A flesh-eating screwworm is in the US. How worried should we be?

A flesh‑eating fly thought eradicated decades ago has been found in Texas, its first return to the United States in generations. The U.S. Department of Agriculture confirmed the presence of New World screwworm in a 3-week-old calf in Texas on June 3, marking the first known U.S. case in decades and the closest the parasite has come to reestablishing itself north of the Mexico border since its eradication in the 1960s. So far, the Texas case appears isolated, with no additional detections reported.

Still, authorities are treating it as a worst‑case scenario. A quarantine zone roughly 12 miles wide has been established around the McAllen Ranch and surveillance has ramped up across the region, according to the USDA. Texas Gov. Greg Abbott warned residents to brace for a “challenging summer" during a June 4 press conference, as agencies mobilize to prevent the parasite from gaining a foothold.

Experts say the coming 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 potentially billions in losses for ranchers and the broader food supply. Because the parasite can infect any warm-blooded animal, the public concern extends beyond beef prices. Wildlife, pets and even people could find themselves occupied by flesh-eating larvae - but experts say that's not reason to panic.

Here's why. More: Flesh-eating screwworm found in Texas cattle: Explaining the outbreak What are New World screwworm flies? How do they spread? The New World screwworm (NWS) is a species of parasitic fly that is somewhat unique in that it eats the live, healthy tissue of warm-blooded animals, said Dr.

Casey Locklear, veterinarian and parasiticide lead at Elanco Animal Health, hence the "flesh-eating" moniker. The flies are around the same size as a common housefly, but complete their lifecycle by laying eggs inside already open wounds. The original point of entry can be as small as a tick bite or scratch, but it provides the perfect environment for a female to lay hundreds of eggs at a time. Screwworms can also enter the body via regular openings, including the corner of the eye, nose, ears and mouth.

Newborn and infant animals are also at risk via their umbilical cord site. Once laid, those eggs hatch into maggots that then burrow into live tissue and muscle to feed, creating the deep wounds that give the parasite its name. The painful, foul-smelling wounds will continue to expand and grow until, without treatment, they can become deadly thanks to secondary infection or associated illness, according to the Centers for Disease Control and Prevention. Once they've fed and grown, the maggots drop 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 be infected by screwworm.

While it can be transmitted to humans, it is much more likely to be found in livestock and wildlife than in people or pets, especially because symptoms become more readily apparent and are generally treated earlier in the latter two categories, said Locklear. Risk factors include spending a lot of time outdoors during the day, having any open wounds or small breaks in the skin and medical conditions or treatments that cause bleeding or sores. "If you live, work, or spend an extended amount of time with (or near) livestock or other warm-blooded animals in areas where NWS flies are present, you could also be at a higher risk for NWS," according to the CDC. How to identify New World screwworm Adult NWS flies have orange eyes, a metallic blue or green body and three dark stripes across the back, according to the USDA.

The telltale signs of a screwworm infection are the growing wounds or lesions and the presence of feeding maggots on still-alive animals. Some other species of maggot can be found on live animals, said Dr. Jennifer Bloodgood, assistant professor of practice and wildlife veterinarian at Cornell University, but the difference is what they're eating. NWS feeds on healthy, living flesh, while most other maggot species feed on dead tissue.

It can be difficult to discern the difference, especially in wild animals you cannot closely observe. Because of this, Bloodgood and public health officials encourage the public to report any suspected cases. How to avoid and treat screwworm Several drugs have been approved for the treatment of screwworm in pets and livestock. In the first-ever FDA emergency use authorization for a veterinary medicine, the agency approved Elanco Animal Health's drugs Credelio, Credelio CAT and Negasunt Powder for treating screwworm in dogs, cats and livestock, on Oct 24.

Other drugs for preventing and treating screwworm in livestock have also been granted emergency authorizations since then. One over-the-counter injectable (Dectomax/Dectomax-CA1) was approved as recently as May 19. If you suspect your pet or a wild animal may be infected with screwworm, contact your local veterinarian. Treatment for pets can be as simple as one chew tablet to kill the maggots and professional wound cleaning.

"A wound that's unexpected, a wound that's enlarging, has discharge, may have a distinctive foul odor to it. Please contact your veterinarian if you notice that," Locklear told USA TODAY. Keeping your animals free of ticks, fleas and other parasites can also help prevent wounds resulting from itching and, therefore, prevent the making of an opportunity for screwworms, she added.

While the risk to humans is low, the CDC has some tips for people concerned about prevention, including: Keeping any open wound or broken skin, no matter how small or mild, clean and covered.Sleeping indoors with closed windows or screened windows, and sleeping beneath a bednet or tent if outside.Protecting skin from insect bites by wearing long-sleeved shirts and pants, hats, and socks while outdoors and using an EPA-registered insect repellent. Screwworm was previously eliminated from the U.S. Why is it back? Screwworm was once endemic to the United States, which was declared free of indigenous screwworms around 1966, according to the USDA.

It was eradicated from North America altogether in the early 2000s. It is still endemic in South America, and there have been isolated cases of people in the United States becoming infected through international travel. The last recorded outbreak was a small one in 2017 in the Florida Keys. NWS was previously eradicated through the systemic release of sterilized flies.

Female screwworm flies can lay 200 to 300 eggs at a time and up to 3,000 eggs, says the CDC, but they only mate once to do so. Releasing male flies sterilized via radiation into target areas means females that mate with them will lay infertile eggs, causing a gradual population decline. Worm approaches the border: USDA confirms New World screwworm in Texas calf, first in decades NWS has slowly crept through South America back up to the U.S. border over the course of years, with outbreaks occurring in Central America and Mexico, which provides a large portion of the U.S. cattle supply.

Experts began sounding the alarm again in late 2025, when it was detected near the border. As a result, preparations were made, allowing the USDA to employ a response plan quickly after detection in Texas. Two driving factors behind the spread are climate change and travel, said Bloodgood. The relative ease of modern travel means the flies can hitch a ride on infected animals or people and end up far from home if those animals or people hop on a car, train, plane, boat, etc.

The flies also prefer a more tropical climate and don't survive as well in areas that stay cooler or have longer bouts of cold weather, she said. As the environment continues to warm, more places, including the southern U.S., have become suitable for the parasite's survival, according to a 2025 Nature study and a 2019 study in the Journal of Medical and Veterinary Entomology. "I think it's fair to say that with climate change, we're going to be seeing more areas that stay warmer longer and have more favorable environments for the flies to live and maintain its population year round, which is going to be critical for it to survive," said Bloodgood. "We'll see it progressively coming farther out of the tropical areas." How worried should we be about screwworm?

We should be aware but not panicked over the reappearance of screwworm in the U.S., according to Locklear, Bloodgood and public health agencies. You should know what to look out for, but you don't need to go into full-blown quarantine. "As a veterinarian in Texas, the threat is real.

However, knowledge is power and vigilance is power, especially with this parasite," said Locklear. "Typically, when we're talking about parasitic infestations, we're talking about something we can't beat. With this one, we really can take proactive measures." And while the headlines may be anxiety-inducing, this is not something the United States hasn't dealt with before, said Bloodgood. Agencies like the USDA and state health departments have already been collaborating effectively to stop the spread.

"It's important (to know) that this doesn't spread like an infectious disease like we're used to, like COVID or other viruses. It's a parasite, so it's not contagious between animals," explained Bloodgood. "We've eradicated this before. We have drugs for this.

We have the sterile insect technique .... People should (just) inform themselves of what screwworm is and then be vigilant for it." Contributing: Eduardo Cuevas, USA TODAY.