USDA officials are seeking to ease concerns that New World screwworm’s arrival in Texas is the precursor to a major outbreak. On the evening of June 3, Agriculture Secretary Brooke Rollins confirmed a case was detected in a 3-week-old calf near La Pryor, Texas.
Rollins contends the Trump administration’s efforts over the past year have kept screwworm at bay far longer than anyone anticipated. In response to the Texas case, USDA is releasing 4 million sterile flies along the ground near the detection site. This is in addition to the 4 million sterile flies already being deployed aerially in the same area.
Sterile flies have proven to be the most effective way to combat New World screwworm outbreaks. Over time, sterile male flies mate with wild female flies, causing the screwworm population to crash over time.
Since NWS was first detected in Mexico in late 2024, USDA has worked to increase sterile fly production. The agency also announced plans to build a new sterile fly production facility in South Texas. That facility will reportedly not be completed until late 2027 at the earliest.
In addition to deploying sterile files, USDA and the Texas Animal Health Commission have established a 20-mile infested zone around the detection site. For now, no warm-blooded animals will be allowed out of the zone without being first inspected by an animal health official.
“While this development is a serious threat to our livestock and wildlife, it hasn’t caught us off guard,” Rollins said during June 4 testimony before the House Agriculture Committee. “We were ready and prepared.”
A history of destruction
New World screwworms are flies that lay eggs in open wounds or body openings in cattle and other animals. When those flies hatch, the larvae feast on the host animal’s flesh, causing severe damage. If left untreated, the animals may eventually die.
Reports of NWS in the U.S. date back to 1842. USDA ramped up efforts to combat the pest in the 1930s. By 1966, in large part due to the release of sterile male flies, USDA declared the nation free of indigenous screwworm flies.
The pests returned in the early 1970s, significantly impacting livestock operations in Texas and the Southwest. Since then, the U.S. has been mostly screwworm-free apart from a handful of cases brought in by imported animals. Those outbreaks were easily contained with minimal impact to U.S. livestock and wildlife.
In 1986, eradication efforts had pushed NWS completely out of Mexico. Twenty years later, the flies had been pushed all the way south to Panama’s Darien Gap. At the time, it was hoped that the remote stretch of dense swamps and rainforest between Central and South America would serve as a natural buffer against the return of screwworm. Unfortunately, that didn’t prove to be the case, as the pests once again began making their way north.
No cause for panic?
In a joint June 3 statement, Senate Agriculture Committee Chair John Boozman, R-Ark., and House Ag Committee Chair Glenn “GT” Thompson, R-Pa., said the detection should not be a cause for panic. They said USDA’s efforts have strengthened the country’s defense against screwworm’s serious threat to animal health and the livestock industry.
“By working closely with producers, animal health officials, and our federal and state partners, we will take the necessary steps to protect and safeguard America’s livestock industry,” the two lawmakers stated. “We have successfully eradicated this threat to livestock, wildlife and animal health before, and we’re confident we will do so again.”
Gregg Doud, president of the National Milk Producers Federation, called the return of screwworm to the U.S. a “disappointing milestone.” Still, he noted that dairy producers have been preparing for more than a year in case this day should come.
“It’s important to remember that this development has no effect on food safety, and that measures to combat both the screwworm and its spread are in place and time-tested,” said Doud in a June 3 statement. He also called on officials to avoid regulatory responses that cause economic harm that “could be greater than screwworm itself.”
What should farmers do?
Bud Dinges, executive director of Texas Animal Health Commission, is calling on farmers to immediately report any suspicions of screwworm. During a June 3 press call, he reiterated that a screwworm outbreak is not a food safety issue. That’s because flies do not infest meat, fruits, vegetables or other food sources.”
Dinges said owners need to look at their animals as often as possible, monitoring wounds that can be covered and treated. He emphasized that screwworm is not an infectious disease. The flies must lay their eggs in or near a wound for an animal to become infested.
“This is also a highly treatable condition,” Dinges said. “We have many tools in our tool kit today to prevent devastating impacts.”