A Maryland case of New World screwworm in a human being illustrates an important fact about the flesh-eating parasite that has the agricultural world on edge.
Screwworms have no respect for borders, and, lacking a favored food source, they will infect any animal host they can. That includes a hapless human just traveling through.
The human case in Maryland was found in a patient returning from travel in Central America. The U.S. Department of Health and Human Services officially announced the nation’s first confirmed human case of New World screwworm on Sunday, though the case was actually diagnosed Aug. 4.
The individual has since recovered, and there is so far no indication of any transmission to humans or other animals. But alerts have gone out across the nation, nonetheless, to other agricultural stakeholders. That includes Wyoming agriculture and veterinary officials, who told Cowboy State Daily they are mainly pushing awareness right now, as well as heightened vigilance across the livestock industry.
“There’s not much to be done in Wyoming as of now, other than to monitor for possible emergence of screwworm,” Wyoming Department of Agriculture spokesman Derek Grant told Cowboy State Daily Monday morning. “We are keeping our eyes out with our federal, state, and local partners through their normal monitoring.”
Wyoming State Veterinarian Dr. Hallie Hasel said her office and the Wyoming Livestock Board have sent educational information out to livestock producers about screwworm, including photographs and videos. They encouraged people to report anything unusual they notice with any animal, whether livestock, wildlife, or pets.
“We have to depend on our veterinarians and our producers to report anything they see that is unusual about a wound,” Hasel said. “There’s a lot of information out there in the public now, we have education brochures and there are a lot of educational webinars, and a lot of photos being handed out to let people know what this looks like.”
An adult cow with an advanced screwworm infestation. 
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) 
An unidentified animal that died with new world screwworm. (The Natural History Museum via Alamy) 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) 
Screwworm maggots. (U.S. Department of Agriculture)
The Stories Grandpa Tells
Many of today’s Wyoming ranchers have no direct experience with screwworm, but they’ve heard stories of just how devastating an outbreak here could be.
Among them is Tyler McCann, who ranches near Lander, Wyoming.
“My father-in-law was alive during the last screwworm, I don’t know if you’d call it a pandemic or what, but he vividly remembers them having to doctor cattle and all the effort they had to put into eradicating the screwworm,” he said. “So being just one generation removed from that, we’re very concerned about what the implications would be if we started seeing that here.”
In McCann’s conversations with a retired state veterinarian, he believes Wyoming is less vulnerable than places like Texas, simply owing to its more northern latitude.
The screwworm is a tropical and subtropical parasite, requiring warm, moist conditions, rather than very cold or very dry weather. Geographical barriers like oceans and high mountains can also restrict their natural distribution.
“We feel very secure because of our geography and being that far away,” McCann said. “But there’s always a possibility, and we don’t know how effective what they used to treat this was in the 1960s. We’re curious if it wasn’t more cyclical, but we’re definitely going to be listening to any advice that the veterinarian has, if it does cross the border, and we’re going to be very mindful.”
McCann said his operation typically screens cattle in the fall for a variety of problems. He’s added the potential for screwworm to that checklist.
“As we’re weaning calves and processing cattle for where they’re going to end up in the winter, while we’ve got them in the corral in a chute, that’s when we typically administer vaccines or wormers or any of those kinds of things,” he said. “We’ll definitely be keeping an eye on this as we go into the fall. It’ll be a great opportunity to look them all over.”
The Stuff Of Horror Films
Mark Eisele, former president of the National Cattleman’s Beef Association, and a Wyoming rancher, is also keeping a closer watch on his herds. He is very familiar with the dangers of screwworm for American agriculture and, in his position as a past president, has been active in efforts to ensure the pest doesn’t return.
“It really is a horrific critter,” Eisele told Cowboy State Daily on Monday. “Most parasites don’t kill their hosts. There’s this courtesy thing that I keep you alive, you keep me alive, I don’t kill you, that sort of thing.”
But screwworms don’t follow that norm at all. Screwworms are the stuff of a living nightmare. While the parents of screwworms look and behave much like black flies commonly found in the United States, they have a crucial difference.
The adult females lay their eggs directly on open wounds or sores, 200 or so at a time. Lacking that, a nasal cavity will do just as well.
The larvae that hatch from those eggs aren’t all that interested in dead tissue like typical maggots would be. They want living flesh. They burrow down deep into living tissue using sharp mouth hooks, screwing themselves deeper and deeper into their hosts for their suppers, until, ultimately, they kill it.
It’s from this behavior they get their name, screwworm. All of this may sound like the stuff of horror flicks, but there’s nothing fictional about it. This is a real, live, gruesome pest. And it will cost the U.S. agricultural industry billions of dollars if this miserable creature manages to gain a foothold here once again.
Building A Biological Wall
Screwworms were actually once common in the southern United States up until the 1950s or so, and they cost the agricultural industry millions annually. The pests ranged from Florida to California, infecting all sorts of living, warm-blooded animals, not just cattle. They feasted on deer, squirrels, pets, and even the occasional human as well.
Their scientific name, C. Hominivorax, in fact, means “man eater” in Latin. They earned that name after an outbreak on Devil’s Island, which was an infamous 19th-century French penal colony in South America.
In the 1950s, the United States Department of Agriculture started an ambitious effort aimed at eradicating screwworms in the United States with sterile flies.
