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Published : 16 Aug 2025, 10:56 AM
He was only eight years old in 1973, but fifth-generation Texas rancher Kip Dove remembers spending countless days trotting up to sick and dying cattle on horseback that year during the last major outbreak of flesh-eating screwworm. He carried a bottle of foul-smelling, tar-like medicine in his saddlebag and a holstered revolver to shoot any animals too far gone to treat.
Surrounded by baying cattle dogs and cowboys, the infested cattle kicked and bit at their open wounds, staring wild-eyed at the truck headlights illuminating them and giving off the unmistakable smell of rotting flesh, he recalls.
Now surrounded by a healthy herd of longhorn cattle, Dove is anticipating the return of screwworm, the parasitic fly that eats livestock and wildlife alive. From 1972 to 1976, a screwworm outbreak in the United States infested tens of thousands of cattle across six states, cost tens of millions of dollars to contain, and was only defeated after a massive eradication effort.
Today, the parasitic flies are pushing northward from Central America again after being officially eradicated from the US in 1966, threatening $1.8 billion in damage to Texas’ economy alone, according to a US Department of Agriculture estimate. An outbreak could further elevate record-high beef prices by keeping more calves out of the U.S. cattle supply.
Ranchers in central Mexico are discovering the dreaded fly’s maggots burrowed in their cattle for the first time in a generation, and a factory in Panama is losing a race against time to breed sterile flies, the most powerful tool to quell an outbreak. As cases in livestock – and occasionally in humans and house pets – increase, it's more likely than not that the fly will infest the US again, Dr Thomas Lansford, assistant state veterinarian at the Texas Animal Health Commission, and other experts told Reuters.
“I don’t know what we’re going to do,” Dove said, folding his arms, scarred from decades of riding horses and chasing cattle through thorny brush.
Female screwworm flies lay hundreds of eggs in wounds on any warm-blooded animal. Once the eggs hatch, the larvae use their sharp, hooked mouths to burrow through living flesh — feeding, enlarging the wound and eventually killing their host if left untreated. A tiny scrape, a recent brand or a healing ear tag can quickly become a gaping wound, carpeted with wriggling maggots.
“The smell is bad, and some of the wounds are horrific. You have humongous holes in these animals teeming with worms,” Dove said. “I don’t know if I could handle it if it happens now."
Washington has halted cattle imports from Mexico and invested millions in setting up a new sterile fly production plant in Metapa, Mexico. But it will take roughly a year to come online. So, cattle producers in the U.S. are stockpiling insecticides, making contingency plans and sounding the alarm that a shortage of skilled ranch labour will hamstring their ability to detect and treat screwworms.
Treatment is low-tech and onerous: vets and ranchers must scrape each worm out of the infested animals by hand before spraying the wounds with an insecticide.
In 1973, Dove was a child who could rope cattle for treatment until 2 a.m. and head to school the next morning. Now at age 60, injuries accumulated from years of ranching would make it more difficult to do the exhausting work of managing cattle during an outbreak, he said.
Freddy Nieto is the manager at El Sauz Ranch in South Texas, which runs cattle but also offers deep-pocketed clientele the opportunity to hunt wildlife, from whitetail deer and wild hogs to exotic animals. “This might be the worst biological outbreak that we're facing in our lifetime,” he said.
The multi-billion-dollar hunting industry is especially vulnerable since wildlife infested with screwworms are essentially untreatable. They often disappear into the thick brush to die from their wounds.
'IT'S AN OVERWHELMING SITUATION'
In the sweltering heat and humidity of Panama City, a world-renowned biological facility has operated since 2006. Biologists and technicians work in extreme heat surrounded by pungent odors — an overwhelming mix of ammonia and the artificial diet fed to fly larvae — to breed up to 100 million sterile screwworm flies per week.
Flies are fed a carefully formulated mixture of egg, milk, and powdered hemoglobin that mimics the conditions of a wound. The flies are blasted with radiation before they are released into hotspots, where the sterile males will mate with wild females to produce infertile eggs.
