Northward Spread of New World Screwworm Tests Sterile Insect Technique and Cross-Border Biosecurity

Overview

The New World screwworm, Cochliomyia hominivorax, is re-emerging as it spreads north from Central America into Mexico, approaching the U.S. border. The article discusses the sterile insect technique (SIT), historically used to eradicate the fly from North and Central America, and explains why SIT alone may not suffice under current conditions. It notes renewed SIT production efforts, including a new facility at Moore Air Base in Edinburg, Texas, and refurbishment of Mexican rearing facilities expected by 2026. The piece also emphasizes the need for international cooperation, sustained funding, and robust surveillance to prevent a costly reinvasion of the United States. Author: Nature

Overview

The article examines the northward spread of the New World screwworm (Cochliomyia hominivorax) from its historic eradication zone in the southern United States and Central America to two Mexican states that border Texas, Tamaulipas and Nuevo Leon. The parasite lays eggs in open wounds and in natural body openings of warm-blooded animals, including cattle and occasionally humans, with larvae causing myiasis and potentially death of the host. Historically, large-scale eradication in the 1950s through 1960s used the sterile insect technique (SIT): rearing, sterilizing, and releasing large numbers of sterile males so wild populations would fail to reproduce. This strategy, deployed widely in Florida and then westward, helped eliminate screwworm from the US, Mexico, and Central America, and a barrier at the Darién Gap in Panama helped maintain the containment for decades. Author: Nature

"SIT alone may not be enough to control the current outbreaks." - Nature

SIT and Eradication History

SIT operates on the principle of releasing sterile males that outnumber wild males, leading to a population crash and eventual local extinction. The approach requires robust mass-rearing infrastructure, safe sterilization (usually via radiation), and careful management of population genetics to avoid unintended ecological consequences. In the United States, SIT and accompanying surveillance enabled the country to push the fly back and keep it at bay for many years, including through coordinated efforts with Mexico and Central American countries. The Darien Gap barrier in Panama, maintained with continuous releases and surveillance, formed a critical transnational shield. The article notes these achievements as the pest became re-entrenched in the region, undermining previously successful barriers. Author: Nature

"Insects don’t respect borders, and international cooperation is required for management at a geographically relevant scale." - Nature

Current Outbreaks and Geography

In recent years, screwworm control has weakened, with outbreaks spreading across Central America and northward through Mexico, reaching Tamaulipas and Nuevo Leon—two states sharing a border with Texas. The resurgence is linked to several factors, including budget cuts, reduced foreign aid, weakened surveillance, and challenges in preventing the illegal movement of cattle. The article cites potential implications for US livestock producers, including substantial economic costs should an outbreak reinvade Texas. The geographic dynamics underline why cross-border collaboration remains essential, even as individual countries face fiscal and political constraints. Author: Nature

"The recent breakdown of screwworm control has seen thousands of cases confirmed in animals and humans across Panama, Costa Rica, Nicaragua, Honduras and Mexico." - Nature

Control Challenges and Funding

Key challenges to re-establishing SIT include the high costs of maintaining barriers, rearing facilities, and surveillance networks, along with political and budgetary constraints. The UN Food and Agriculture Organization’s health security program reportedly reduced its screwworm surveillance as US funding was withdrawn in March 2025, compounding regional vulnerabilities. With SIT potentially insufficient on its own, there is discussion of alternative or supplementary measures, such as insecticides, while regional preparedness is retooled. The article argues that restoring regional control requires renewed funding, training of veterinary entomologists, and international cooperation to manage cross-border incursions effectively. Author: Nature

"Loss of control over the illegal movement of cattle, lacking veterinary inspections, may also have been a contributing factor." - Nature

Policy Implications and Next Steps

New facilities are underway to produce sterile screwworms, including a facility at Moore Air Base in Edinburg, Texas, and refurbishment of rearing facilities in Mexico intended to restart production by summer 2026. The article discusses the geopolitical reality that border control alone cannot guarantee containment; instead, sustained international collaboration and investment are essential to prevent reinvasion and to prepare for future outbreaks. It notes that a preventative barrier in Panama was likely cheaper than the long-term costs of repeated outbreaks across the region and into the United States, highlighting the economic and strategic value of maintaining early intervention capabilities. Author: Nature

"But given that the pest is now re-entrenched in Central America, it may be too late to quickly reestablish regional control using SIT. As such, a fall back on insecticides seems like the only fix for immediate problems." - Nature

Lessons and Conclusions

The piece closes with reflections on how pests ignore political borders and the importance of international cooperation, sustained funding, and political will to manage cross-border threats. It cautions against neglecting expertise in veterinary entomology and emphasizes preparing for future outbreaks with a diversified toolkit, including SIT and chemical controls where appropriate. It also underscores the need for ongoing surveillance and border controls to stamp out incursions as soon as they occur, minimizing the risk of reinvasion and mitigating potential economic losses for livestock producers. Author: Nature