How much damage does the flesh-eating screwworm pose to Texas? Curious Texas answers
A flesh-eating maggot is threatening Texas’ booming cattle industry, nationwide grocery prices and the state’s diverse wildlife.
As the New World screwworm inches closer to the U.S.’s southern border, federal and state officials have announced a multi-pronged attack against the pest.
The screwworm is actually a parasitic fly that burrows into open wounds of animals and deposits larvae. Although officials are largely concerned about wildlife and cattle, the pest can also feed on pets and humans.
This is not the first time the screwworm has wreaked havoc. One reader asked just how much damage the screwworm has historically caused and how much danger it poses today. Curious Texas dove into the topic.
Curious Texas
History of screwworm
Decades ago, screwworms were an annual scourge for cattle ranchers and dairy farmers, particularly in Texas and the Southeast United States. The flies are metallic greenish-blue with three stripes that run down the top of the body just behind its head and large orange eyes.
Related:First human case of flesh-eating screwworm confirmed in U.S., HHS says
Native to South and Central America, the flies thrive in warm tropical environments. Although they can only fly about 10 miles or so, they easily travel via infected animals.
In fact, they arrived in the Southeast U.S. in 1933, by way of a shipment of infected animals from the Southwest, according to the U.S Department of Agriculture’s National Agricultural Library, one of the world’s largest such collections.
“In the early stages, you might not notice these tiny maggots, which are smaller than a grain of rice,” said Phillip Kaufman, a professor and head of entomology at Texas A&M University. “So you move your animal 300 miles, and you’ve just moved screwworm 300 miles.”
Related:What to know about New World screwworm flies — and what’s at stake for Texas livestock
The results were catastrophic. Treating infested livestock was a “highly unpleasant, labor-intensive job,” according to the agricultural library. Ranchers had to check livestock often for infested wounds, and infested animals had to be treated and confined.
If left untreated, the damage is often fatal. In 1935 in Texas, screwworms resulted in 180,000 livestock deaths in fewer than half the counties, according to the library. Losses to livestock reached up to $100 million a year in the U.S. and extensive losses to wildlife occurred from the 1930s to 1950s. In some years, infestations killed up to 80% of fawns in Texas, according to figures from the Texas Wildlife Association.
Screwworms were eradicated in the U.S. in the 1960s after scientists developed a method of breeding sterile male flies to mate with fertile females. Only female screwworms burrow into animals. Because the females only mate once in their weekslong adult lives, the population eventually dies off. That technology is still used today.
A calf is evaluated by a veterinary during a veterinary inspection in Hermosillo, Sonora State, Mexico, Monday, July 28, 2025. (AP Photo/Fernando Llano)
Fernando Llano / AP
How close is the screwworm to Texas?
Although screwworms were eradicated, they have continued to pop up. In 2016, a screwworm infestation killed more than 130 endangered deer in Florida.
Federal and state officials are now monitoring an outbreak that has reached 370 miles from the Texas-Mexico border. Crews are dropping sterile flies across infected zones. If screwworms arrive in Texas, the state’s $15 billion-a-year cattle industry and diverse wildlife in South Texas would be at risk, Kaufman said.
“It could be very, very bad,” Kaufman said. “It’s important to understand this fly will lay eggs in any mammal and occasionally a bird. It doesn’t care if it’s a cow, dog or person.”
A sample of screwworms are displayed at a veterinary clinic in Tapachula, Chiapas state, Mexico on July 4, 2025.
Daniel Becerril / REUTERS
To stop the spread, U.S. Secretary of Agriculture Brooke Rollins recently announced construction of a new $750 million facility in the Rio Grande Valley town of Edinburg that will be capable of producing 300 million sterile flies a week, tripling the current output.
The new facility will complement an $8.5 million fly dispersal facility also being built in Edinburg. In addition, the U.S. plans to spend $100 million to identify and deploy new technologies, such as traps and lures, to fight the pest.
“The threat is real, and the time to act is now,” Rollins said at a news conference.
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