Judy McCullough was just a child when the flesh-eating new world screwworm harried her grandfather's cattle ranch in Oshoto, Wyoming, more than 70 years ago. But she hasn’t forgotten the havoc it wrought.
She recalls barn walls splattered with blood and the pungent smell of a chemical tar that was nearly impossible to escape.
“The blood would just spray out and decorate the walls of the barn,” McCullough said, explaining how ranchers resorted to castrating and dehorning calves late in the year during the winter hiatus of the screwworm fly.
“When you dehorn a little calf, there’s very little blood at all. But when it's an 800-pound yearling, it’s a big bloody deal, let me tell you,” she said.
Tar mixed with chemicals like DDT was applied to cattle sores to prevent flies from depositing eggs, and it was costly in unforeseen ways.
“Little boys would run around with tar buckets and sticks and put tar on the brands and cuts and where you dehorn,” she said. “That tar ruined everybody's clothes.
“You’d put it on the calf, but then it gets on the wrestlers, and then it gets on the rope, and then it gets on the roper and his horse and his saddle it gets on the guy branding them. We just had to throw our clothes away every time.”
She thought she’d seen the last of it in 1966 when screwworm was eradicated in the United States. But it’s beginning to look as though the parasite may visit her ranch again.
What is screwworm?
New world screwworms (NWS) are fly larvae with uniquely sharp teeth, and they’re able to infect a variety of species, including humans, though they’re most commonly found in cattle.
Different from other parasites, they don’t merely inhabit a host, but rather consume their host until it dies.
They were eradicated from North and Central America in the late 20th century with measures like Sterile Insect Technique (SIT), in which sterilized flies are disseminated in screwworm hotspots to squelch out reproductive populations.
But in 2022, the parasite snuck across a biological barrier in Panama as the result of illegal cattle shipments by criminal cartels, many believe.
The larvae have been inching their way northward since.
Feds and cattle industry advocates are scrambling to head the problem off, but some say it's not a matter of if the screwworm arrives, but when.
Last week, U.S. Secretary of Agriculture Brooke L. Rollins announced plans for the construction of anSTI facility in Edinburg, Texas, along with $100 million made available for innovative solutions, which may include widespread cattle immunizations.
The threat is poised to restructure the American marketplace for meat, but not all industry groups see eye to eye about how the new marketplace should operate.
Meat Market Shakeup
Claims of the screwworm reaching Missouri spread online last spring. The claims turned out to be false, but their impact on the cattle futures market was very real, causing prices to fall by 6% overnight.
“Our industry is highly susceptible to rumors and to disease threats,” said Bill Bullard, CEO of R-CALF USA, an industry group representing cattle ranchers.
“We’ve called for an investigation by the Commodity Futures Trading Commission into that false report, because it had a tremendous impact on the cattle futures market, and on cash prices too, which means we had producers who were harmed financially as a result of that false report,” Bullard said.
It reveals the potential impacts cattlemen will face if screwworm crosses over the U.S. border.
Paradoxically, however, the threat of NWS has so far played to the favor of ranchers in Wyoming and other U.S. states, as an embargo on Mexican cattle has helped drive demand for domestic beef.
In 2024, the U.S. imported 1,249,000 cattle from Mexico, along with 797,000 from Canada, which made up around 22% of the American meat market.
With Mexican cattle no longer part of the equation, U.S. processors and feedlots have become more reliant on domestic operations, a welcome development for states like Wyoming.
This comes as the nominal price for beef is already at an all-time high. Yet, these strong prices have come for all the wrong reasons, said Bullard.
For decades, ranchers experienced depressed domestic cattle prices due to increased international competition, market consolidation and severe drought in the western U.S.
This resulted in a perennial production decline that put the collective national herds at their lowest number since the 1950s.
The ‘Big Four’
In 1980, there were 1.3 million beef cow operations in the United States. By 2022, there were 622,000 operations, with an average of 44 head per, according to the latest census report; it also showed that in the five-year period from 2017 to 2022, the country lost 107,000 cattle operations.
“We've been displacing our domestic cattle producers with imported products, and the only way that we're going to reverse that is if we begin to manage those imports,” said Bullard, who lays much of the blame on corrupt influence by the “Big Four” meatpacking companies, which control around 80% of nation’s boxed meat and largely call the shots on cattle import dynamics.
The Big Four are lobbying to lift the embargo while pop-up processing plants are already in the works to see Mexican cattle get across the border in the form of boxed meat.
