How a shortage of factory-bred flies left the US exposed to the flesh-eating screwworm

A shortage of factory-bred sterile flies has left the US exposed to the flesh-eating New World screwworm, which broke through a containment barrier in Panama in early 2023 and spread through Mexico. The Trump administration closed the border to cattle imports for over 15 months starting in May 2025. Mexico and Panama produce about 150 million sterile flies weekly, far below the 500 million benchmark. The US has no production plant; one in Texas is expected by spring 2027.

A shortage of factory-bred sterile flies has left the United States exposed to the flesh-eating New World screwworm, according to Reuters interviews with 15 current and former officials. The parasite broke through a containment barrier in Panama in early 2023 and spread through Mexico, reaching the US. The Trump administration closed the border to cattle imports for over 15 months starting in May 2025, disrupting a multibillion-dollar trade. Mexico and Panama currently produce about 150 million sterile flies a week, less than a third of the 500 million per week recommended as a benchmark. The US has no sterile-fly plant of its own; one under construction in Texas is not expected to start production until spring 2027. The outbreak has been hampered by political transitions, funding delays, and bureaucratic hurdles. Mexico has recorded about 51,000 cases, and the US has recorded 120 cases to date.