A flesh-eating parasite not seen in American livestock since the United States declared it eradicated sixty years ago has returned to the country. The US Department of Agriculture confirmed Thursday that a three-week-old calf in Zavala County, Texas tested positive for New World screwworm — the first confirmed detection in a US bovine since 1966 and a potential threat to an estimated $1.8 billion of Texas's agricultural economy.
What Is New World Screwworm
New World screwworm, Cochliomyia hominivorax, is not a simple external parasite like a tick or bot fly. The female lays her eggs in open wounds or orifices of warm-blooded animals — livestock, wildlife, and in rare cases, humans. The larvae burrow into living flesh and cause progressive, potentially fatal tissue destruction. Untreated animals can die within days. The parasite affects cattle, horses, sheep, goats, and deer with particular severity, and it spreads rapidly when conditions are favorable.
USDA's Animal and Plant Health Inspection Service confirmed the detection in a calf whose larvae were found in its umbilical area, a common entry point. No additional cases had been confirmed as of Friday morning, but APHIS activated ground personnel in Texas immediately, establishing a 20-kilometer quarantine zone around the affected property and implementing movement controls for livestock in the surrounding area.
A 60-Year Eradication Now in Question
The USDA eradicated New World screwworm from the United States in 1966 through a decades-long sterile insect technique program — one of the most ambitious agricultural biosecurity campaigns ever undertaken. Billions of radiation-sterilized male flies were released into the wild to overwhelm the fertile fly population and prevent reproduction. The program eliminated the parasite from the continental United States and has since been maintained in Central America and parts of Mexico as a biological buffer zone to prevent reinfestation from the south.
That buffer appears to have been breached.
"We are treating this as the most significant agricultural biosecurity incident since the foot-and-mouth scare of 2001," said Dr. Lorraine Castillo, a veterinary entomologist at Texas A&M University in College Station. "One confirmed case does not tell us how long the parasite has been here or how far it has traveled. The quarantine is necessary, but it may not be sufficient."
The Economic Stakes
The numbers at risk are stark. A full New World screwworm outbreak in Texas would threaten an estimated $1.8 billion in economic activity in the state alone, according to a USDA risk assessment prepared in 2024 when cases were detected in cattle in Panama. Texas maintains roughly 12 million head of cattle — approximately 15 percent of the entire US national herd — making it by far the most exposed state to a potential outbreak.
Beyond Texas, the threat extends to deer and other wildlife populations. The white-tailed deer, abundant across the South and Midwest, is highly susceptible to New World screwworm. A wildlife-reservoir outbreak would be significantly harder to contain than a localized livestock event, because sterile fly releases and active surveillance are far less effective across wild populations distributed over hundreds of square miles.
Where It Came From
Zavala County is in deep South Texas, roughly 80 miles southeast of Eagle Pass along the Mexican border — the region where animal and agricultural movement across the border is heaviest. The USDA said Friday it is working with Mexican agricultural authorities to trace the parasite's movement north, comparing recent diagnostic data from both countries to determine the likely origin and pathway.
The pest had been detected in cattle in Panama in 2024 and in parts of northern Mexico more recently. Entomologists warned at the time that the Central American buffer zone was under increasing pressure and that the risk of a US detection was elevated. Those warnings went largely unheeded in terms of preventive action.
"We have known this threat has been creeping north for a few years," said Derek Munoz, a rancher operating 3,000 acres in Uvalde County, adjacent to Zavala. "It hit Panama, it hit Mexico, and most of us figured it was a matter of when, not if. That does not make it any less serious when it actually happens."
USDA Response and What Comes Next
APHIS said it is expediting targeted releases of sterile flies in the quarantine zone to disrupt any breeding that may have already occurred, and has expanded surveillance protocols statewide. Federal officials are urging livestock producers across Texas and neighboring states to inspect animals daily for signs of infestation — unusual wound activity, excessive fly attention, or behavioral signs of discomfort — and to report suspicious cases to APHIS immediately.
The response window is narrow. New World screwworm reproduces rapidly in warm temperatures, and South Texas in early June is well within the parasite's optimal thermal range. Agricultural officials say the next two to three weeks will determine whether this remains an isolated incident or the beginning of a broader infestation that could take years and hundreds of millions of dollars to contain.