New World Screwworm Returns to U.S., Triggering Broader Surveillance Measures

Federal agricultural researchers have verified a new wave of New World screwworm outbreaks in multiple states, reigniting worries over the parasite’s ability to spread among U.S. livestock and wildlife.

The New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae consume living flesh, producing painful lesions that may progress to serious infection or fatality without treatment. It chiefly attacks cattle, sheep and goats, and occasionally wildlife, resulting in considerable economic damage for producers.

While a vigorous eradication effort during the 1960s and 1970s removed the pest from the contiguous United States, sporadic re‑emergences have been noted in recent years, highlighting the vulnerability of that achievement. This latest cluster of incidents may indicate a gap in the surveillance system that has safeguarded the country for decades.

Scientists from the USDA’s Agricultural Research Service are now employing an array of monitoring tools to chart the flies’ spread. By examining the genetic profiles of collected specimens and integrating those results into geographic information system (GIS) models, they aim to trace the insects’ pathways and locate hotspots where future outbreaks are most probable.

Veterinary authorities emphasize that early detection is crucial. Rapid treatment of afflicted animals can curb transmission to neighboring herds, and increased vigilance by ranchers can accelerate reporting of questionable wounds.

The USDA is working with state agriculture agencies to broaden trap networks and to evaluate reinstating the sterile‑insect technique—a strategy that previously eradicated the pest by releasing millions of sterile males to interrupt reproduction.

Ongoing surveillance and swift action will be vital to prevent the screwworm from re‑establishing a foothold, protecting both animal health and the agricultural economy.