A Dangerous Cattle Parasite Returns to Texas After 60 Years

On June 3, 2026, the U.S. Department of Agriculture confirmed something that hadn't happened in six decades: the New World screwworm was found in a cow in Zavala County, Texas. This parasitic fly—Cochliomyia hominivorax—had been officially gone from the continental United States since 1966. Now it's back, and federal and state officials have activated emergency containment protocols they've been preparing for months.

The outbreak didn't come out of nowhere. Cases have been spreading northward through Mexico since January, getting closer to the Texas border each time. Just days before the Texas detection, authorities found the screwworm in a sheep in Mexico's Coahuila state, only 31 miles from the U.S. border. The pattern suggested it was only a matter of time.

Why This Matters: What Screwworm Does

New World screwworm larvae are parasites that burrow into the living flesh of warm-blooded animals—cattle, sheep, and other livestock. They cause a disease called myiasis, essentially an infestation that can spread rapidly and cause serious damage if untreated. The World Organization for Animal Health classifies it as a notifiable disease, meaning countries are legally required to report cases immediately. It's the kind of threat that can destabilize livestock operations and affect food supply chains.

Response Infrastructure Ready to Deploy

The good news: the USDA has been expecting this. Since February, a new sterile fly production facility has been operating at Moore Air Base in Edinburg, Texas—about 150 miles from where the Texas case was found. USDA Secretary Brooke Rollins and Texas Governor Greg Abbott opened this facility specifically in case the parasite crossed the border.

This facility is historic. It's the first domestic sterile fly production center the United States has operated since the original eradication campaign in the 1960s. Until now, North America relied on a joint U.S.-Panama facility called COPEG that has successfully kept the screwworm out of the United States by containing it in South America since the 1980s.

The strategy is called sterile insect technology. Scientists breed thousands of male screwworm flies in the lab and sterilize them. These males are then released in affected areas where they mate with wild females. The offspring don't survive, so the population gradually shrinks. It's been proven to work, but only if officials can release enough sterile males across all infected areas before the wild population gets established.

Tracking the Outbreak's March North

The current outbreak started in southern Mexico and moved steadily toward Texas through early 2026. In January, Texas Agriculture Commissioner Sid Miller issued the first warning after a cow tested positive near González, Tamaulipas—215 miles south of the Texas border. Each month brought the parasite closer.

By April 20, cases appeared in Nuevo León state, just 62 miles away. A week later, the screwworm was detected in Coahuila, 119 miles from the border. Cattle ranchers across South Texas watched this progression closely. They attended preparedness seminars at livestock clinics, learning what to watch for and how to respond.

A Success Story—But Will It Work Again?

The United States has actually eliminated New World screwworm before. The first eradication, completed in 1966, stands as one of the greatest achievements in agricultural pest control. The country even handled a recent incursion: in 2017, the parasite was discovered in the Florida Keys and was successfully eradicated again using the same playbook federal officials are now deploying.

The broader context here is important. This isn't a crisis unfolding in a vacuum. The USDA has institutional memory stretching back decades. The Panama facility has kept the screwworm suppressed in South America for forty years. The question now is whether the new domestic facility in Texas can do the same thing in real time, against an active outbreak spreading through major cattle country.

What Happens Next: Disruptions to Expect

The detection in Zavala County has triggered mandatory quarantines and livestock movement restrictions across South Texas. Cattle in affected areas and buffer zones are now under enhanced surveillance. This happens to coincide with peak marketing season, when ranchers typically move cattle for sale and breeding—so supply chains serving major processing plants could face delays and complications.

Cattle shipments moving out of affected regions will need special certification and additional screening. Export markets are also at stake: countries that import American beef and breeding cattle have strict rules about animals coming from areas where screwworm has been detected. Those animals require extra testing and documentation.

The scale of economic disruption will depend on how well containment works and how large the quarantine zones need to be. The 2017 Florida case stayed relatively localized, so the broader economic impact stayed limited. This outbreak is different—it's advancing through the heart of Texas ranching country, not an island.

Federal and state officials are confident that the early detection and pre-positioned infrastructure give them the best shot at containment. But sterile insect technology only works if they can saturate affected areas with enough sterile flies, fast enough, before the wild population becomes established. The coming weeks and months will show whether the lessons learned from the 1960s and 2017 can be applied successfully in 2026.