Sonora's Ranchers Are Holding the Line on a Parasite While the US Border Stays Closed

The Unión Ganadera Regional de Sonora is reinforcing surveillance along its eastern border to keep screwworm out of the state's cattle herds while producers wait for the US border to reopen to Mexican livestock exports. The state's cattle sector has been caught between two threats since late 2024: a flesh-eating parasite moving north through Mexico and a closed American border that has gutted the export market on which Sonora's ranchers depend.

The screwworm, Cochliomyia hominivorax, is a parasitic fly whose larvae burrow into open wounds on warm-blooded animals, consuming living tissue. It was largely eradicated from North America through decades of sterile insect technique programs in which irradiated flies were released to interrupt the reproductive cycle. But recent years have brought outbreaks in southern Mexico, particularly Chiapas, marching north through livestock corridors toward the US border. The US responded by closing the border to Mexican cattle imports in late 2024 after screwworm detections triggered an animal health emergency.

Sonora is one of Mexico's largest cattle-producing states, with roughly 1.9 million head according to the most recent agricultural census. It is also the primary pipeline for live cattle exports to the United States, a trade worth hundreds of millions of dollars annually that ranchers depend on for their margins. The border closure has forced Sonora's cattle into an oversaturated domestic market, driving live-cattle prices down and leaving feedlots operating below capacity.

The UGRS, which represents Sonora's cattle producers, said it has stepped up surveillance operations along its border with Chihuahua to detect and contain any screwworm cases before they reach Sonora. The organization continues to press federal authorities for accelerated reopening protocols at the US border. "Continuamos a la espera de la reapertura de la frontera para la exportación hacia Estados Unidos," the organization stated.

The surveillance push is a stopgap born of necessity. Sonora cannot export live cattle to the US without USDA clearance, and USDA clearance requires Mexico to demonstrate screwworm-free status in the export zone. The state's cattlemen have been watching herds in Chihuahua and northern Durango for signs of infestation while the diplomatic and veterinary bureaucracies negotiate inspection and certification timelines that have already stretched longer than most ranchers expected when the first border restrictions came down. Those negotiations have dragged on for months, slowed by divergent views between the US and Mexico on what constitutes sufficient surveillance coverage along the proposed export corridor. USDA inspectors want testing protocols that Mexican agricultural officials call excessive given the absence of confirmed screwworm cases in northern Mexico.

Screwworm infestations can devastate cattle operations. The fly lays eggs in wounds as small as a tick bite. The hatched larvae burrow into flesh, causing deep lesions that attract more flies. A single infestation can kill an untreated animal within two weeks. The economic toll of the outbreaks detected in southern Mexico in recent years has been substantial in the affected states, though exact figures are disputed between federal authorities and livestock organizations.

Mexico's agriculture ministry and USDA have conducted joint inspections in states along the proposed export corridor, and there have been incremental steps, like the reopening of some border facilities for limited shipments of feeder cattle. But the full-volume trade that Sonora depends on remains paused. Some Sonora ranchers have looked south, selling cattle to buyers in Sinaloa and Nayarit who feed the domestic market, though prices there run fifteen to twenty percent below US-bound rates, squeezing margins for producers who built their operations around the export pipeline. For now, the UGRS fights on two fronts: watching the eastern border for the worm and pushing federal officials to get the northern border back open.

The state's ranchers would prefer the full reopening of US markets, but they know the science on screwworm will determine the timeline as much as any political negotiation in Washington or Mexico City. A certified screwworm-free zone could open the gates. A new detection near Sonora could close them again. The UGRS patrols both borders at once: one against politics, the other against a parasite that does not care about either. For Sonora's ranchers, the cattle keep eating, the border stays shut, and the surveillance trucks keep running the eastern roads looking for any sign of the worm that could make everything worse.