U.S. and Mexico Unite Against New World Screwworm Threat

Despite ongoing trade discussions, the United States and Mexico have found common ground in their fight against a shared enemy: the New World Screwworm (NWS). This parasitic fly, known for its devastating impact on livestock and other mammals, poses a significant threat to both nations’ economies and animal health.

The NWS is a fly whose female lays eggs in open wounds or body openings of mammals, including cows, deer, birds, pets, and even humans. Once hatched, the larvae, or maggots, burrow into the wound, feeding on the host’s tissue. This "screwing" action, characterized by the use of sharp mouth hooks, causes extensive damage and can lead to severe infections, potentially resulting in death.

The United States successfully eradicated the NWS parasite in 1966 through the release of sterile male flies and pupae. This strategy aimed to disrupt the reproductive cycle of the fertile screwworm flies, eventually leading to their elimination. The U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) spearheaded this effort.

However, the threat of reintroduction remains a concern, as the NWS parasite persists in Central and South America, creating the potential for northward spread into Mexico and, subsequently, the United States.

In November 2024, the United States temporarily halted livestock imports from Mexico after a NWS detection in a cow at an inspection checkpoint near the Mexico-Guatemala border. In response, the USDA increased the delivery of sterile flies and bolstered surveillance programs. These measures proved effective, and livestock imports resumed in February 2025.

Recently, tensions arose in April 2025 when Mexican authorities began curtailing flights of USDA planes dispersing sterile flies, limiting them to 7-day stays. Additionally, Mexico imposed import duties on essential aviation parts, dispersal equipment, and sterile fly shipments. These actions prompted a letter from U.S. Agriculture Secretary Brooke Rollins to Esteban Moctezuma Barragán, Mexico’s ambassador to the U.S., expressing concerns about the potential impact on NWS eradication efforts.

Fortunately, a potential U.S. livestock ban was averted when Mexico reached an agreement to allow additional flights and waive custom duties. This resolution, announced by Secretary Rollins on April 30, highlighted the importance of cooperation in addressing this shared threat.

"I am happy to share Mexico has continued to partner in emergency efforts to eradicate the New World Screwworm," Rollins stated in a press release. "This pest is a devastating threat to both of our economies, and I am pleased to work together with Mexico in good faith to protect the livelihoods of our ranchers and producers who would have been hurt by this pest."

Adult screwworm flies share similarities with common houseflies, but they possess distinct characteristics. They are roughly the same size or slightly larger, with orange eyes, a metallic blue or green body, and three dark stripes along their backs, according to APHIS.

The larvae’s feeding behavior is what gives the parasite its name. As they consume the host’s tissue, the maggots burrow or "screw" into the wound or opening, causing extensive damage with their sharp mouth hooks.

Following the report of an infected cow in Mexico, APHIS issued a warning to states along the southern border, urging them to monitor livestock and pets for signs of NWS infestation. APHIS and the Texas Parks and Wildlife Department have outlined several warning signs in wildlife, birds, and pets.

The collaborative efforts between the United States and Mexico are crucial in containing and eradicating the NWS parasite. The economic implications of an uncontrolled outbreak could be devastating, impacting livestock production, trade, and the overall health of both nations’ agricultural sectors. The ongoing partnership, as demonstrated by the recent agreement on flight access and customs duties, is essential for safeguarding the livelihoods of ranchers and producers on both sides of the border. The timely detection and reporting of potential infestations are key to preventing widespread outbreaks. By working together, the U.S. and Mexico can protect their animal populations and economies from the threat of the New World Screwworm.