US to fight New World screwworm by dropping millions of flies

The Herald Scotland

10-07-2025

"This can kill a thousand-pound cow in two weeks," said Dr. Michael Bailey, president-elect of the American Veterinary Medical Association. "The federal government is being very aggressive in working to contain this."
It's not as crazy as it sounds. A similar effort worked well decades ago.
As the U.S. Department of Agriculture said in a release, "The U.S. has defeated NWS before and can successfully do it again."
The effort works by sterilizing male screwworm flies so that when they mate with females the resulting eggs are infertile. Over time so few fertile eggs are created that the fly is eradicated altogether.
An effort across the U.S., Mexico and Central America that began in the 1950s was so successful this flying menace was totally eradicated north of Panama, except for a few tiny outbreaks. The U.S control program cost $32 million and was ended in the 1960s.
But this past November, Mexico's Chief Veterinary Officer notified the U.S. Department of Agriculture's Animal and Plant Health Inspection Service that a New World screwworm had been found in a cow in southern Mexico close to the border with Guatemala.
This set off blazing alarm bells for the entire U.S. cattle and beef industry.
So once again, USDA is ramping up its protective program to blanket the potential path of this reemerging threat onto the United States with a rain of sterile males to stop what USDA calls "a devastating pest."
What is the New World Screwworm?
So far the screwworms have only been seen in Mexico.
"We have not seen any screwworms cross over the border into the U.S. and we're trying hard to prevent that," said Bailey.
By dropping the sterile, fully developed flies, the USDA plans to keep the maggots from ever reaching the border.
The New World Screwworm, or the NWS for short, is about the size common housefly. It has orange eyes, a metallic blue or green body and three dark stripes along their backs, according to the Animal and Plant Health Inspection Service.
The fly gets its name for how its maggots feed on the living flesh of an animal by burrowing, or screwing, into an open wound, creating "extensive damage by tearing at the hosts' tissue with sharp mouth hooks," the Animal and Plant Health Inspection Service states.
This results in long, deep wounds in the surrounding flesh that are also susceptible to secondary bacterial infections, causing severe pain, and potentially death if left untreated. Livestock, pets and wildlife are all threatened, as are humans in rare cases.
"To give you the heebie-jeebies, you can actually see the skin moving. And it smells rank," said Bailey.
Animals at greatest risk include those that have recently given birth, have open wounds, or have undergone surgery or management procedures such as dehorning or branding, according to the American Veterinary Medical Association.
It's dangerous enough that veterinarians are urged to wear disposable gloves when treating affected animals.
How do sterile male flies help?
The screwworm first became a significant problem in the United States in 1933, according to the USDA. By the early 1960s, Southeast cattle producers had lost between $50-100 million because of screwworm infestations.
Beginning in the 1950s, the federal government established the Sterile Insect Technique, a method that utilized radiation to sterilize male flies. These mass-produced sterile males were released into target areas. They mated with females, resulting in unfertilized eggs.
No fertile eggs meant no more screwworms. Similar programs have been created to release sterile mosquitos.
The screwworm program was a stunning success. The insect was officially eradicated in the United States by 1966, at a cost of $32 million to complete - protecting the U.S. cattle and beef industry, worth more than $80 billion today.
With the screwworm's return to Mexico, USDA on June 18 announced the launch an $8.5 million sterile fly dispersal facility in South Texas. It's anticipated to begin producing sterile males by the end of the year.
In addition, USDA is working with Mexico to begin producing sterile screwworms. The agency is investing $21 million to renovate an existing fruit fly production facility in Metapa, Mexico. It will be able to produce between 60 and 100 million sterile flies each week within a year.
It's the first step towards producing the estimated 400 to 500 million flies needed each week to re-establish the new New World Screwworm barrier at the Darien Gap at the border of Panama and Columbia - and stop the movement of the flies.
Pets, especially dogs, are vulnerable to the screwworm
Wildlife, including deer, are vulnerable to the maggots as are pets - and especially dogs.
"There's a treatment availalbe, but it takes several days in quarantine if a dog comes down with this," Bailey said. The treatment can only be used in non-food animals, which is why cattle simply have to be euthanized if they're infected.
The goal is for the new effort will stop the screwworms long before they threaten the United States again.
"Hopefully this becomes another 'I remember when' story for veterinarians," said Bailey.
Contributing: John Bacon, David Baratz, Greta Cross, Janet Loehrke, George Petras and Mike Snider.