Panama until its recent re-emergence in southern Mexico late last year.

Escalating Border Restrictions and Strategic Responses

The U.S. Department of Agriculture (USDA) initially closed its southern border to live cattle, horses, and bison imports in May. While the USDA announced on June 30 plans to reopen select ports of entry in July and September, these intentions were swiftly reversed following a new, alarming development. A screwworm infestation was reported approximately 185 miles northeast of Mexico City, a location significantly further north than previous cases and roughly 370 miles from the Texas border. This discovery prompted an immediate and decisive re-closure of the border.

USDA Secretary Brooke Rollins affirmed the agency’s steadfast commitment to protecting the nation’s livestock. “Thanks to the aggressive monitoring by USDA staff in the U.S. and in Mexico,” she stated, “we have been able to take quick and decisive action to respond to the spread of this deadly pest.” In a strategic move to bolster containment efforts, Secretary Rollins announced plans for a nearly $30 million investment dedicated to establishing new facilities for breeding and dispersing sterile male flies. This initiative aims to intensify the disruption of the screwworm’s reproductive cycle, thereby reducing the wild population and mitigating the risk of spread.

Mexico’s Counter-Narrative and Data

Conversely, Mexican President Claudia Sheinbaum voiced criticism regarding the U.S. decision, describing it as “totally exaggerated.” She asserted that Mexican authorities are rigorously adhering to all established protocols to manage the northernmost infestation. President Sheinbaum also provided data indicating a positive trend in Mexico, reporting 392 infected animals, which represents a nearly 19% reduction since June 24. This suggests, from Mexico’s perspective, that domestic containment measures are proving effective in controlling the outbreak.

Future Initiatives and Policy Calls

Looking ahead, the USDA’s long-term strategy includes the operationalization of a new fly factory in southern Mexico by July 2026, designed to supplement the output from an existing complex in Panama. The agency also plans to establish a holding site in southern Texas for sterile flies imported from Panama, enabling rapid deployment along the border if necessary. Concurrently, U.S. Representatives Tony Gonzalez of Texas and Kat McCammack of Florida have urged the Trump administration to expedite the approval of existing anti-parasite treatments for New World screwworm infestations in livestock, noting that current labeling requirements hinder their immediate and effective use in addressing the threat.