A case of the New World screwworm has been confirmed in northern Mexico, positioning the flesh-eating parasite less than 70 miles from the U.S. border. Mexican agricultural authorities reported on Sept. 21, 2025, that an 8-month-old cow in Sabinas Hidalgo, Nuevo Leon, tested positive for the parasite, marking the northernmost detection in an outbreak that has been moving north through Central America and Mexico.
The detection has elevated concerns for the U.S. livestock industry, as the parasite can be devastating to warm-blooded animals, including cattle, wildlife, and pets. The infected animal was part of a larger shipment, and its proximity to a major commercial route into Texas has prompted a strong response from U.S. officials, who reaffirmed border closures and plans to combat the parasite’s spread. The incident highlights the persistent threat of a pest eradicated from the U.S. decades ago and the challenges in preventing its re-entry.
Details of the Detection
Mexico’s National Service of Agro-Alimentary Health, Safety, and Quality (SENASICA) confirmed the screwworm case in a feedlot in Sabinas Hidalgo. The location is significant as it lies near the major highway connecting Monterrey, Mexico, to Laredo, Texas, one of the busiest commercial corridors in North America. This detection represents a major jump for the parasite, as the previous northernmost case, reported in July 2025, was approximately 370 miles farther south in the state of Veracruz.
According to preliminary reports, the infected calf was part of a shipment of 100 cattle transported from a region in southern Mexico with known screwworm activity. Mexican officials stated that the other 99 animals in the group were not infected. Mexico’s agriculture secretary characterized the finding as an “isolated case,” noting the parasite was discovered in its larval stage, meaning no adult fly had emerged to spread the infestation further.
U.S. and Mexican Response Measures
The U.S. Department of Agriculture (USDA) has reiterated its commitment to a “five-pronged plan” to protect the nation’s livestock. In response to the July detection, the USDA had already closed the border to imports of Mexican cattle, bison, and horses. U.S. Agriculture Secretary Brooke Rollins stated that the department would pursue the aggressive release of sterilized male screwworm flies in the affected region of Nuevo Leon to disrupt the parasite’s life cycle.
Long-term U.S. strategy includes a $750 million investment in a new sterile insect production facility in the Rio Grande Valley and another $100 million for new technologies like advanced traps and lures. Surveillance has also been heightened, with thousands of traps monitored in Texas, Arizona, and New Mexico since July. For its part, Mexico is evaluating further actions and is expected to soon open a new plant for producing sterile flies to combat the outbreak.
Data at a glance
- Sample size: 1 infected cow out of a shipment of 100.
- Effect size / change: The parasite moved approximately 300 miles closer to the U.S. border between July and September 2025.
- Time frame / location: Detected on Sept. 21, 2025, in Sabinas Hidalgo, Nuevo Leon, Mexico.
- Instrument / dataset: Confirmed by Mexico’s National Service of Agro-Alimentary Health, Safety, and Quality (SENASICA).
The Biology of a Voracious Parasite
The New World screwworm, or Cochliomyia hominivorax, which translates to “man-eater,” is the larva of a parasitic blowfly. Unlike other maggots that consume dead tissue, these larvae are obligate parasites, meaning they feed exclusively on the living tissue of warm-blooded animals. Adult female flies lay their eggs in open wounds, even ones as small as a tick bite. When the eggs hatch, the larvae burrow into the host’s flesh, feeding and growing, which can lead to severe tissue damage, secondary infections, and often death if left untreated.
The United States, in cooperation with Mexico and other Central American nations, successfully eradicated the pest from North America in the 1980s using the sterile insect technique. This method involves releasing massive numbers of sterilized male flies into the wild. When these males mate with wild females, no offspring are produced, causing the population to collapse. The reappearance of the pest in southern Mexico last year and its steady march northward threatens to undo decades of progress.
Economic and Agricultural Implications
The northward advance of the screwworm poses a significant economic threat to the American cattle industry. The National Cattlemen’s Beef Association has called the parasite a “critical and urgent threat,” emphasizing the speed at which it has moved through Mexico. An infestation in the U.S. could lead to substantial losses from livestock deaths, expensive veterinary treatments, and intensive surveillance and quarantine efforts.
The current suspension of livestock imports from Mexico is already impacting the market, contributing to higher U.S. cattle and beef prices. The proximity of the latest case to a vital trade artery like the Laredo crossing underscores the potential for disruption not just to livestock transport but to broader commercial activity if more drastic containment measures become necessary. The USDA’s significant financial commitments to sterile fly production and border surveillance reflect the high stakes involved in preventing the parasite from re-establishing itself on U.S. soil.