The New World screwworm’s return to the United States after more than six decades marks a significant challenge for the livestock industry and public health officials. The parasite, which feeds on the living tissue of warm-blooded mammals, was once widespread in the American South and Southwest before being declared eradicated in 1966 through the sterile insect technique.
As of June 12, 12 active cases had been confirmed in the US, according to UPI. The initial case was discovered June 3 in a 3-week-old calf on a ranch in Zavala County, near the southern border. Since then, the screwworm has been found in cattle across the region, as well as in goats in Edwards and Gillespie counties, Texas, and in a dog in southeastern New Mexico.
“We have taken advantage of every day we’ve been given,” Dr. Phillip Kaufman, professor and head of the Department of Entomology at Texas A&M, told UPI. “We are going to win this battle. We’ve done this before.”
Kaufman emphasized that the infestation did not come as a surprise. The United States has tracked the fly’s northward spread through Central America for months. After being contained for decades in a “biological barrier” in the Darien Gap between Panama and Colombia — established in 2004 — the fly broke containment in 2022 and moved rapidly.
“In about two years it moved that distance,” Kaufman said. “That’s a distance when we did the eradication program, it took about 17 years to eradicate. It recolonized very, very quickly.”
The fly reached Mexico in November 2024. As of this week, Mexico reported 2,021 active animal cases, including 30 cases in the border state of Coahuila, according to UPI.
USDA Secretary Brooke Rollins said the arrival was “as expected” and that “the nation’s food supply is 100% safe.” Kaufman echoed that reassurance.
Ranchers are on the front lines. Stephen Diebel, president of the Texas & Southwestern Cattle Raisers Association, told UPI that the best preventive measure is close monitoring of livestock. “The key to early detection is monitoring livestock and wildlife, knowing the signs and reporting suspicious cases immediately,” he said.
Diebel noted that the Texas & Southwestern Cattle Raisers Association has joined 13 other trade organizations to form the Screwworm Coalition of Texas, aimed at connecting animal health officials with producers. Most rancher questions, he said, concern movement protocols in infested zones, where livestock can be moved only after passing an inspection by the Texas Animal Health Commission.
The screwworm’s lifecycle makes it particularly destructive. A single female can lay up to 300 eggs at one time — as many as 3,000 in its 10- to 30-day life span — depositing them in open wounds, eyes, ears, and genital openings of warm-blooded mammals. When the eggs hatch, the maggots burrow into living tissue and begin feeding.
“They burrow into living tissue and start eating it,” Kaufman said. “As they grow for five to seven days progressively larger they eat more and more tissue and can cause more and more damage.”
The primary weapon against the screwworm is the sterile insect technique, pioneered by Texas A&M alumnus Edward Knipling. The technique exploits the fact that female screwworm flies mate only once; if she mates with a sterile male, her eggs never hatch. “We call it reproductively killing her,” Kaufman said. “She will continue to live but will not contribute to more maggot infestations.”
Sterile fly production facilities in Panama, Mexico, and Texas raise flies for release. Two dispersal facilities operate in Mexico and one in Texas, according to the UPI report.
It remains unknown how the screwworm breached the original biological barrier or how it entered the United States, underscoring the importance of livestock movement restrictions, Kaufman said. The parasite does not distinguish between domestic animals and wildlife — it can also lay eggs in deer and feral swine.
The New World screwworm has been present in the Americas since before human arrival. Kaufman noted that written records from Spanish conquistadores mention the fly, and that it was documented in Texas as early as the 1840s.
The first infected calf, the rancher who owns it told Kaufman, is expected to make a full recovery.
“One thing we didn’t have before was time,” Kaufman said. “We’ve had time to prepare.”