Each screwworm female only mates once. And if that mating occurs with a sterile male, she cannot produce any viable eggs.
Wyomingite Larry Friedman was among researchers who were directly involved in that ground-breaking work. He had planned to work in Wyoming, but instead was assigned to drive to remote, Spanish-speaking ranches in the 1960s, helping to identify and eradicate screwworms in Texas.
The effort was ultimately a huge success, and soon expanded to Mexico, which eradicated them in the 1970s, and to Central America, which achieved the same in the early 2000s, pushing the New World screwworm flies all the way back to the Isthmus of Panama.
There, the United States and Panama have been collaborating to build a kind of biological wall, dumping nearly 15 million sterile flies on that narrow strip of land every week.
Inspectors, meanwhile, travel thousands of square miles on horseback, by motorcycle and boat, looking for any stray screwworm infestations north of that invisible, biological wall.
It costs millions, but has saved billions, by keeping the blowflies from moving north with their flesh-eating babies.
Unknown Breach
The wall concept worked well for the past 20 or so years until 2023, when the pest somehow breached containment at what’s known as the Darien Gap in Panama.
It’s not known exactly how the breach occurred. Are the screwworm flies adapting to existing sterile fly technology? Are warmer temperatures changing its normal range? These are just some of the questions occupying researchers right now.
One thing that is known. Since breaching containment, the pest has moved much more rapidly than expected into Central America and Mexico. It’s now about 370 miles from the Texas border.
Experts have pegged the rapid movement of this pest to infested livestock traveling through the middle of Central America as the most likely contributing factor in the rapid spread.
Individually, screwworm flies only travel 12 miles in their month-long lifetime. But they can travel much greater distances while developing inside their hosts, whether livestock, wildlife, or, in more rare instances, human beings.
Each new generation of fly is sexually mature within a couple of weeks. And each adult female can lay up to 2,800 eggs over her lifespan.
Sterile fly factories continue to be the main focus for efforts to re-establish containment of this pest and keep it out of America.
In Panama, there’s a facility sterilizing 100 million new flies every week, and, at another factory in southern Mexico that will open in 2026, another 100 million sterile flies will be produced.
U.S. Building New Sterile Fly Factory
The United States is going to add to those efforts. U.S. Agriculture Secretary Brooke Rollins just announced last week a sterile fly production facility in Edinburg, Texas, at Moore Air Force Base. It will produce up to 300 million sterile flies per week, and its work will be coordinated with facilities in Panama and Mexico, Rollins said.
The U.S. Department of Agriculture will also be investing up to $100 million in researching viable innovations to accelerate the pace of sterile fly production. There will be longer-term research projects looking for new techniques, new traps, new lures, and new therapeutics to bolster responses to screwworms.
Human patrols will increase as well. The USDA will hire more mounted patrol officers — so-called “Tick Riders” —to physically patrol borders looking for any signs of infestations, along with other animal health experts, who will patrol the border in vehicles.
Detector dogs are being trained who can sniff out screwworm infestations in livestock or other animals along the border and at various ports of entry. The United States will work closely with Mexico to lock down animal movements to prevent further spread.
“The construction of a domestic sterile fly production facility will ensure the United States continues to lead the way in combating this devastating pest,” Rollins said. “If our ranchers are overrun by foreign pests, then we cannot feed ourselves. USDA and Customs and Border Protection are constantly monitoring our ports of entry to keep NWS away from our borders. We are working every day to ensure our American agricultural industry is safe, secure, and resilient.”
Complacency Is Still A Danger
These are all measures Eisele expressed confidence in on Monday.
“We’re taking really serious measures with the sterile fly program to keep them out of the country,” he said. “I think it’s going to be successful, I genuinely do, because we’re working on both sides of the border and we have the cooperation of the Mexican government.”
That doesn’t mean, however, Eisele added, that people should get complacent. As the Maryland traveler who contracted screwworm demonstrates, Texas and other southern states aren’t the only places that face this risk.
“That person went to three or four practitioners before someone figured it out,” Eisele said. ‘So, we’ve got to do a little work with the medical profession to make them understand what they’re looking at, what they need to do to deal with it, and to be aware of that.”
Same thing with veterinarians, Eisele added.
“Veterinarians who are used to maybe their cat, dog, small animal practice aren’t used to seeing this,” he said. “So, the first thing that they’ve got to ask is, have you traveled abroad? Have you traveled to other countries South or Central America? That is the first critical question or step that needs to be taken.”
The oncoming cool season buys many states in America some time, Eisele added, to get ahead of things.
“We’ve got a chance to stop this,” he said. “It will take a while to get the fly facilities up and running, but they will be up and they’re doing a lot of stop gap measures in between. I think diligence of both the Mexican ag officials and the USDA are really going to help us out in this pinch.”
But that diligence can’t stop with officials, Eisele added. Everyone must raise their awareness, even ranchers in Wyoming. Screwworms have been found as far north as North Dakota back when they were still common in the United States. And they’re really only as far away as the nearest airport carrying a traveler from one part of the world to another.
“The real point of this message is, we all need to be paying attention on all fronts, human, domestic, pet, wildlife, and obviously agricultural livestock,” he said. “Because all of those factors, if we have a missed opportunity in any of those positions, we could end up with screwworm.”
Renée Jean can be reached at renee@cowboystatedaily.com.
