Flesh-eating screwworms have returned to the United States.
Experts have discovered New World screwfly larvae in a calf in South Texas, the first confirmed case in the United States in decades.
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A carnivorous fly thought to have been exterminated decades ago has been discovered in Texas, returning to the United States for the first time in generations.
On June 3, the U.S. Department of Agriculture confirmed the presence of New World screwworm in a 3-week-old calf in Texas. This is the first confirmed case in the United States in decades, and the closest the parasite has come to recolonizing north of the Mexican border since its eradication in the 1960s.
So far, the Texas case appears to be isolated and no additional detections have been reported. Still, authorities are treating this as a worst-case scenario. A quarantine zone approximately 19 miles wide has been established around the McAllen Ranch, and surveillance has been increased throughout the region, according to the Department of Agriculture.
Texas Gov. Greg Abbott warned residents at a June 4 press conference to prepare for a “tough summer” as government agencies mobilize to prevent the parasite from gaining a foothold. Experts say the next few weeks will be critical. With increased surveillance, rapid treatment and strict containment, authorities hope to stop the outbreak before it spreads beyond a single case and avoid potential billions of dollars in losses to ranchers and the broader food supply.
The public’s concerns go beyond beef prices, as the parasite can infect any warm-blooded animal. Wild animals, pets, and even humans can be taken over by the flesh-eating larvae, but experts say that’s no reason to panic. Here’s why:
What is the New World screwworm fly? How does it spread?
The New World screwworm (NWS) is a somewhat unique type of parasitic fly in that it feeds on the living, healthy tissue of warm-blooded animals, hence its nickname “carnivorous,” said Dr. Casey Locklear, veterinarian and director of parasite control at Elanco Animal Health.
This fly is about the same size as the common house fly, but completes its life cycle by laying eggs inside an already open wound. The initial point of entry can be something as small as a tick bite or scratch, but it provides the perfect environment for the female to lay hundreds of eggs at a time. Screw maggots can also enter the body through normal openings such as the corners of the eyes, nose, ears, and mouth. Newborn and infant animals are also at risk via the umbilical cord site.
Once laid, these eggs hatch into maggots, which burrow into living tissue and muscles to feed, creating the deep wounds that give the parasite its name. The painful, foul-smelling sores continue to enlarge and grow, and if untreated, secondary infections and related illnesses can become fatal, according to the U.S. Centers for Disease Control and Prevention.
Once fed and grown, the maggots fall from the affected animal and burrow into the soil after a few days, later emerging as mature flies. The entire life cycle takes place over 10 to 30 days.
Any warm-blooded animal can become infected with screwworms. Although it can be transmitted to humans, Locklear said it is much more likely to be found in livestock and wildlife than in people and pets, especially because symptoms appear more easily and are generally treated earlier in the latter two categories.
Risk factors include spending a lot of time outdoors during the day, having open or small wounds on the skin, and medical conditions or treatments that cause bleeding or sores.
According to the CDC, “If you live, work, or spend long periods of time with livestock or other warm-blooded animals in areas where NWS flies are present, you may also be at increased risk for NWS.”
How to identify New World screwworms
According to the U.S. Department of Agriculture, adult NWS flies have orange eyes, a metallic blue or green body, and three dark stripes on their backs.
Obvious signs of screwworm infection are enlarged wounds or lesions and the presence of maggots that parasitize living animals. Dr. Jennifer Bloodgood, a clinical assistant professor and wildlife veterinarian at Cornell University, says other species of maggots can be found on live animals, but the difference is in what they feed on. NWS feeds on healthy, live flesh, while most other maggots feed on dead tissue.
It can be difficult to tell the difference, especially in wild animals that cannot be observed up close. For this reason, Bloodgood and public health officials are encouraging the public to report any suspected exposure.
How to prevent and treat screwworms
Several medications are approved to treat screwworms in pets and livestock. The FDA has approved Elanco Animal Health’s Credelio drug in the first emergency use authorization for an animal drug., Credelio CAT and Negasunt Powder launched on October 24th to treat screwworms in dogs, cats, and livestock.
