New World Screwworm Southwest Outbreak Triggers Quarantine Zones and International Trade Suspensions
The outbreak of the highly destructive, flesh-eating parasite New World screwworm (Cochliomyia hominivorax) in the U.S. Southwest has escalated, prompting significant state-level quarantine measures and international trade restrictions. While agricultural and public health officials are beginning to see signs of containment, the outbreak continues to pose a major economic threat to the livestock industry.
Updated Case Counts and Regional Spread
As of July 16, 2026, the U.S. Department of Agriculture (USDA) has confirmed 39 New World screwworm infestations since the parasite's reemergence in June. The vast majority of cases are concentrated in Texas, with one case confirmed in New Mexico:
- Texas: 38 confirmed infestations.
- New Mexico: 1 confirmed infestation.
The two most recent infestations were confirmed on July 14, 2026, in sheep located in Pecos County, Texas.
Containment Efforts and Slowing Case Rates
A key milestone was reached in mid-July 2026: inactive cases now outnumber active ones for the first time, signaling that rigorous containment efforts are taking effect. The rate of new detections has significantly slowed:
- June 2026: 31 confirmed cases.
- July 2026 (first half): Only 4 new cases confirmed.
To maintain this progress and prevent the parasite from becoming permanently reestablished in the U.S., the Texas Animal Health Commission and the USDA have established a massive quarantine zone across South and West Texas. Officials are strictly monitoring animal movement and enforcing movement restrictions on livestock with suspicious or untreated wounds.
International Trade and Economic Fallout
The reemergence of the screwworm has triggered swift international regulatory reactions, highlighting the severe trade risks associated with the pest:
- Canada: The Canadian government has suspended the importation of certain animals from Texas due to the risk of introducing the screwworm. Additionally, animals imported into Canada from any other U.S. state must now be accompanied by a USDA Accredited Veterinarian's official NWS Certification Statement confirming they are free of the parasite.
- State-Level Alerts: Multiple non-border states, including Delaware, Wisconsin, and Illinois, have issued high-alert warnings to veterinarians and pet owners, advising them to inspect animals returning from the Southwest.
Eradication Strategy and the Sterile Insect Technique
The primary tool for combating the outbreak remains the sterile insect technique (SIT). The USDA and state teams are releasing millions of sterile male flies to mate with wild females, causing the population to collapse. While this technique successfully eradicated the pest from the U.S. in the 20th century, the current outbreak demonstrates the ongoing vulnerability of the southern border to reintroduction from Central and South America.