Federal officials confirmed the detection of New World screwworm, a fly whose larvae infest living tissue, in a three-week-old calf in Zavala County, Texas. This marks the state's first confirmed case since the early 1980s and the first in U.S. livestock in several decades, signaling a new phase in the parasite's northward spread through Central America and Mexico that began in 2023.
While human infestations are rare in the United States, with no locally acquired human cases reported by the Centers for Disease Control and Prevention, the detection poses a significant challenge for ranchers and the U.S. animal agriculture sector. Given that beef prices are already near record highs, a widespread screwworm outbreak could have considerable economic repercussions.
Texas officials are prioritizing containment and investigating whether this represents an isolated incident or indicates the presence of adult screwworm flies in the region. The Texas Animal Health Commission (TAHC) reported no additional confirmed cases and is conducting extensive animal and fly surveillance within a 12-mile radius of the initial detection site. Animals within this zone require inspection before they can be moved.
TAHC has maintained fly traps along the Texas-Mexico border since July 2025, collecting over 54,000 suspicious flies, none of which were identified as New World screwworm. The Zavala County case has shifted the state's approach from precautionary measures to active containment. Sterile flies are being released through ground chambers at the infestation site, with aerial dispersal expected to follow. This strategy aims to overwhelm wild screwworms with sterile mates, preventing reproduction, mirroring the method used for past eradications.
It remains unclear how the calf became infected, as officials are unaware of any recent animal movement from the ranch or known links to Mexico or other affected areas. Experts suggest that if the calf did not travel from an affected region, adult screwworm flies must be present in the local area to have laid eggs on it.
Understanding the Threat
The New World screwworm fly, scientifically known as Cochliomyia hominivorax, is characterized by its blue-gray body and attraction to fresh wounds or natural openings in warm-blooded animals. Female flies lay eggs that hatch within 12 to 24 hours, with the resulting larvae burrowing into flesh to feed on living tissue, causing pain and severe damage. Untreated infestations can lead to significant wounds, blood loss, secondary infections, and potentially death in animals.
Historical Eradication and Current Challenges
Screwworms historically caused significant economic losses to cattle ranchers in the U.S. Starting in the 1950s, the U.S. Department of Agriculture (USDA) developed the sterile insect technique (SIT), which involves releasing millions of sterilized male flies to mate with wild females, thereby preventing offspring. This method led to the eradication of screwworms from the U.S. by 1966.
The SIT was subsequently applied in Mexico and Central America, with a program known as COPEG maintaining a barrier in the Darién Gap between Panama and Colombia since 1998. However, increased cattle smuggling and climate shifts have contributed to outbreaks spreading across wider areas of Mexico and Central America, overwhelming the capacity of existing sterile fly production facilities.
The current sterile fly production plant in Panama operates at full capacity, producing approximately 100 million flies weekly, but only half are males, which are essential for population suppression. To address this, the USDA is renovating a facility in Mexico and developing new production capacity in Texas. Advanced genetic tools, such as a male-only strain called NovoFly, are also under development but require regulatory approval and field testing.
Economic impacts from screwworm-related export restrictions have been substantial in Mexico, and a widespread outbreak in Texas could cost ranchers and the economy billions annually. The current response is unfolding amidst staffing reductions and budget cuts within USDA agencies responsible for animal health surveillance and outbreak response, raising questions about the nation's preparedness for emerging animal health threats.