Screwworm epidemiology concept macro insect orange segmented body spiky reflection nature Screwworm epidemiology concept shown by macro image
Flesh-eating pests sighted moving north, but have been eradicated before
New World screwworms once threatened livestock, pets and people across the southern United States. Decades after a successful eradication effort, the pests may be returning.
The United States was officially declared free of screwworms – a flesh-eating, parasitic fly – in 1966 after years of effort from the U.S. Department of Agriculture. But a new wave of the flying species has killed seven people and infected at least 1,400 more across Mexico and South America in January.
This northward expansion could threaten the U.S. border, prompting the Centers for Disease Control and Prevention to issue a health alert.
“The New World screwworm is a parasitic fly, which is different from something like a mosquito or a housefly, which we might find annoying,” said Tim Menzel, University of Mississippi instructional assistant professor of biology. “It’s the parasitic nature of the screwworm which makes them so dangerous, because their host is going to be livestock, pets or even possibly humans.”
New World screwworms lay eggs in open wounds, where the young hatch and feed on flesh as they grow. This feeding cycle can exacerbate wounds, lead to infections and even be fatal to hosts.
Although screwworms have not been spotted in the U.S. yet, Menzel said it’s important to know what to do if you suspect screwworm activity.
“If you see wounds that have little maggot-like things crawling around in them, that would be a good indication that something is wrong,” the Ole Miss professor said. “It could be screwworms, or there are other parasitic flies that can do that.
“Whether it’s screwworms or one of these other flies, the animal needs to go to a veterinarian.”
Veterinarians and doctors are required to report suspected screwworm infestations, which helps the USDA and other federal bodies respond and prevent the parasite’s spread.
“Other signs that you could look for would be an animal with a lot of open sores,” he said. “You can’t always see the larvae. They can get down under their skin. But they’ll have a foul smell to them, and the animal might exhibit unusual or concerning behavior.
“If any of those things are happening, that animal needs to see a veterinarian, and a veterinarian should be able to determine if screwworm is the cause of it.”
To prevent screwworms from implanting larvae, all open wounds should be covered when outdoors, Menzel said. The same bug repellents used for mosquitos work well for repelling adult screwworm flies.
“It is a real threat, and you can see that in the response that our government is unleashing right now,” he said. “They have really been moving quickly to address concerns.”
The USDA in February announced that it would release glowing, sterile male screwworm flies in Mexico close to the border. Female screwworms mate only once in their lives, and if that mating is with a sterile male, she will not produce any offspring. This is one of the techniques used to eradicate screwworms in the United States in the 1960s.
“Those efforts required a lot of research in the background before they could be actualized,” Menzel said. “It was almost like an American moonshot for entomology; it was a technological capability that we did not have before this outbreak.”
The USDA has also approved emergency medication for screwworm treatment in dogs, cats and cattle and dedicated $100 million to protect animal and human lives from the threat. In May 2025, the U.S. also suspended live animal imports from Mexico to reduce the risk of importing infected livestock.
The Mississippi Board of Animal Health also issued an alert about the threat of screwworms, stating all suspected cases should be reported to their offices.
“This is a very serious threat, but we’ve dealt with it before and they haven’t been spotted in the United States,” he said. “Right now, it is not something that we need to be overly worried about in our daily lives.”
For more information on screwworms, visit screwworm.gov or the Mississippi Board of Animal Health.
By Clara Turnage
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