Summary (tl;dr)

The flesh-eating New World screwworm has been confirmed in a calf in Zavala County, South Texas, marking the first U.S. case in decades and triggering immediate quarantine and eradication efforts to protect livestock and wildlife.

Essential Background

The New World screwworm (NWS) is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals, leading to severe injury or death if untreated. The United States successfully eradicated NWS in 1966 using the sterile insect technique, a method where millions of sterile male flies are released to prevent wild females from reproducing. This eradication pushed the pest south, historically containing it in Panama. However, outbreaks began re-emerging in Central America in 2023, spreading northward through countries like Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and into Mexico by late 2024, gradually approaching the U.S. border. The last major U.S. outbreak prior to this was in the Florida Keys in 2016, affecting wild deer. In August 2025, a human case was confirmed in a Maryland resident who had traveled from El Salvador, though no domestic transmission occurred.

The Full Story

On June 3, 2026, the U.S. Department of Agriculture (USDA) confirmed the presence of New World screwworm in a three-week-old calf near La Pryor in Zavala County, South Texas. This detection, confirmed by the USDA's National Veterinary Services Laboratories in Ames, Iowa, represents the first U.S.-originated case in decades. The confirmation comes after a period of heightened alert, with Mexican officials reporting cases, including one in a goat in Coahuila, Mexico, near the Texas border, just days earlier on May 31. In response to the growing threat, Texas Governor Greg Abbott issued a disaster declaration in February 2026. Following the confirmed case, federal and state partners, including the Texas Animal Health Commission, immediately established a 20-kilometer infested zone around the detection site, implementing quarantines, animal movement controls, and intensified surveillance. Efforts to combat the screwworm also include the opening of a new sterile fly dispersal facility in southern Texas in early 2026 and the ongoing construction of a larger $750 million production facility at Moore Air Base near Edinburg, Texas, expected to produce 300 million sterile flies weekly by 2027. Emergency Use Authorizations for various animal treatments have also been issued.

Why It Matters

The re-emergence of the New World screwworm in Texas poses a significant threat to the state's $15 billion cattle industry and the broader U.S. livestock and wildlife populations. These parasites can infest open wounds in any warm-blooded animal, including livestock, pets, and even humans, causing immense suffering and potentially fatal outcomes if left untreated. The economic implications are substantial, as historical screwworm outbreaks caused billions of dollars in losses before their eradication. Immediate and decisive action is crucial to contain and eradicate the pest, preventing its establishment and widespread impact on animal health, food supply, and economic stability. The current efforts, including the sterile insect technique, quarantines, and treatment authorizations, are vital to protect the nation's animals and agricultural economy.

Geographic Location

• La Pryor, Zavala County, Texas, United States (first U.S. New World screwworm case confirmed in a calf; establishment of a 20-kilometer infested zone and quarantine)
• Ames, Story County, Iowa, United States (USDA's National Veterinary Services Laboratories confirmed the screwworm sample from Texas)
• Moore Air Base, Edinburg, Hidalgo County, Texas, United States (site of a new, larger sterile fly production facility under construction to combat screwworm)
• Coahuila, Mexico (screwworm case detected near the Texas border in May 2026, indicating the immediate threat preceding the Texas detection)
• Panama (site of an established sterile fly production facility whose output has been expanded to combat the northward spread)