They breed in open sores and orifices then eat their way through flesh. They squirm visibly under the skin while producing horrendous odors that rise from wounds like sewer gas belching out of a manhole.
Their scientific name, C. hominivorax, translates literally as “man-eater,” and they are slowly inching their way closer to you.
Known as New World Screwworm, these fly larvae can infect a variety of species, including humans, though they’re most commonly found in cattle. It was eradicated from North and Central America in the 20th century.
But in 2022, the maggots began rearing their heads in parts of Central America, and they’ve been moving north since.
In November, the parasite was discovered in a cow in southern Mexico, triggering a cattle embargo that was later conditionally lifted.
This month the larvae were found in cattle in the Mexican states of Oaxaca and Veracruz, 700 miles from the Texas border, and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) responded by indefinitely shutting down animal imports originating from or transferring through Mexico.
The U.S. Secretary of Agriculture Brooke L. Rollins this week released a statement on the USDA efforts to thwart larva invasion, including a $21 million emergency investment to revamp a screwworm sterilization facility in Metapa, Mexico.
“The investment I am announcing today is one of many efforts my team is making around the clock to protect our animals, our farm economy, and the security of our nation’s food supply,” Rollins said.
But the threat of spreading screwworm has only grown.
Industry experts believe it’s only a matter of time before the maggots make their way to the U.S., raising the prospect of new winners and losers in the North American cattle sector while promising to raise consumer prices at a moment when the cost of beef is already at record highs.
An adult cow with an advanced screwworm infestation. 
A cow infested with screwworm. (U.S. Department of Agriculture) 
An unidentified animal that died with new world screwworm. (The Natural History Museum via Alamy) 
Screwworm maggots. (U.S. Department of Agriculture) 
A fly laying eggs in the wound of a livestock animal. The larvae that hatch become screwworms, which can eat an animal or person alive. (U.S. Department of Agriculture) 
A horse with an infestation of screwworms on its back. (U.S. Department of Agriculture)
Short-Term Boost, Long-Term Headache
The U.S. is experiencing record high beef prices in response to steady growth in consumer demand, which has driven happy profits for livestock producers, according to David Anderson, agricultural economist at Texas A&M University, who says the threat of screwworm is all but certain to drive beef prices higher still.
Though the increase bodes well for producers in the short term, the potential spread of NWS to domestic livestock in the long term is likely to kick the American meat market in the seat, as mitigation measures would inevitably raise overhead on ranches far and wide.
Reflecting on ranch life during the 1960s eradication effort, Anderson explained how cowboys had to work additional hours to maintain healthy herds.
“Older ranchers will tell you it was terrible,” Anderson told Cowboy State Daily. “They had to be out there all the time, constantly roping, constantly [inspecting] the animals, doctoring them and cleaning out wounds.
“Then you’d turn them loose and do the whole thing again the next day. It was an everyday process, because if you didn't find those animals, they ended up dying.”
At a time when good hands are already hard to come by, as Platte County cattle rancher Steve Shockley previously explained to Cowboy State Daily, additional labor challenges could have profound impacts on both large and small operations around the West.
Higher Prices For Feedlots, Higher Prices On The Grill
Memorial Day weekend kicked off what the industry refers to as “grilling season,” a period of predictably higher seasonal demand for beef. This year's grilling season arrived on a tide of record high prices, and with beef import restrictions in place, butcher bills are set to jump even more.
Nor will the price hike be limited to consumers. Whether or not the screwworm makes its way to the U.S., feedlot operations will see costs increase as the supply of feeder cattle has already constricted significantly.
The U.S. imports roughly 1.3 million cattle from Mexico annually. Those animals are brought in shy of full maturity and destined for feedlots, where they’re fattened before going to slaughter. If the import border remains closed, that’s 1.3 million head left out of American markets.
“By closing the border, we have effectively cut the supplies of feeder cattle for sale in our market, and that’s going to put upward pressure on those prices,” Anderson said, adding that the first wave of impact is around the corner.
“If you think about those calves that we did not import in December and January, we're getting very close to the point where those cattle would be headed to a packing plant. That’s going to cut supplies when we already have record high prices for consumers,” Anderson said.
Too Much Or Not Enough?
The border closing has spurred some division among industry leaders.
The Southwestern Cattle Raisers Association, one of the sector's most influential lobbies, strongly supports hard import restrictions and believes minimizing the movement of cattle is necessary to stave off the spread of screwworm.
Although some in Wyoming think the USDA’s full closure is too much.
Mark Eisele, Cheyenne-based rancher and former president of the National Cattlemen’s Beef Association, believes that the inspection process between USDA and its partner agencies in Mexico is sufficient to keep at least some imports trickling in safely.
Eisele speculates that the shutdown of trade may be meant in part as a political message to the Mexican government.