Until 2023, the sterile flies were dropped into the Darien Gap, a sliver of jungle between Panama and Colombia, to maintain a biological barrier against northward spread. Now they're being sent to Mexico.
Screwworms cannot fly more than 12 miles on their own, but they can cover large distances inside the flesh of their hosts, such as cattle, horses and deer. The flies have already passed through the narrowest stretches of land in Central and North America – the Darien Gap in Panama and the isthmus in Mexico — meaning that exponentially more need to be released to control the outbreak.
The US eliminated screwworms in the 20th century by flying planes over hotspots to drop red-striped boxes packed with sterile flies, sometimes called “cupcakes” by ranchers. The USDA constructed a fly production plant in Mission, Texas, in 1962, that pumped out 96 trillion flies until it was decommissioned in 1981. Now the USDA is planning to resurrect the plant to disperse sterile flies, while Texas officials have scattered 100 screwworm traps along the border. USDA inspectors known as Tick Riders who patrol the border on horseback to guard against another pest, the cattle fever tick, have also been tasked with conducting screwworm preventive treatment for all cattle and horses they find in the border area.
At the heart of the problem is an unworkable math equation. The USDA estimated 500 million flies need to be released weekly to push the fly back to the Darien Gap. At its maximum, the Panama plant produces just 100 million.
"It's an overwhelming situation at this point," Dr Lansford said. “Screwworm is obviously doing well in Mexico, and they’re up against the same challenges we are.”
Alfonso Lopez, a livestock veterinarian in Tapachula, Mexico, told Reuters he sees new cases of screwworm every day. He showed a tube containing worms collected from a newborn calf hours earlier. The worm's body features distinctive rings that make it possible to twist and burrow into an animal's flesh, earning its "screw" moniker. When removed from the tube with a pair of tweezers, one worm rears its head, still alive.
Chiapas state is ground zero in Mexico's outbreak. Infestation in livestock started emerging here last year and cases in the country are now increasing by roughly 10 percent each week. There have been nearly 50,000 cases reported from Panama to Mexico so far, according to the Panama-United States Commission for the Eradication and Prevention of Screwworm.
Rancher Julio Herrera in Tapachula checks his animals regularly for wounds but he said his efforts can only go so far until the government addresses what he considers the root of the problem. He and other experts say increased migration of cattle and people from Central America has fueled the expanding outbreak.
Chiapas State Agriculture Secretary Marco Barba said federal authorities are reviewing the issue of illegal livestock crossings.
"No country is immune," Barba said in an interview with Reuters in state capital Tuxtla Gutierrez. The state government has launched a highly-publicised campaign encouraging producers to check their herds carefully for any sign of screwworm and report cases.
'THERE ARE NO COWBOYS ANYMORE.'
Even with government action, many US ranches don’t have enough skilled labour to monitor and treat their herds for screwworm. They need cowboys who can tell if cattle are sick just by looking at them, who don’t get squeamish elbow deep in a birthing cow, who can lasso and tie temperamental bulls.
Isaac Sulemana, a rancher and attorney in Sullivan City, Texas, estimated his ranch would need at least 10 cowboys to monitor pastures during an outbreak. He only has two.
Preventing deaths during a screwworm outbreak requires ranchers to adopt a punishing routine of monitoring every single head of cattle every single day. But as Dove lurched down a bumpy two-track dirt road looking in vain for his scattered cattle, the challenges of locating animals – even 1,000-pound ones – on a sprawling ranch were laid bare.
“You look at this,” Dove said, pointing toward the dense thickets of mesquite, catclaw and prickly pear that mark Texas cattle country. “Just take a look at that and think about going and getting your cattle out of that when they don't want to be got.”
In the meantime, ranchers are preparing for the worst.
In May, third-generation West Texas cattle and sheep rancher Warren Cude entered a barn where his dad kept old canisters of screwworm medicine and jars filled with dead screwworms. He added new bottles of wound spray and insecticides to the collection.
“We’re repeating history after 50 years. We didn’t learn from the first time and we let those facilities go and now we’re having to do everything again to combat something we eradicated 50 years ago,” Cude said.
