Why It Matters

The USDA confirmed the first detection of New World screwworm in a Texas calf in June, marking the first U.S. case associated with the current outbreak. Since then, dozens of additional cases have been confirmed in Texas and New Mexico.

For decades, New World screwworm was held south of North America by a biological barrier maintained through continuous releases of sterile flies in the Darién Gap between Panama and Colombia. That barrier failed beginning in January 2023. The screwworm spread beyond Panama, advanced through Central America and reached southern Mexico in November 2024.

The Big Picture

USDA confirmed the first U.S. case in Zavala County, Texas, on June 3. By Aug. 10, USDA had confirmed 45 U.S. detections, including 44 in Texas and one in New Mexico. USDA-linked tracking data showed the total had risen to 46 confirmed animal cases by Aug. 16.

Mexican authorities have reported more than 20,000 New World screwworm cases since late 2024, including more than 1,200 active cases during 2026.

USDA has responded with expanded surveillance, sterile fly releases, quarantines, animal movement controls and eradication activities. It is also moving ahead with a phased reopening of selected southern livestock ports, beginning with Douglas, Arizona, on Aug. 24. Santa Teresa and Columbus, New Mexico, are expected to follow if Mexico continues implementing the joint action plan and surveillance does not indicate increased risk.

The economic stakes are substantial. USDA estimates that an outbreak roughly comparable in scale to the 1976 Texas outbreak could cost livestock producers about $732 million annually and result in roughly $1.8 billion in losses to the broader Texas economy after adjusting the historical losses for inflation. USDA cautions that changes in livestock populations, veterinary practices and other factors make a direct comparison difficult.

Texas A&M AgriLife has separately estimated that establishment of the pest could cause about $2.1 billion in losses to Texas' cattle industry and $9 billion to the state's hunting and wildlife industry.

Wildlife presents a particular challenge. Wild animals are more difficult to monitor, inspect and treat than livestock, and established wildlife reservoirs could complicate eradication efforts. CRS identified wildlife surveillance and coordination among USDA, the Interior Department, states and tribes as a continuing policy issue.

Some ranchers and livestock businesses have pushed for southern ports of entry to reopen because restrictions on Mexican cattle have disrupted cross-border trade and tightened feeder-cattle supplies. Other livestock stakeholders have urged USDA to move cautiously to avoid increasing the risk of further screwworm spread.

USDA's phased reopening attempts to balance those concerns. Every animal entering through reopened ports will undergo USDA inspection, and the agency says it can suspend reopening if surveillance, audits or Mexico's implementation of the joint response plan indicate increased risk.

The Bottom Line

USDA continues surveillance, movement controls and sterile fly releases. The COPEG facility in Panama produces about 100 million sterile flies per week, while USDA is expanding additional production and dispersal capacity in Mexico and Texas.

Maintaining or eventually reestablishing the biological barrier in the Darién Gap requires continuous sterile fly releases because wild screwworm populations remain established farther south in South America.

Researchers are also pursuing new tools that could make eradication more efficient. Scientists have developed male-only screwworm strains, established CRISPR/Cas9 genome-editing techniques in the species and investigated gene-drive approaches that could alter inheritance or suppress wild populations. USDA is also testing genetically engineered male-only flies as a potential supplement to conventional sterile insect techniques.

Those technologies remain experimental and raise regulatory questions. Under the federal Coordinated Framework for the Regulation of Biotechnology, oversight could involve USDA, the Food and Drug Administration or the Environmental Protection Agency depending on the technology and its intended use. CRS found that no single federal agency has primary responsibility for regulating gene-drive organisms, leaving the applicable pathway to be determined case by case.

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