U.S. Closes Southern Border to Livestock Imports Amid New World Screwworm Fly Threat

The U.S. has taken the precautionary measure of closing its southern border to livestock imports once again, citing the northward spread of a flesh-eating parasite in Mexico. The parasite in question is the New World screwworm fly, which lays eggs in wounds on warm-blooded animals. The larvae that hatch from these eggs are unique among flies for their habit of feeding on live flesh and fluids rather than dead material. This poses a significant threat to the U.S. beef industry, as the economic losses from such an infestation could be substantial. The U.S. had previously eradicated the pest in the 1970s through a program of breeding and releasing sterile male flies to mate with wild females, effectively controlling the population. However, the fly has since resurfaced in southern Mexico, prompting renewed concerns.

The U.S. Department of Agriculture (USDA) had initially closed the southern border in May to imports of live cattle, horses, and bison. However, in late June, it announced plans to reopen three ports of entry this month and another two by September 15. This decision was based on the assumption that the parasite had been contained. Unfortunately, recent reports indicate that an infestation has been detected 185 miles northeast of Mexico City, approximately 160 miles further north than previously reported cases. This location is about 370 miles from the Texas border, raising alarms about the potential spread into the U.S.

In response to the new developments, the USDA has closed the border again, emphasizing the need for vigilance. Secretary Brooke Rollins stated that aggressive monitoring by USDA staff in both the U.S. and Mexico has enabled quick and decisive action to respond to the spread of this deadly pest. The USDA's plan includes spending nearly $30 million on new sites for breeding and dispersing sterile male flies. These sterile males, once released into the wild, would mate with females, causing them to lay eggs that won't hatch, thereby reducing the fly population. The USDA hopes to have a new fly factory operational in southern Mexico by July 2026, supplementing fly breeding at an existing complex in Panama. Additionally, a site in southern Texas is planned for holding sterile flies imported from Panama, ready for release along the border if necessary.

Mexico's President Claudia Sheinbaum has criticized the U.S. decision, suggesting that the threat to the beef industry is being exaggerated. Sheinbaum asserted that Mexico is following all established protocols to deal with the northernmost case of the infestation. Mexican authorities reported that the country has 392 infected animals, a decrease of nearly 19% since June 24. Sheinbaum emphasized that all scientifically necessary measures are being taken to control the situation.

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 fly infestations in livestock. They highlighted that current labeling requirements are preventing the use of these treatments, which could be crucial in mitigating the spread of the parasite. The situation underscores the complex interplay between agricultural health, economic interests, and international relations, as both countries work to contain the threat posed by the New World screwworm fly.