New World screwworm (NWS) has officially landed on U.S. soil, ending more than six decades without the destructive livestock pest and putting dairy producers on alert.
USDA recently confirmed a case in South Texas near La Pryor. The infestation was identified in a 3-week-old calf and confirmed by USDA’s National Veterinary Services Laboratories in Ames, Iowa. Federal animal health officials have activated response efforts as they work to contain the pest and prevent further spread.
As expected, @USDA_APHIS confirmed the detection of a New World Screwworm (NWS) fly in a 3 week old bovine in Zavala County, Texas. @USDA and Texas Animal Health @TAHC officials are taking immediate action to contain and eradicate NWS from the area.
— Secretary Brooke Rollins (@SecRollins) June 4, 2026
For more… https://t.co/GJkUJl0XEI
The confirmation marks a significant development for the U.S. livestock industry. NWS was eradicated from the U.S. in the 1960s through an aggressive sterile insect release program that ultimately pushed the pest south through Mexico and into South America. However, detections have steadily moved north through Mexico in recent years before crossing the U.S. border.
For dairy producers, the discovery serves as a reminder that vigilance and early detection will be critical if additional cases emerge.
“This is not just another fly,” says Keith Poulsen, Ph.D., director of the Wisconsin Veterinary Diagnostic Laboratory at the University of Wisconsin. “The New World screwworm is the larval stage of a fly that actually burrows into live tissue. That’s what makes it unique and what makes it a genuine threat to cattle, other livestock and even people.”
While the adult fly is roughly the size of a common housefly, the pest can spread through the movement of livestock, wildlife, pets, vehicles and even people. Because infestations can be difficult to identify in their early stages, experts say awareness and surveillance will play a major role in limiting its impact.
Why New World Screwworm is Different
Unlike flies that feed on manure or dead tissue, female NWS flies target living animals. They lay eggs around wounds and other vulnerable areas, often depositing 200 to 300 eggs at a time.
Within 12 to 24 hours, those eggs hatch into larvae that burrow into living flesh and begin feeding. The larvae use hook-like mouthparts to tunnel deeper into tissue, creating increasingly severe wounds as they grow.
As the larvae feed, wounds enlarge and often develop a foul-smelling discharge. Animals can experience severe inflammation, secondary infections and significant pain.
“They’re irritated because it’s painful,” Poulsen explains. “When those eggs hatch and the larvae start feeding, it creates abscesses. Abscesses are painful, and they can lead to sepsis or a generalized infection.”
NWS is considered an infestation rather than an infectious disease. It does not spread directly between animals, nor is it considered a food safety concern.
“This is certainly considered a pest that we are trying to control, not a consumer safety issue at all,” says Stephen Diebel, president of the Texas & Southwestern Cattle Raisers Association. “We continue to have the safest, most wholesome protein product on the market today.”
Dudley Hoskins, USDA undersecretary for marketing and regulatory programs, echoes that message.
“The screwworm is not contagious,” Hoskins says. “It does not spread directly from animals to people or from person to person. And the screwworm does not pose a food safety risk. It spreads only when a screwworm fly lays eggs in a wound, not through meat, poultry or dairy products.”
However, officials emphasize the pest remains a serious livestock health threat because of the damage it can cause if cases are not identified and treated quickly.
Dairy Industry Reaction
National dairy organizations say the confirmation of NWS in the U.S. is a significant development, but not an unexpected one.
“The return of New World screwworm to the United States decades after its initial eradication is a disappointing milestone, but it’s also one for which dairy producers have been preparing for more than a year, in collaboration with USDA and across agriculture,” says Gregg Doud, president and CEO of the National Milk Producers Federation.
Doud emphasizes NWS does not pose a food safety risk and notes animal health officials and industry organizations have spent months preparing response plans should the pest cross the border.
“It’s important to remember that this development has no effect on food safety and that measures to combat both the screwworm and its spread are in place and time-tested,” he says.
Now that USDA has confirmed a U.S. case, Doud says dairy producers are committed to working closely with federal, state and local officials to limit the impact on livestock operations and ensure producers have access to the latest information and resources.
He also encouraged policymakers to rely on science-based decision making when considering animal movement restrictions and other regulatory responses.
“We urge officials to follow scientific guidance in any decisions affecting animal movements and regulatory responses to avoid causing economic harm that could be greater than the screwworm itself,” Doud says. “We also ask that adequate resources be provided to combat this problem, which causes suffering in animals and creates risk for producers.”
Warning Signs Producers Shouldn’t Ignore
Poulsen says producers do not need specialized equipment to detect potential cases. Instead, successful surveillance relies on careful daily observation.
“We call it active observational surveillance,” he says. “You already check your animals every day. Now you just add a heightened awareness for very specific signs.”
Warning signs may include:
- Animals that appear uncomfortable or agitated
- Head shaking, tail swishing or kicking at the body
- Animals isolating themselves from the herd
- Open sores or wounds that worsen quickly
- Foul-smelling lesions
- Clusters of abscesses rather than a single wound
- Visible larvae within a wound
“You’ll often see chains of abscesses,” Poulsen says. “They can range from blueberry-sized to golf ball-sized, and you’ll usually see more than one.”
