A pest that was eradicated from the United States 60 years ago could soon rear its ugly head once again.
New World screwworms – flesh-eating, parasitic flies – once threatened livestock, pets, and people across the southern U.S. until being effectively removed in 1966. But a new wave of the flying species has since trekked northward with a vengeance, killing seven people and infecting at least 1,400 more across Mexico and South America in January 2026.
The migration could pose a threat to southern states, prompting the Centers for Disease Control and Prevention to issue a health alert over potential risks. 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.
“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 are known to lay eggs in open wounds, where the young hatch and feed on flesh as they grow. This feeding cycle can make wounds much worse, leading to infections and can 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 one suspects the species’ 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,” Menzel 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.
“Other signs that you could look for would be an animal with a lot of open sores. 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.”
Veterinarians and doctors are required to report suspected screwworm infestations, which helps the U.S. Department of Agriculture and other federal regulatory agencies respond to prevent the parasite’s spread. To prevent screwworms from implanting larvae, all open wounds should be covered when outdoors, experts say. The same bug repellents used for mosquitoes are also said to work well to repel adult screwworm flies.
The USDA announced in February 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.
Federal officials have 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.
“This is a very serious threat, but we’ve dealt with it before, and they haven’t been spotted in the United States,” Menzel said. “Right now, it is not something that we need to be overly worried about in our daily lives.”