Mexico's agriculture ministry has confirmed a new outbreak of New World screwworm (Cochliomyia hominivorax) in Nuevo León, a key northern border state. The detection, announced on October 6, 2025, marks the second confirmed case in the area within two weeks and intensifies fears of a northward spread toward the United States.

The infested calf, originating from southern Mexico, was intercepted in Montemorelos, south of Monterrey. Mexican authorities reported that none of the 84 other animals in the shipment were infected. The screwworm larvae were found dead or dying, thanks to mandatory antiparasitic treatments, including ivermectin.

Mexico's sanitation agency SENASICA now requires that ivermectin be administered at least 72 hours before cattle transport, under the oversight of the International Regional Organization for Animal and Plant Health. Officials say the larvae's unviable state demonstrates the success of current inspection protocols.

U.S. Agriculture Secretary Brooke Rollins labeled the case an "isolated incident", though it occurred roughly 100 miles south of the previous Nuevo León case. She promised that U.S. inspectors would be on-site within hours to verify containment.

Rollins has criticized Mexico's response, citing insufficient enforcement on cattle movement restrictions and inadequate fly trap monitoring. Her remarks reflect growing frustration in the U.S. livestock sector, already impacted by import halts on Mexican cattle since May 2025.

Mexico has recorded 6,703 cases of screwworm-infested animals as of September 13, up from 5,086 in mid-August - a 32% increase in under a month. The Mexican government confirmed that the latest case is not linked to the earlier Nuevo León detection.

Although the U.S. continues to report zero new domestic screwworm cases, the proximity of these outbreaks underscores a serious biosecurity threat. The pest, known for its flesh-eating larvae, was eradicated in the U.S. in 1966 through the sterile insect technique, yet remains active in parts of Latin America.

To respond, Mexico and the U.S. are accelerating cooperation, including plans for a new sterile fly production facility by 2026, aimed at biological control of the pest.

For U.S. ag professionals, the spread of screwworm near the border represents a critical challenge in maintaining herd health, market stability, and international trade flow.