The first 24 to 48 hours can be especially challenging because larvae may be difficult to detect. As they continue feeding, however, wounds typically enlarge and develop noticeable drainage and odor.
“If you see larvae or maggots coming out, that’s a real telltale sign,” Poulsen says.
Young Calves Face Big Risk
The first confirmed U.S. case highlights that calf management may deserve particular attention.
Fresh umbilical cords can provide an attractive site for NWS flies to deposit eggs, making newborn calves especially vulnerable. Naval care should include:
- Remove bedding or any other foreign material from the navel and dip the navel with 7% iodine tincture or a commercial navel dip product.
- Change the solution and clean the dip cups between each calf.
- Do not cut or clamp the navel cord unless directed by a veterinarian.
- Dipping is preferred over spraying to ensure even coverage of the navel cord.
- Navel dipping can be repeated once the calf is moved to a permanent housing location or within 24 hours.
- Check the navel daily for pain, swelling or foul odor.
- Navels will typically take 7 to 10 days to heal.
Other wounds can also serve as entry points, including dehorning sites, surgical incisions ear tags and accidental injuries. Because larvae can be difficult to detect during the early stages of infestation, routine observation of calves and prompt attention to wounds become especially important.
“The consequence of missing it is very, very high,” Poulsen says. “It is much easier to control it earlier than later.”
In regions where NWS is endemic, animals are often inspected every few days. Untreated infestations can become severe quickly, and cattle may die within two weeks if cases go unnoticed and untreated.
What Treatment Could Look Like
While the first line of defense against NWS is prevention, veterinarians emphasize that producers should understand what treatment options are available if a case is detected.
“You can’t treat your way out of screwworm,” is a common message from animal health experts. Prevention remains the primary strategy and includes minimizing wounds, maintaining strong fly control programs, closely monitoring animals and reporting suspicious lesions quickly.
If an infestation does occur, treatment focuses on removing the larvae, cleaning and managing the wound and preventing reinfestation. Veterinarians may also recommend antibiotics, pain management and additional supportive care depending on the severity of the case.
“A veterinarian is central to the preparation and treatment plan,” says Diane Kitchen, DVM, veterinarian manager for bovine and cervidae programs with the Florida Department of Agriculture.
She encourages producers to sit down with their veterinarian before NWS becomes a problem and discuss questions such as: How often should animals be inspected? What products will be used? What steps should be taken if a case is suspected?
Federal officials have been working to expand the toolbox available to producers. The Food and Drug Administration has conditionally approved several products for the prevention and treatment of NWS infestations and has established emergency-use pathways to make additional products available if needed.
Among the products currently available are injectable and topical treatments designed to kill larvae and prevent reinfestation. Some products are approved for use in dairy cattle, including lactating cows and replacement heifers, while others carry restrictions based on animal age or production status. Because product approvals and authorizations continue to evolve, producers should consult their veterinarian and review current FDA guidance before using any treatment.
Experts caution against treating entire herds simply because NWS has been detected in a region.
“Blanket treatment of animals is not recommended,” says Julia Herman, DVM, beef cattle specialist veterinarian with the National Cattlemen’s Beef Association. “We need to be really responsible in how we use these products.”
Overuse of parasiticides can contribute to resistance and reduce the effectiveness of tools producers rely on to control other pests. Instead, regulators and veterinarians recommend identifying affected or high-risk animals, treating them appropriately and continuing close observation of the herd.
Review Biosecurity Plans Now
Although the greatest risk remains in Texas and the southern U.S., animal movement means producers across the country should stay informed.
“We move a lot of animals back and forth,” Poulsen says. “Wisconsin alone moves about 6,000 animals per month to and from Texas.”
Transporting calves or cattle from affected areas could potentially move NWS into regions where the pest would not otherwise be expected. Experts encourage producers to review biosecurity plans, evaluate protocols for incoming animals and discuss response procedures with their veterinarians.
“If animals are coming from a high-risk area, they may need to be treated before transport and monitored closely after arrival,” Poulsen says.
Producers may also want to evaluate how wounds are managed on the farm. Whenever possible, procedures that create open wounds should be carefully monitored during periods of elevated risk, and facilities should be inspected for sharp objects or equipment that could injure animals.
Vigilance Will be Critical
Federal and state animal health officials are expanding surveillance, testing and monitoring efforts designed to identify cases quickly and prevent the pest from becoming established.
If producers suspect NWS, experts encourage them to contact their veterinarian immediately rather than attempting to diagnose the pest on their own. Because NWS is a reportable foreign animal disease, veterinarians and animal health authorities play a key role in coordinating testing and response efforts.
Laboratory confirmation typically involves microscopic identification of larvae. Producers should work with veterinarians and animal health officials before collecting or submitting samples.
For now, the message from veterinarians and regulators is one of preparedness rather than panic.
“This isn’t about fear,” Poulsen says. “It’s about knowing what we’re up against and using every resource we have to protect our animals and our industry.”
As response efforts continue in Texas, dairy producers across the country are being encouraged to pay close attention to calves, wounds and animal behavior. Early detection, rapid reporting and strong veterinary partnerships will be among the most important tools for limiting the impact of NWS.
“We’re at the beginning of this, not the end,” Poulsen says. “That’s exactly why we’re having these conversations now.”
