A parasitic fly that attacks warm-blooded animals and threatens the agricultural sector has been detected in a young sheep in Mexico within 31 miles of the U.S. border, according to the U.S. Department of Agriculture.
New World Screwworm Detected Within 31 Miles of U.S. Border
The detection of the New World screwworm in a six-month-old sheep in Coahuila state marks the closest the parasite has approached the United States during the current outbreak, despite containment efforts by the USDA and Mexico. Female screwworm flies deposit hundreds of eggs inside open wounds on warm-blooded animals. Once hatched, the larvae utilize sharp, hooked mouths to burrow into living flesh, feeding and enlarging the wound, which can kill the host if left untreated.
The discovery escalates risks for the American beef industry and cattle producers, who have anticipated potential cross-border spread as the pest moved northward through Mexico. Washington has blocked cattle imports from Mexico for over a year to prevent the screwworm from crossing the border. Experts warn that an entry into the U.S. could further elevate beef prices by reducing the cattle supply, with a potential outbreak in Texas causing an estimated $1.8 billion in economic damage to the state. The U.S. cattle herd currently sits at its lowest level in 75 years, alongside record high beef prices.
Biological Threat and Vulnerability of Livestock
The New World screwworm, scientifically known as Cochliomyia hominivorax, is capable of affecting any warm-blooded animal, including birds, mammals, and humans, though it exhibits a strong preference for bovines. According to Gaby Maier, a Cooperative Extension specialist for beef cattle herd health and production at the UC Davis School of Veterinary Medicine, mortality rates can reach up to 100 percent in newborn calves.
Data from an outbreak in Panama indicated that 83 percent of screwworm cases occurred in bovines, while human cases remain relatively rare. The severe damage inflicted by the larvae has drawn comparisons to severe horror narratives due to the destructive nature of the feeding process.
California commodities face significant exposure should the parasite cross the border. Dairy products and milk rank as California’s top gross-value commodity at $8.61 billion in 2024, while cattle and calves rank fourth at $4.98 billion. Daniela Bruno, a University of California Cooperative Extension dairy advisor, noted that dairy farms face severe risks because dairy cows produce milk daily that requires immediate processing. Quarantines or plant closures can lead to rapid milk spoilage and mandatory dumping, unlike beef cattle which can be held back from the market during quarantines.
Sterile Fly Production and Control Strategies
To combat the resurgent parasite, Mexico and the United States inaugurated a sterile fly production facility in Metapa de Domínguez, Chiapas, on June 27, 2026. The facility began operations the following day, supplementing flies previously sourced from a COPEG plant in Pacora, Panama.
The Metapa facility utilizes the Sterile Insect Technique, which involves radiating pupae to sterilize the insects without modifying their genes or making them radioactive. These sterile males retain the ability to fly and mate with wild females, but produce no viable offspring, thereby reducing the pest population over successive generations. The plant aims to reach a production capacity of 100 million sterile flies per week by the end of 2026.
Following the completion of its first production and sterilization cycle, Mexico released an initial batch of 2.5 million sterile flies in the municipality of Coronado in southern Chihuahua. Experts emphasize that sterile fly releases must be combined with active measures including livestock inspection, wound care, trapping, case management, and transit controls.
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