June 10, 2025 
Last week, as my senior horse was being treated for an aggressive skin infection, I asked my vet about turnout.
After nearly a month of rain in New York, my gelding was certainly ready for the chance to munch spring grass and feel the sun on his face. But I worried about the flies aggravating the scabby, healing lesions on his ankles. Turns out, I had a right to be.
While my vet assured me that my horse had little to fear from the typical fly-season pests, she said it might not stay that way for long. “We don’t have screwworms here—yet,” she warned.
This was still days before U.S. Equestrian’s most recent warning, and the first I was hearing about this freaky pest with a Freddy Krueger-sounding name. But horse owners, especially those in the southern United States, do have plenty to fear from the New World screwworm, Cochliomyia hominivorax. Or, more specifically, from its larva.
“The New World screwworm is currently of great concern in the U.S.” says Sally DeNotta DVM, PhD, DACVIM, a clinical associate professor at the University of Florida. “This is a dermatological parasite; an insect that infects the skin of literally all mammalian hosts, but mostly horses, cattle, dogs, and even people.”
Adult screwworm flies lay their eggs (hundreds of them) in open wounds, which hatch, like typical fly larva. The problem, Dr. DeNotta explains, is that while we sometimes see generic maggots in the wounds of living and dead animals and even people in the U.S.—horrific as they are—those are actually the good guys.
“The difference between a normal fly larva, which we might see on roadkill anywhere in the U.S., the ones that are endemic in this country, only feed on dead tissue, so they actually do a biological service,” she says. “The New World screw worm is a different kind of maggot in that they eat live, healthy flesh.
“Not to be too sci-fi, but they are literally a flesh-eating parasite.”
Equestrian vs. Predator
New World screwworms are native to South America and occur sporadically in parts of Central America as well. They were initially introduced into the U.S. in the 1930s, then successfully irradicated in the 1960s thanks to a revolutionary biological control method known as the ‘sterile insect technique’—one of the most successful ever employed.
Because screwworm female flies mate only once in their lifetime, sterile male flies—produced in a special facility in Panama—were introduced in the millions to disrupt their reproduction, gradually reducing their population. By 1966, the New World screwworm was declared eradicated in the U.S. and, with a few exceptions, it’s pretty much worked ever since.
“We’ve remained New World screwworm-free for all these years, but over the past couple of years, it’s been getting closer and closer to the U.S. border by way of Mexico—with recent detection in Oaxaca and Veracruz, about 700 miles from the U.S. border,” says Dr. DeNotta.
In response, in May of this year, the U.S. Department of Agriculture (USDA) closed all land border crossing stations for horses, cattle, and bison along the Mexico/US border. Recently, however, the USDA’s Chief Veterinary Officer, Dr. Rosemary Sifford, indicated that a change in policy is coming.
“It’s hard to say exactly when,” she said, “but [imports will resume] for sure before the end of the year, unless something really dramatically changes.”
While New World screwworms are expected to be found in the coming months in quarantine facilities for livestock species including horses, cattle, and bison, they will likely cross the border via wildlife, according to Dr. DeNotta.
“I do think there’s a high likelihood of it being picked up in a quarantine situation, when animals are being inspected,” she says. “It’s infected wildlife that is harder to monitor and manage.”
Knowledge = Power
So when it comes to detecting New World screwworms in our own animals, just what should horse owners be on the lookout for?
Maggots, for one, as well as large, foul-smelling wounds which cause draining and weight-loss. If not properly treated, it is possible that animals can die from secondary infections or toxicity from their infections within two weeks.
“They look like any other wound that has maggots,” Dr. DeNotta explains. “Because they’re eating live tissue, though, it’s quite painful and annoying to the host. So, one of the things we tell people to look for is an animal that looks very bothered by the infection. They can be restless, they can bite at it; cattle will stomp their feet, things like that.”
The good news? For the properly initiated, identifying a New World screwworm infection isn’t that hard. And, when it’s caught early in an otherwise healthy horse, Dr. DeNotta says the prognosis is very good.
“The insect, itself, is susceptible to many insecticides such as ivermectin—very typical anti-parasite treatments and [the animals] can be cleared completely. There’s no lingering infection. It’s not a virus or bacterial infection that’s going to take up shop and stick around,” she says. “Infected animals can be treated and cleared and have perfectly good survival rates.”
The larger problem, especially in the southern United States, however, is control.
A Call to Arms
While we can expect one-off cases of New World screwworm infections to pop up like some horror-movie game of Whac-A-Mole, addressing the larger problem is likely to involve both limited resources and significant expense.
Only one facility in North America—that same one in Panama—currently makes the sterile flies necessary to combat the problem, and obtaining them isn’t cheap. For example, it is estimated that the cost for a single eradication campaign in endangered miniature deer native to the Florida Keys in 2016 cost the U.S. more than $10 million.
“We’re going to need a lot more sterile flies than they’re likely able to produce currently,” Dr. DeNotta says.
“People are trying to get ready to not only have better surveillance—because you have to know where they are in order to target with the sterile flies—but also to increase production of the sterile male flies. Right now, they’re only created in Panama, and we certainly need to be able to make them here in the U.S. if the disease is going to be in the U.S.”
To that end, there are currently several efforts in development at the university-level at places such as Texas A&M and the University of Florida. In addition, the ‘Stop Screwworms Act,’ which proposes the creation of one or more new sterile fly breeding facilities in the U.S., is currently making its way through Congress.
While New World screwworms can’t overwinter in regions such as the northeast, Dr. DeNotta says she worries about snowbird communities—particularly the equestrian circuits in Florida, where horses are routinely shipped north for summer horse shows.
“If they have an animal in Florida that has a screwworm infection, and they take it up for the New York summers, then it could potentially bring disease there,” she says.
“Anyone in the Southern United States needs to be aware and keep a really close eye out for any infected-looking wound with signs of any fly larva, at all, in any of their animals.”
And, as ever, having the right facts at hand is everything.
“The way I’m trying to make an impact now is just informing veterinarians and horse and animal owners to understand the risks, and recognize that it is coming, and to keep their eye out, and be really vigilant,” Dr. DeNotta says.
“If you have an animal that you think may be infected, call your veterinarian immediately. If your veterinarian thinks it’s screwworm, they can contact their state animal health official, and the process will go from there.”
