The New World screwworm is a parasite that doesn’t just infect livestock — it can cost ranchers thousands per animal in treatment and lost productivity. Once eliminated from the continental US through a decades-long sterile-fly program, it’s back, and federal agencies are now in full response mode.
The Customs and Border Protection agency is actively supporting the federal screwworm response, mobilizing resources to stop the fly at the border before it can establish itself in US herds. CBP is facilitating the movement of sterile flies, training frontline personnel, and sharing critical intelligence with livestock producers and USDA officials.
The screwworm (Cochliomyia hominivorax) is a flesh-eating parasitic fly native to Central and South America. When a female lays eggs in open wounds on livestock — or occasionally humans — the larvae burrow into living tissue, causing severe infection and often death if untreated. For decades, the fly was nearly eliminated from US territories through the Sterile Insect Technique, a biological control program that releases millions of sterile males to prevent reproduction. The program worked so well that screwworm became a non-issue for American ranchers.
But climate instability is changing the equation. Warming trends have expanded the screwworm’s range northward, and its proximity to the porous southern border means livestock operations across the Southwest are now at risk. Unlike a market downturn or equipment failure, a screwworm infestation is an immediate, cascading crisis: animals must be treated with costly larvae-killing insecticides, wounds must be managed to prevent secondary infection, and affected livestock can become unmarketable.
For family operations and smaller ranches, a single outbreak can mean the difference between survival and bankruptcy. Large industrial feedlots have the capital and infrastructure to absorb treatment costs and implement quarantine protocols. Smaller producers, especially in rural border communities where ranching is a cultural and economic cornerstone, are far more vulnerable.
The federal response signals the severity of the threat. Beyond CBP’s border coordination, the USDA’s Animal and Plant Health Inspection Service (APHIS) is expanding the sterile-fly release zones and intensifying surveillance at ports of entry. But keeping a parasitic fly out of an entire continent is a race against climate and biology.
Ranchers should watch for the telltale signs of screwworm infestation: open wounds that suddenly worsen, foul-smelling discharge, and dramatic animal distress. Early detection and treatment are critical. The USDA recommends immediate wound management and insecticide application, followed by veterinary supervision to prevent secondary infection.
For now, the federal screwworm response remains coordinated and proactive. But as climate zones shift and the border remains porous, this is a reminder that the threats facing American agriculture are not just economic or regulatory — they’re biological, and they’re arriving from the south on the wings of a warming world. The sterile-fly program that once saved US ranching may need to do so again, this time against longer odds.
Related coverage: NRCS staffing cuts undermine farm resilience, farmworkers facing health hazards from climate heat, Trump administration’s dismantling of water protections, and how automation is reshaping farm labor.
