June 13, 2026

New World screwworm found in Texas calf

By Martha Blum
Cowboys push a cow out of its spot to a veterinarian inspection at a ranch in northern Mexico. The New World screwworm had been declared eradicated in the United States decades ago, thanks to efforts such as the breeding of sterile flies, awareness campaigns and efforts to control the pest in foreign countries. But there have been a surge of cases in South America — and it was detected in South Texas on June 3 — that public health experts and the U.S. Department of Agriculture have been watching closely.

Cowboys push a cow out of its spot to a veterinarian inspection at a ranch in northern Mexico. The New World screwworm had been declared eradicated in the United States decades ago, thanks to efforts such as the breeding of sterile flies, awareness campaigns and efforts to control the pest in foreign countries. But there have been a surge of cases in South America — and it was detected in South Texas on June 3 — that public health experts and the U.S. Department of Agriculture have been watching closely. (AP photo/Fernando Llano/AP)

LA PRYOR, Texas — A 3-week old calf in South Texas has been confirmed to be infected with New World screwworm. The detection was announced on June 3.

A 12-mile control area has been established around the detection in La Pryor, about 100 miles southwest of San Antonio and roughly 45 miles north of the U.S.-Mexico border.

“We expedited the targeted release of millions of sterile New World screwworm flies by immediately deploying 4 million flies in ground release chambers, in addition to the 4 million flies per week already being released aerially in the area,” said U.S. Agriculture Secretary Brooke Rollins on June 4.

Just days before the calf was found in Texas, a case of the parasite was found 31 miles south of the Texas border in a sheep.

“The New World screwworm are flesh-eating larvae that are laid in a wound,” said Dr. Jim Lowe, a professor in the Department of Veterinary Clinical Medicine at the University of Illinois.

“The fly is the primary agent that migrates and it prefers to lay eggs in a wound,” said Lowe during a presentation at the Illinois Beef Association’s Annual Meeting and Awards Ceremony at the Illinois State Fairgrounds in Springfield.

“The fly lays the eggs in a wound, the eggs become larvae, the larva digest the tissue, pupate and come back as flies,” he said. “It is a relatively short time, 24 hours after the fly lays the eggs, there will be larvae and then they feed for five to seven days.”

Therefore, it is important for cattlemen to check their cattle daily.

“The larvae burrow into the tissue, they don’t just eat the surface and they produce a toxin that creates more dead tissue for them to feed on,” Lowe said. “They can take a relatively small wound and make it a big wound in a short period of time.”

Screwworm is not a contagious disease that spreads from animal to animal. Rather, female adult flies lay eggs in fresh wounds of warm-blooded animals. The larvae feed on the host and can damage vital organs or cause serious bacterial infections. Cases can be severe and can even cause the host animal to die.

Screwworm is not a contagious disease that spreads from animal to animal. Rather, female adult flies lay eggs in fresh wounds of warm-blooded animals. The larvae feed on the host and can damage vital organs or cause serious bacterial infections. Cases can be severe and can even cause the host animal to die. (USDA photo/Agricultural Research Service/AP)

Cattle infected with the parasite act sick.

“The signs of sickness are lethargy and isolating themselves, so it looks like respiratory disease,” Lowe said.

“The wound is going to look disgusting and you may or may not see the larvae, depending on how far they have burrowed in,” he said. “The wound is going to smell because it is rotten.”

For prevention, cattlemen should avoid creating wounds intentionally during fly season.

“Don’t create a wound for the fly,” Lowe said. “Move calving and processing to the cold season, which might really change what you are doing from a management strategy.”

Animals that are infected can be treated.

“We are going to treat the wound and use fly control,” Lowe said. “Treatment is not that hard and there is no animal-to-animal spread.”

“Ivomec and Dectomax have been approved for screwworm control, and Exzolt is a new product that is available with emergency authorization,” he said. “And we need additional classes of drugs because we will get resistance relatively quickly.”

Jim Lowe

Lowe (AgriNews photo/Martha Blum)

It is important for all cattlemen to have good relationship with their veterinarian, Lowe said.

“I cannot urge you enough to call a veterinarian if you see it,” the professor said.

“Screwworm is reportable, so if you see a wound call a veterinarian so he can get the right people involved,” he said. “We need to know where it is to get control.”

The female fly only mates once, so if it mates with a sterile fly, it will not produce fertile eggs and that stops the lifecycle of this parasite.

“We can create a buffer with the sterile flies and then push them south, but we have to know where the screwworm is to do that,” Lowe said.

“Producers have to be the first line of defense,” the veterinarian said. “If we see it, we have to say something — that is the only way we are going to stay ahead of this, by working cooperatively.”

“We move a lot of cattle and quite a few Southeastern cattle show up in this state,” Lowe said. “If we get penetration of the screwworm past Texas, that is the most likely way it will move up into Illinois, not the fly migrating itself.”

“It is not here yet, but we have to think about inspecting cattle regularly,” he said.

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Martha Blum

Martha Blum

Field Editor