The New World screwworm fly has returned to the United States, threatening the $113 billion cattle industry for the first time in over 50 years. According to the US Department of Agriculture (USDA), five confirmed cases have been reported in Texas and New Mexico as of June 2026. Federal officials are deploying sterile fly releases and strict quarantine zones to prevent a national outbreak.
Why is the New World Screwworm a threat to livestock?
Unlike common house flies, the New World screwworm is a parasitic pest that feeds on the living tissue of warm-blooded animals. According to the USDA Animal and Plant Health Inspection Service, female flies deposit eggs in open wounds or mucous membranes. Once hatched, the larvae—or maggots—burrow into the host’s flesh, causing severe damage that can be fatal if left untreated.
University of Notre Dame associate research professor Lee Haines notes that standard ranching practices, such as dehorning, shearing, or even simple scrapes from corrals, create the entry points these flies require. Texas & Southwestern Cattle Raisers Association president Stephen Diebel warns that even a minor injury, such as a tick bite, can leave an animal vulnerable to infestation.
The US successfully eradicated the screwworm in the 1960s by releasing massive swarms of sterile male flies. Because females mate only once, they were unable to produce viable offspring, eventually crashing the population.
How are officials responding to the Texas outbreaks?
Texas State Veterinarian Bud Dinges has established a 20km quarantine zone across parts of Zavala and Uvalde counties to restrict the movement of potentially infested livestock. According to Zavala County Sheriff Eusevio Salinas, state officials are currently operating road checkpoints to enforce these movement restrictions.
The USDA is scaling up its “sterile insect technique” to combat the spread. Rear Admiral Michael Schmoyer, a member of the USDA response team, reports that the agency is currently releasing 4 million sterile flies per week in South Texas, supplemented by the distribution of 4 million pupae. Additionally, the federal government is investing $750 million into a new production facility in South Texas, capable of generating 300 million sterile flies weekly by next fall.
What are the future trends for parasite control?
The return of the screwworm highlights a growing concern regarding climate change and its impact on pest migration. According to Lee Haines, warmer temperatures are allowing tropical species to thrive in regions where they were previously killed off by annual cold snaps. As these environmental barriers weaken, the geographic range for the screwworm is likely to expand.
Furthermore, the logistical struggle of keeping pace with the fly is evident in the divergence between regional programs. While the US-Panama program successfully contained the fly for years, entomologist Edward Burgess of the University of Florida notes that many sterilization programs outside of Panama have shuttered, making it difficult to maintain a regional buffer. The current crisis suggests that future livestock protection will require a permanent, high-capacity infrastructure for sterile fly production rather than reactive, short-term measures.
Frequently Asked Questions
- Does the screwworm threaten the human food supply? No. US Agriculture Secretary Brooke Rollins stated that the fly does not infest food, and the current outbreak is not expected to damage beef production.
- Can humans get screwworm? Yes. While the pest primarily targets livestock, it can infest any warm-blooded animal, including humans. The CDC reports over 2,000 human cases across Central America and Mexico since 2024.
- What should ranchers do to protect their herds? Experts recommend proactive injections and increased vigilance. Ranchers are advised to treat all wounds—including those from ear tagging—and monitor animals closely for signs of infestation.
Stay updated on quarantine boundaries. If you operate a ranch in South Texas, check the USDA Animal and Plant Health Inspection Service website regularly for maps of active quarantine zones and livestock movement requirements.
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