New World Screwworm Detected in Texas Livestock for First Time in Decades

US federal and state officials confirmed this week that New World screwworm, a fly whose larvae burrow into living tissue of warm-blooded animals, was found in a three-week-old calf in Zavala County in Southwest Texas — the state's first confirmed case since the early 1980s. The parasite has been advancing northward through Central America and Mexico since 2023, and its appearance in Texas signals a new phase of that resurgence. Authorities have established a roughly 12-mile containment zone around the detection site, restricting the movement of livestock and deploying sterile flies on the ground, with aerial dispersal expected to follow. The sterile insect technique — flooding an area with non-reproductive flies to prevent wild screwworms from producing offspring — is the same method the US used to eradicate the parasite decades ago. It remains unclear how the calf became infected, as officials say they are unaware of any recent animal movement off the ranch or known links to Mexico. Entomologists note that if the animal was not moved from an affected area, adult screwworm flies must already be present in the region, though an established population has not been confirmed. A widespread outbreak could cost Texas ranchers up to $1.8 billion annually, and in Mexico, export restrictions related to the parasite have already cost cattle exporters more than $1.3 billion.