US and Mexico work together to eradicate new world screwworm
For the second time in under six months, the U.S. is closed to cattle, sheep and bison imports through ports of entry along the southern border. Secretary of Agriculture Brooke Rollins announced the closure on Sunday, May 11.
“The United States has ordered the suspension of livestock imports through ports of entry along our southern border after the continued spread of the new world screwworm (NWS) in Mexico. Secretary [Julio] Berdegué and I have worked closely on the NWS response; however, it is my duty to take all steps within my control to protect the livestock industry in the United States from this devastating pest,” Secretary Rollins said in a USDA press release. “The protection of our animals and safety of our nation’s food supply is a national security issue of the utmost importance. Once we see increased surveillance and eradication efforts, and the positive results of those actions, we remain committed to opening the border for livestock trade. This is not about politics or punishment of Mexico, rather it is about food and animal safety.”
USDA stated that the U.S. and Mexico continue efforts to interdict and eradicate NWS in Mexico and work in good faith. However, despite these efforts and the economic impact on both countries due to this action, there has been unacceptable northward advancement of NWS and additional action must be taken to slow the northern progression of this deadly parasitic fly. As such, effective immediately, the USDA Animal Plant Health Inspection Service in conjunction with Customs and Border Protection will restrict the importation of live animal commodities originating from, or transiting Mexico.
This import suspension will persist on a month-by-month basis, until a significant window of containment is achieved. USDA will continue constant collaboration with Mexico, including a review of the latest data and metrics in two weeks.
“Our teams have been in daily communication discussing how we can build on the good work that has been accomplished to improve our strategy toward eradication,” said the press release. “Any livestock currently in holding for entry into the United States will be processed normally, this includes an APHIS port veterinary medical officer inspection exam and treatment to ensure they are not carrying NWS.
“Effective eradication, which remains the shared goal and stands in the best interest of both the U.S. and Mexico, requires robust, active field surveillance with education and outreach to ensure prevention, treatment, and early detection; controlled animal movement to limit spread; and sustained sterile insect dispersal. Suspending livestock transport through southern ports of entry will assist in the effort to limit northbound transport of NWS through livestock commerce, and will allow the U.S. to reassess whether current mitigation standards remain sufficient.”
The northward spread of NWS is possible through natural wildlife movements, including wildlife that transit the border region without impediment. USDA stated that it is taking all possible actions to monitor for and limit the northward movement of NWS. The USDA Tick Riders are monitoring livestock and wildlife along the southern border region between the ports of entry for the presence of NWS.
In the 1950s and 1960s, the U.S. successfully led efforts resulting in the eradication of NWS in the U.S. and Mexico, at a cost of billions of dollars. Screwworm has moved north over the past two years, spreading throughout Panama and into Costa Rica, Nicaragua, Honduras, Guatemala, El Salvador, Belize, and now Mexico. USDA/APHIS considers the NWS a serious threat to animal health, the U.S. food supply, and national security.
“NOBODY WINS”
Ron Gill, professor and extension livestock specialist for Texas A & M AgriLife Extension visited the Chihuahua Port of Entry recently where he observed livestock inspection protocols firsthand. Gill express mixed emotions over the border closure.
“Nobody wins. I don’t think anything would come across on cattle being imported, but with that said, there is still movement on trucks from further south to the border, so closing it makes sense,” he said.
He believes closing the border is necessary due to the urgency of the NWS situation, not just for the health of livestock in the U.S. but also for animals in countries to our south.
“Every country is feeling the impact of the NWS,” Gill said. “We need to get more coordinated and aggressive in our approach to stop the advancement of flies.”
While he believes the inspection protocol is stringent enough to prevent screwworm infested animals from crossing the U.S southern border, Gill has heard concerns that reporting south of the border may not be as timely as some would like or need. He also believes that an increase in the production and distribution of sterile male flies is needed to slow the spread of the NWS.
There is a possibility that a fly production plant would be developed in Texas, Gill said.
“The distribution of sterile flies continues to be brought up in every discussion I hear about the border closing,” he said. “The only production facility is in Panama. It was originally set up to produce about 25 million sterile flies per day. I read a report this morning that 100 million sterile flies are being shipped out of this facility every day.”
Gill experienced a screwworm outbreak when he was a teenager, and helped doctor infested animals on the family ranch.
“It’s just a never ending battle, and pretty gruesome to observe,” he said. “We can catch our cattle and sheep and goats and treat them, but with the wildlife population, there’s not much you can do. It would really decimate the wildlife population and increase the workload on ranches trying to find and treat affected animals before the screwworms do extensive damage to the animal. That’s the hard part, especially in brush country, where it can be hard to get cattle checked and treated as needed.”
The border closure will eventually impact consumers and the U.S. beef supply, Gill said, but he is hopeful that the situation will be resolved.
SCREWWORMS EAT ANYTHING
Cuero, Texas, rancher and former R-CALF Region V Director, Stayton Weldon, experienced nearly a decade of screwworm infestations in his youth.
“It usually started in the early spring and went all summer long,” he recalled. “As long as the cows were having calves, not only the calves got the screwworms, the cows did too. If there was any afterbirth, or even if it was just blood it didn’t make any difference what it was or how it got there, they would lay those eggs and you’d have to catch them and start doctoring them once a day until you got rid of the screwworm and they started healing up.”
Any blood on the cow’s vulva would attract the flies, who would come lay their eggs, and the newly hatched larvae would migrate into the cow’s vagina.
“You could doctor a cow one day, and might be able to turn her out the next morning if there was no worm activity,” he said. “You’d have to look for them, and you didn’t need a microscope.”
