How did the New World screwworm invade the United States?

The New World screwworm, a parasitic fly whose larvae burrow into the flesh of warm-blooded animals, has been detected in 32 livestock across Texas since early June 2026, marking the first U.S. livestock cases in roughly five decades. The fly’s arrival and limited spread remain a mystery to health experts, who warn it poses a serious threat to the nation’s cattle herd. The U.S. Department of Agriculture has quarantined affected areas in Texas, restricted livestock movement from the state, and ramped up border and wildlife surveillance, including the use of drones and artificial intelligence. A new sterile screwworm fly production facility in Metapa, Mexico, opened in June 2026 to help curb the parasite’s reproductive cycle. The first confirmed case was a calf in Zavala County, Texas, at the start of June. Since then, detections have included sheep and goats, with 14 cases now listed as inactive as of July 5. A single infected dog in New Mexico—recovered and with no travel history to Mexico—raises concerns about undetected local transmission and a potentially wider presence of the pest in the Southwest. Officials and experts have dismissed the idea that the fly could have traveled long distances on its own, suggesting instead that infected livestock or humans moving animals were likely responsible for its entry and spread. Livestock shipments from Mexico, where screwworm presence accelerated in late 2024, have been largely halted since the fly was confirmed there. Still, alternative routes—such as wild animals or uninspected pets—remain possible. The USDA has added over 100 full-time employees dedicated to screwworm response in the past 15 months, and Agriculture Secretary Brooke Rollins has defended the agency against criticism that staff cuts and relocation plans weakened its preparedness. Meanwhile, political figures have framed the outbreak as a consequence of immigration policy or criticized the USDA’s response, while scientists emphasize that the actual transmission pathways remain speculative. Eradication efforts could take years, and understanding how the pest entered and spreads within the U.S. is critical to limiting damage while containment measures are scaled up.