Also hurt by prolonged drought in the U.S. West, the diminished supply of cattle is what’s caused meat prices to rise. But cattleman would rather see their fortunes improve for other reasons.
“The legitimate screwworm threat has forced the United States to begin an introspective look at our ability to be self-reliant in beef production,”
Bullard said. “If we manage imports, we can provide the economic incentive for our industry to rebuild.”
For now, the embargo is poised to strengthen the price for domestic beef, but the threat of screwworm may yet decimate large and small entities alike.
Officials are pulling out the stops for a forceful response, yet as the focus remains on cattle, some are worried that a bigger threat lies in other species.
Cattle Not The Biggest Concern
In the mid-2010’s, an endangered sub species of white-tailed deer known as Key deer were nearly wiped out in the Florida Keys by NWS. An aggressive STI effort pushed the screwworm out by 2017 and saved the species.
Because the breakout was isolated on islands it made an STI especially effective, but a similar instance on the mainland would be much harder to contain, according to Mark Eisele, former president and current officer of the National Cattlemens Beef Association.
The example of the Key deer shows how the scope of the problem goes beyond cattle, and Eisele believes wildlife and humans are the bigger concerns in advancing the spread of NWS.
“It's tourists and their pets, and wildlife. Those are our biggest concerns,” Eisele said. “Imagine someone goes to Puerto Rico for a vacation.
“They take the family dog or the kids get scratched while they're hiking. They get an infection. They come home and now you've got screwworms in your neighborhood, and no one understands what they're looking for.”
APHIS Cutbacks
The threat of NWS coincides with cuts at the bureau that stands at the forefront of screwworm containment efforts.
The Animal and Plant Health Inspection Service (APHIS) is charged to safeguard U.S. agricultural sectors by preventing the introduction and spread of pests and diseases, among other responsibilities.
As many as 1,300 APHIS employees, including 300 veterinarians, have taken the Trump administration's offer for early buyouts, creating anxiety among some in the ag sector who worry it could impact the nation’s ability to stem the tide.
“We have made it crystal clear to the administration and to Secretary (of Agriculture Brooke) Rollins that they have got to help us on this, because this is not just an agriculture problem,” Eisele said.
“There were some legitimate reductions, but there is a flood of people leaving,” he added. “So we have asked the administration to ease up on agriculture and USDA workers to make sure it has the staff they need.”
Ivermectin
It's unclear if the agency will be able to do more with less, but there are hopeful innovations in the treatment of screwworm likely to enhance eradication efforts this time around, including ivermectin.
Ivermectin is a broad-spectrum antiparasitic drug that has proven effective in the fight against Texas fever tick, a parasite that transmits a bloodborne pathogen known as babesia, which destroys red blood cells.
APHIS has controlled the spread of fever tick using ivermectin-laced feed and mineral licks distributed along counties that abut the Mexican border. They also put ivermectin in deer feeders.
It has not been cleared for systemic oral use in cattle, with the exception of a few southern U.S. counties, but some are pushing to deploy ivermectin on a large scale.
Dr. Max Thornsberry, DVM and board member of R-Calf USA, is among those leading the effort to approve widescale oral use in cattle.
“We have a medication that is 100% effective against screwworm larvae, that's ivermectin, as well as some ivermectin analogs.” Thornsberry said.
There will be some caveats if the drug is approved for mass application. For one, he said there is what's referred to as a “withdrawal period” of 35 days between when an animal has consumed the drug and when it can be slaughtered for consumption.
There is also an analog drug known as Eprinex that can be poured over the backs of cattle and absorbed into the blood through the skin. There is no withdrawal period with this analog, but that doesn’t mean it’s an efficient solution to screwworm.
“I don't know if you work with cattle, but if you work cattle every three weeks, they’d see you coming and they would run the other way just as hard as they could run,” said Thornsberry, who still lives on the same Missouri ranch where he was born in 1953. “The problem is the practicality of doing it.”
It shows that screwworm will likely increase labor needs at a time when good hands are already tough to come by, according to Platte County rancher Steve Shockley.
Prepare for the worst?
Judy McCullough, who now lives on a ranch in Moorcroft and who may yet see the ravages of NWS again in her lifetime, believes that the best thing to do is keep a tight and secure watch on the U.S. border.
In the meantime, she’s found herself in a state of Biblical contemplation.
“If you read the Bible, you will see it says that in the last days there will be a fly that will come upon and eat the people's flesh while they're alive,” she said. “I’m not saying that’s what this is, but the thought crossed my mind, because this is a dangerous fly.”
Zakary Sonntag can be reached at zakary@cowboystatedaily.com.