One cow, Weldon recalls, had a persistent infestation of screwworms in her reproductive tract.
“I’d put her in the chute to doctor her, and it got to where she was kind of gentle, so I’d rope her and tie her up close to a post and doctor her. It got to where I could just walk into the pen, walk up behind her and just doctor her and she would just stand there. She had it really bad: it wasn’t just on the surface, it was in her.”
The screwworms would eat anything.
“In another instance, we didn’t catch a cow in time, and they had half of her head eaten off: her eye, her jaw all the way back to the hinge. We had to shoot her, there wasn’t any way we could save her. They had probably gotten into her brains.”
Some cows got ticks on their ears, and screwworms moved in right behind them. They had to remove the part of the ear where the screwworms were.
“Those dad gum ticks would get in their ears, and the screwworms would get in where the ticks were,” Weldon recalled. “You can’t believe how many ears were almost completely cut off. If you didn’t do it, they would end up like the cow with half her head gone.”
They could expect every baby calf born to get screwworms.
“My dad had registered Herefords, and nearly every calf would get them, mainly in the navel, because that’s where the first wound was.”
Weldon’s uncle had Brahama cattle, which were less susceptible to screwworms.
“You very seldom had to doctor the Brahama calves, those Brahama cows would lick the worms out; they would lick on and lick on and lick on their calves,” he said.
Weldon became an expert roper, doctoring anywhere from 25-50 calves per day from the time he was 9 years old until NWS was eradicated when he was 18. He can still picture the little fly boxes all over the pasture.
“They would fly over and drop them and when they hit the ground that would release the sterile male flies,” he said.
At 86 years of age, Weldon is still ranching.
“I think the border closure is the most wonderful thing to happen to cattlemen in South Texas, New Mexico, and all along the border,” he said. “They need to shut it down tight as a tick. If we get screwworms back, it will be very detrimental on ranchers’ bottom lines.”
If screwworms return, he will sell out.
“I will not go through it again,” he said. “It’s horrible. I cannot have it come back into my cattle or I’m finished.”
Weldon does not see the border closure as merely political.
“Screwworm maggots eat live flesh, that’s what makes them so bad. They are too detrimental to everyone. We need to be scared to death of them coming back. It’s horrible, it really is. The damage they do is really indescribable, and I have tried to describe it in the most indelicate way I could.”
NCBA SUPPORT
Since November 2024, when the first case of new World screwworm was reported in southern Mexico, the National Cattlemen’s Beef Association and state affiliate partners have been closely monitoring the situation. Colin Woodall, NCBA CEO, shared his organization’s views on the NWS situation.
“The best way for Mexico to prevent the spread of new world screwworm is through the use of sterile insect technique, which releases sterile male flies to mate with wild female flies, ensuring no offspring and ultimately leading to eradication,” he said. “Unfortunately, the Mexican government had created unnecessary bureaucratic hurdles that made it difficult for the planes carrying these sterile flies to land and properly distribute them in southern Mexico. The flies have continued advancing north and now they are roughly 700 miles away from the U.S. southern border. That is too close for comfort, which is why NCBA supports Secretary of Agriculture Brooke Rollins’ decision to protect the American cattle industry by closing the border to imports of cattle, bison and horses.”
The NCBA supports the recent border closure, in spite of short-term economic impacts, particularly in states that share a border with Mexico where cross-border trade has been a part of the economy for generations, Woodall said. However, these impacts will pale in comparison to the damage that would be caused by new world screwworm if it returns to the United States and we are forced to fight the pest on U.S. soil.
“The new world screwworm is a nasty parasite that was eradicated from the United States back in the 1960s,” he said. “It cost millions of dollars and caused unimaginable suffering for cattle and other animals. It also required years of collaborative effort by livestock producers and the government to eradicate it. The damage that would be caused by a new infestation of new world screwworm cannot be overstated.”
Preventing the migration of NWS into the United States is paramount, Woodall said.
“Allowing new world screwworm to reach the United States would cause millions of dollars of direct economic damage to cattle producers here, not to mention the toll it would take on producers having to deal with the treatment and eradication of screwworms,” he said. “The collateral damage to wildlife, pets, consumers and other segments of our nation can’t be calculated, but they would be immense and completely avoidable if we prevent new world screwworm from continuing its northward movement in Mexico. We cannot allow this pest to return to the United States.”
Woodall said that conversations around an enhanced EID program are separate from the fight against NWS.
“EIDs and new world screwworm are two completely separate discussions,” he said. “EIDs help the cattle industry respond to diseases that spread rapidly between cattle, like foot-and-mouth disease (FMD). In animal health emergencies like a FMD outbreak, it is important for animal health officials to determine where cattle have been to isolate the spread of the disease.”
“New world screwworm is a pest that doesn’t require animals to be in close proximity, it can easily impact different herds miles apart. The screwworm can also infest other warm-blooded wild animals and even humans. The spread of new world screwworm is best prevented by the release of sterile flies. NCBA and state affiliate partners are focusing our efforts on maximizing the production of sterile flies at an existing facility in Panama while also working with Congress and USDA to open a sterile fly facility in the United States.”
NCBA and Texas & Southwestern Cattle Raisers Association are working with Congress to pass the STOP Screwworms Act. This bill would fund a sterile fly production facility in the United States, allowing us to release our own sterile flies that will eradicate screwworm before it can devastate the U.S. cattle industry.
Woodall encourages producers to remain vigilant and closely inspect their cattle. For more information on what to look for, visit ncba.org/NWS or https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm.
