As New World screwworm moves closer to the southern U.S., dairy farmers, veterinarians and industry leaders are paying closer attention. While it’s not time to panic yet, there is a shared focus on being prepared if the pest crosses the border.
Keith Poulsen, Ph.D., director of the Wisconsin Veterinary Diagnostic Laboratory at the University of Wisconsin, explained the urgency during a recent “Professional Dairy Producers” webinar.
“This is not just another fly,” Poulsen says. “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.”
The U.S. successfully eradicated the screwworm in 1960 using a method called the sterile insect technique. This approach involved breeding and releasing millions of sterile male flies into the wild. When these sterile males mated with wild females, no offspring were produced, slowly shrinking the screwworm population. Over time, this effort drove the pest out of the country.
“It took decades, but we pushed screwworm out of North America all the way into South America,” Poulsen explains.
Today, detections moving north through Mexico have veterinarians and animal health officials paying close attention. With confirmed cases now within roughly 70 miles of the Texas border, the concern is very real.
“We’ve seen this pest before,” Poulsen says. “The fact that it’s moving north again means we need to be ready.”
Why is This Fly Different?
Unlike flies that target manure or dead tissue, the New World screwworm lays eggs in living animals. When those eggs hatch, the larvae burrow into the tissue and feed, creating painful wounds that can rapidly worsen.
“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.”
Animal deaths aren’t usually common, but the damage does not need to be fatal to be costly. Livestock often lose condition, require treatment and can suffer carcass trimming or condemnation later.
“You might see it at slaughter when tissue has to be trimmed out or carcasses are condemned,” Poulsen says. “That’s where a lot of the economic impact shows up.”
What Should You Look For?
Poulsen stresses early detection depends less on new technology and more on careful observation. The good news is producers already do this every day.
“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.”
Producers should watch for changes in behavior, skin condition and overall comfort, including:
- Unusual discomfort or behavior, such as head shaking, kicking at the body, tail swishing or animals isolating themselves from the group
- Animals that just seem off, even if no obvious wound is visible at first
- Open sores or abscesses, especially those that appear suddenly or worsen quickly
- Foul smelling wounds, which can indicate infected or abscessed tissue
- Clusters of lesions, rather than a single isolated wound. “You’ll often see chains of abscesses,” Poulsen explains. “They can range from blueberry sized to golf ball sized, and you’ll usually see more than one.”
Certain areas of the body deserve extra attention, particularly places animals struggle to brush flies away, including:
- Udder cleft and teat base
- Axilla or armpit area
- Neck and dewlap
- Along the topline or shoulders
In more advanced cases, larvae can be visible within the wound.
“If you see larvae or maggots coming out, that’s a real telltale sign,” Poulsen adds.
Identifying the Fly
Adult screwworm flies look different from common horn flies, but Poulsen cautions against relying on visual identification alone.
“They have a metallic green body, large heads and striping on the abdomen,” he says. “But it’s hard to tell for sure just by looking.”
Instead, he encourages producers to involve their veterinarian and diagnostic labs early rather than trying to make the call themselves.
“If you think you see the flies, grab a couple, put them in a collection vial and give them to your veterinarian,” Poulsen says. “We’ll identify them for free. Our lab network is activated and trained to look for this.”
Is This Only a Southern Concern?
Texas and the broader Southwest face the highest risk because warm temperatures allow screwworm flies to survive year-round, giving the pest more opportunity to reproduce and spread. Still, Poulsen cautions that northern states should not assume they are completely safe.
“We move a lot of animals back and forth,” he says. “Wisconsin alone moves about 6,000 animals per month to and from Texas.”
Transporting calves south or bringing in animals from higher-risk areas can inadvertently introduce screwworm to northern herds. Even healthy-looking animals can carry early-stage larvae that are not yet visible.
“The risk up here isn’t zero, but it’s not as high as in the south,” Poulsen says. “The advice for those farms is simple. Look closely. If you see anything, say something.”
Cold weather slows screwworm activity, giving farms some seasonal relief. Still, producers need to stay alert as warmer weather returns.
“Now that we’re in the middle of winter, flies shouldn’t be an issue,” he says. “This season is more about keeping an eye out and staying ready for what spring might bring.”
What Happens if Screwworm Crosses the Border?
While no one can say exactly how regulatory responses would play out if screwworm returns, agencies are already preparing.
Federal and state animal health officials are increasing surveillance, testing and monitoring programs to catch the pest early. Rapid response plans are also being developed, including quarantine measures, movement restrictions and treatment strategies.
“We’re ramping up testing, education and surveillance,” Poulsen says. “It’s not just about detecting insect presence. It’s about rapid response, communication and ensuring we have animal movement protocols ready if needed.”
Public health is also involved.
“We had a human case in Maryland from someone who traveled to an endemic area,” he adds. “Public health found it, treated it and contained it. That tells me the system works, but we still don’t want it in our livestock.”
Making Biosecurity a Priority
Being proactive isn’t just about one pest. Recent research presented at the MILK Business Conference shows many U.S. dairy farms take a reactive approach to biosecurity, leaving operations vulnerable to evolving disease threats. About one-third of farms don’t regularly review their biosecurity plans, even as challenges like highly pathogenic avian influenza, bovine spongiform encephalopathy and New World screwworm continue to emerge.
The survey of more than 300 dairy producers found that while 68% of larger farms have a biosecurity plan, 34% do not review or update it regularly. The good news is that more than 70% of large dairies are working on improvements, showing parts of the industry are taking biosecurity seriously. Having these systems in place can make a real difference when it comes to spotting problems like screwworm early and responding quickly.
Awareness Over Alarm
Poulsen is quick to clarify that being prepared does not mean panicking. While screwworm has not been detected in U.S. livestock yet, thinking ahead can make all the difference.
“If animals are coming from a high-risk area, they may need to be treated before transport and monitored closely after arrival,” he says. On modern dairies, that is easier said than done because quarantine takes space and space costs money. That is why he encourages producers to start planning now.
“Talk to your veterinarian,” he says. “Ask what your biosecurity plan looks like if you have to bring animals in. Do you have a place to isolate them? How long would they stay there? What treatments would you use?”
He emphasizes that the goal is not fear but knowledge and readiness.
“This isn’t about fear. It’s about knowing what we’re up against and using every resource we have to protect our animals and our industry.”
While the threat of New World screwworm seems intimidating, Poulsen’s advice is simple. Keep up with the latest updates, pay close attention to your animals and reach out to your vet or state animal health officials sooner rather than later. Staying ahead now makes all the difference later.
“The consequence of missing it is very, very high,” Poulsen adds. “It is much easier to control it early than later.”
Looking ahead, he sees the current situation as the start of a longer process.
“We’re at the beginning of this, not the end,” Poulsen says. “That’s exactly why we’re having these conversations now.”