Since then, other drugs to prevent and treat screwworm in livestock have also received emergency approval. One commercially available injectable (Dectomax/Dectomax-CA1) was recently approved on May 19th.
If you think your pet or wildlife may be infected with screwworms, contact your local veterinarian. Treatment for your pet consists of simply swallowing a pill to kill the maggots and having the wound professionally cleaned.
“If your dog has an unexpected wound, a wound that’s expanding, or a discharge, there may be a distinct odor. If you notice that, call your veterinarian,” Dr. Locklear told USA TODAY.
Protecting animals from ticks, fleas and other parasites also helps prevent itchy sores and therefore prevents opportunities for screwworm infestation, she added.
Although the risk to humans is low, the CDC offers the following tips for people concerned about prevention:
- Keep open wounds and skin injuries, no matter how small or minor, clean and covered.
- When indoors, sleep with the windows closed or with a screen, and when outdoors, sleep under a bed net or tent.
- Protect your skin from insect bites by wearing long-sleeved shirts and pants, hats, and socks when outdoors, and using an EPA-registered insect repellent.
Screw maggots were previously eliminated from the United States, so why have they returned?
According to the USDA, screwworms were once endemic to the United States, but the United States was declared free of native screwworms around 1966. It was completely eradicated from North America in the early 2000s.
It remains endemic in South America, and there have been cases in the United States of people infected through overseas travel who have become isolated. The last recorded outbreak was small in the Florida Keys in 2017.
NWS has previously been eradicated by systemic release of sterilized flies. According to the CDC, female screwworm flies lay 200 to 300 eggs at a time, up to 3,000 eggs, but only mate once to do so. When male flies sterilized by radiation are released into a target area, the females that mate with the flies will lay infertile eggs, which means that the population will gradually decline.
NWS has slowly migrated back through South America to the U.S. border over the years, with outbreaks occurring in Central America and Mexico, which provide much of the U.S. cattle supply.
Experts started sounding the alarm again after it was spotted near the border in late 2025. As a result, the Department of Agriculture is poised to quickly adopt a response plan following the discovery in Texas.
Bloodgood said the two factors behind the spread of infections are climate change and travel. Because of the relative ease of modern travel, flies can end up far from home if they hop onto an infected animal or person and then jump onto a car, train, plane, boat, etc.
The flies also prefer more tropical climates and don’t survive well in areas that are cool or have long periods of cold, she said. As the environment continues to warm, more places, including the southern United States, are becoming suitable for the parasite to survive, according to a 2025 study in Nature and a 2019 study in the Journal of Medical and Veterinary Entomology.
“Climate change will result in more areas being warmer for longer periods of time, creating more favorable conditions for flies to live and maintain their populations throughout the year, which is critical for their survival,” Bloodgood said. “You’ll see it gradually move away from the tropics.”
How worried should I be about screwworms?
According to Locklear, Bloodgood, and public health agencies, we should be aware that screwworms are making a comeback in the United States, but there’s no need to panic. You need to know what to look out for, but you don’t need to go into full-blown isolation.
“As a Texas veterinarian, the threat is real, but especially when it comes to this parasite, knowledge is power and vigilance is power,” Dr. Locklear said. “Usually when we talk about parasite infestations, we talk about something that can’t be defeated. This allows us to take really aggressive measures.”
And while the headlines may be anxiety-provoking, this is not something the U.S. hasn’t dealt with before, Bloodgood said. Agencies such as the USDA and state health departments are already working together effectively to contain the spread.
“It’s important to know that this doesn’t spread like the infections we’re used to with coronavirus or other viruses. It’s a parasite, so it can’t be transmitted between animals,” Bloodgood explained. “We’ve eradicated this before. We have medicine for this. We have sterile insect technology…People should (just) inform themselves what screwworm is and be wary of it.”
Contributor: Eduardo Cuevas, USA TODAY.
