Screwworms Crossing to the US Threaten to Prolong Pain for Ranchers in Mexico
By Maya Averbuch
The spread of the screwworm is hitting Mexican ranchers with the double shock of surging costs and a ban on cattle sales to their top export market.
Mexico sold over $1 billion of live cattle to U.S. buyers in 2023, according to U.S. import data. That was before the deadly fly re-emerged in the country, prompting the U.S. to shut its border to cattle from its southern neighbor in 2024. Since then, Mexican meat industry group Comecarne estimates the value of lost cattle exports at nearly $2.8 billion.
Though the sale of packaged beef — which wasn’t included in the ban — got a boost, a survey from Mexico’s meat producer association AMEG covering the first half of this year showed a 12% per kilogram drop in the price of young cows sent to feedlots. Falling prices are compounded by higher costs for veterinary checks, health certifications and medicine needed to prevent and treat injuries caused by the New World screwworm.
Despite the ban, the screwworm appeared in the U.S. last month, with about 20 confirmed cases in cattle out of the country’s herd of 86 million. If there’s a major outbreak of the pest — which was eradicated from the US about five decades ago — the Texas farm sector could suffer more than $700 million in annual losses.
It’s already extracted a toll on the industry in Mexico, which has recorded over 20,000 screwworm cases in cattle over the past couple years, out of a total herd of nearly 37 million, according to official data. There’s been another 13,000 recorded cases in other animals.
“When the United States sneezes, to us it feels like a heart attack, because we depend on them,” said Ronaldo Ocampo, a 65-year-old rancher in southern Chiapas state whose herd has suffered cases of infestation.
A bull on Ocampo’s farm worth nearly $10,000 developed a screwworm infection last year, requiring him to pay for a vet’s emergency visit. His shepherd dog lost part of a paw after the pest’s larvae dug into a small cut. Several calves died after the worms burrowed into their umbilical cords, and Ocampo failed to catch the infection in time.
He estimates the screwworm cost him close to $8,000 — on top of the losses from falling prices at a nearby cattle yard, where he says rates per kilogram are down 20% since the U.S. import freeze.
The screwworm fly lays its eggs near the wounds of warm-blooded animals. If left untreated, the parasite’s larvae begin feeding on living flesh, leading to injuries that can be fatal.
Over half of Mexico’s cattle are free-range, and in the southern states where the screwworm infestation started, most ranchers run relatively small operations with just a few dozen cows on average.
“It’s not only a problem because the animals die,” Ocampo said. “The price has fallen for a young bull. It’s really affecting our finances.”
Feeling the squeeze
Breeder Hiram Gómez, 34, said when Mexican cattle prices started falling in January, clients worried the purebred bulls he sells for at least $4,500 were unaffordable.
“My client who’s going to buy a bull from me now needs to sell seven calves, not six” to make the math work, Gómez said. “I’ve said to my clients, unfortunately, we can’t control that factor — the price. But we try to convince them that a registered breeding bull is an investment.”
Though beef prices have risen sharply, some Mexican ranchers say they’ve seen a recent drop-off in livestock prices and lay the blame on the excess cattle that can’t be exported.
Some of that excess supply is instead sold as meat, both locally and to the U.S. Mexico’s beef sales to American buyers during the first five months of this year rose by nearly a quarter compared to the same period in 2025.
Gómez is also feeling the squeeze. Costs to transport his studs have risen since the screwworm’s revival. A permit to move cattle, a vet’s clean bill of health and anti-parasitic medication applied before the trip are all more expensive, almost doubling what he’s forced to cough up.
He argues that basic vigilance can minimize the bill — and screwworm’s risks.
“A small-scale producer checks on his cattle daily,” he said. “The animal behaves strangely when it has a screwworm infestation. If it’s in the vulva, it moves its tail and is restless. If it’s in the ear, you might see it move the tail, or the ear, or shake its head.”
Veterinarian María Elizabeth Imatzu, who works in Chiapas, acknowledges that some ranchers don’t always report screwworm cases.
“People were afraid. They thought, ‘They’re going to come shut down my ranch,’” she said. The screwworm’s opportunistic larvae can bore into the wounds caused by ear tagging, the bloody stumps when horns are removed, or the genitalia of cows who have just given birth.
Tweezers in hand, Imatzu recently extracted dead larvae from a goat’s chest wound at a friend’s property. She then plopped it into a tube for tests. She could see the ridges on its body and hook-like protrusions that made it look like a miniature walrus — there was no doubt it was the screwworm.
Pessimism prevails
Panama and Mexico are producing sterile flies, which when released in the wild can limit the screwworm’s reproduction. The U.S. has earmarked more than $100 million to aid the efforts, but ranchers in Mexico are pessimistic the cross-border trade will reopen even early next year.
Mexican President Claudia Sheinbaum has also pledged to help the ailing sector, announcing a $128 million program to boost meat production from northern Mexico’s bigger cattle stock. Mexican authorities have also distributed free medication through rancher unions.
Gómez is careful with every newborn on his farm. In June, he held a calf so a ranch hand could spray a purple larvicide on its freshly cut umbilical cord. They sprayed the mother’s scraped knees, then walked to a pen to repeat the measure on all the other babies, who were still unsteady on spindly legs.
Beyond all the plans to save the industry — especially the looming release of swarms of sterile flies — Gómez still isn’t banking on a quick fix. He doesn’t see the U.S. lifting its border ban before 2027.
“The situation is complicated,” he said. Though U.S. officials have said the country’s food supply is not at risk, Gómez thinks one factor could hasten change: “If the Americans start to run short of meat.”
© 2026 Bloomberg L.P.