“Mexican cattlemen have tried very hard to meet the inspection requirements, both theirs and also our USDA’s. They’re good people and they’ve got some good cattle. The problem was the Mexican government. The Mexican government was lazy and the corruption caught up to them,” Eisele said.
Containment south of Panama had been maintained through regular airdrop flights of sterilized flies into the Darien Gap. Though he can only cite hearsay, Eisele believes Mexican cartels and corrupt officials have held up airdrop flights of sterilized flies in the affected regions in extortion rackets.
“All we needed to do was keep a flow of those planes. But the cartels were extorting money for every flight of flies that came out of Panama. They were extorting $35,000 a plane,” he said. “So, for all practical purposes, this is really kind of a political closing to make a point that they have got to get their act together.”
The threat of screwworm is not limited to cattle. It can also infect horses, dogs, and big game. If it reaches the U.S., it could hit additional revenue streams on ranches and tourism.
For instance, “high fence” hunting operations, in which sportsmen pay private property owners fees for hunting on their lands, could take a hit if the maggots spread to big game.
As for cattle, the processing sector is likely to look for new trading partners in Canada or Australia if the embargo is upheld in perpetuity, Eisele says.
If opinions differ on the appropriate scope and scale of the response, there is one area of uniform consensus: ramping up fly sterilization efforts.
Sterile Insect Technique
When it first became endemic in the mid twentieth century, the U.S. government attacked the problem using what’s known as Sterile Insect Technique (SIT).
Screwworms were raised in factories, sterilized with radiation and then air dropped by the millions into affected areas until sterile flies choked out the reproductive populations.
By 1966, screwworm was eradicated across America, and by 2006 they were eradicated from every country above a biological containment border established in Panama’s Dorian Gap.
This practice has continued in perpetuity at a biosecure facility in Panama, explained Dr. Maxwell Scott, an entomologist who worked for the USDA’s screwworm program in the 2010’s.
Here, flies are bred in mesh cages, and then eggs are collected on 20-liter trays filled with a larval diet. The diet is composed from a mixture of cattle blood, egg and milk protein, along with a cellulose bulking agent and formaldehyde preservative.
‘Putrid’
Rack upon rack upon rack, maggots by the millions squirm to life. It’s not for the faint of smell.
“I can’t even describe the smell of the place. It’s just unbelievable. Particularly the larval room. You really can’t go in there without a respirator,” Scott told Cowboy State Daily. “It's a putrid smell.”
At the pupal stage, flies are blasted with a targeted dose of radiation, which sterilizes both males and females.
They’re stored in coolers enroute to the Panama airport, from which they’re loaded into planes and parachuted into the Dorian Gap and other affected areas. It’s been a remarkably successful program – until recently.
If the problem gets worse, it could come with a steep price tag for the U.S. and partner countries, Scott says, as the sterilization programs aren’t cheap. Between distribution on planes, facility scientists and surveillance personal in the field, the program cost the U.S. $750 million in the 1950s and 1960s.
In his work with the USDA’s screwworm program, Scott pursued gene-methods with the aim of bringing down the program costs.
The best way to improve the economics of the fight against screwworm, he believes, is through “gene drive” techniques from CRISPR technology, a powerful gene-editing tool that allows scientists to precisely cut and modify DNA within living organisms.
Scott was able to develop a strain of male screwworm with a gene that was lethal to females. The female-killing gene, furthermore, could be “turned off” by adding tetracycline antibiotics to fly diets.
In this way, scientists could focus on male-sterilization exclusively and cut down on the program’s diet costs.
Additionally, because females require higher levels of radiation to sterilize, the current standard of simultaneous radiation means the fitness of male flies can be compromised.
By requiring less radiation, Scott’s modified strains would have a better ability to overwhelm reproductive populations once released, while also lowering radiation-related expenses in the lab.
However, after only a single, limited field test in Panama in 2018, the project’s funding was stopped.
“I was disappointed we didn't get the green light to go ahead with gene drive, because I think in the long term some type of CRISPR-based solution is what’s needed. It's just so much more efficient than SIT.”
His research wasn’t for naught and is currently helping facilitate gene-modification programs for screwworm eradication in Uruguay.
Scott says that the problem of screwworm is liable to worsen in Wyoming with climate change, as the flies' reproductive season widens with warming weather. He’s also concerned that other animals, especially deer, are going to be hit hard by screwworm in the U.S. because they are poorly monitored.
Humans, too, are likely to feel the pain of spreading screwworm.
“It’s not common in humans, but there are cases. They're usually elderly people or homeless people not being looked after,” Scott said, going on to describe what it’s like for such unfortunate souls.
“This is a problem in Uruguay. I worked with a dentist, and he said, particularly when he's in the rural area, he gets people coming in complaining of a toothache. But turns out it's maggots from screwworms infested in their mouth,” he said. “It can be painful. Once they start burrowing in, they put these hooks out and it’s a bit difficult to pull out.”
Zakary Sonntag can be reached at zakary@cowboystatedaily.com.
