New World Screwworm Guide
The National Cattlemen’s Beef Association warned it wasn’t a matter of if — but when — New World screwworm (NWS) would make its way back into the U.S. That day came on June 3, 2026, when NWS was confirmed near La Pryor, Texas, in a 3-week-old calf. These parasitic fly larvae eat the living tissue of warm-blooded animals, posing a threat to livestock health and the U.S. cattle industry. For beef producers, the reintroduction of NWS heightens the need for frequent herd checks, enhanced biosecurity and immediate reporting of any suspected cases.
Check the status of confirmed New World screwworm detections on the USDA dashboard.
Are You Inside A New World Screwworm Infested Zone?
The clock is running for livestock producers in infested zones. If your ranch falls within these designated zones, you must follow strict animal movement restrictions to stop the spread of NWS and protect animal health.
USDA Guidance
New World Screwworm: What Every Cattle Producer Needs to Do Now
NWS is inching closer to the U.S. Based on the USDA-APHIS NWS response playbook, here is what ranchers and livestock owners should know.
- Minimize wounds before they happen. When possible, castrate, dehorn, brand and tag outside of peak fly season. Any open wound is a potential infestation site.
- Immediately treat all wounds with approved insecticides. Newborn navels, fresh brands, tagging nicks and foot lesions are the highest-risk sites. Apply treatment the moment any wound occurs.
- Tighten your calving season. Compressed calving windows reduce the number of vulnerable calves on the ground at any one time, so there are fewer animals to monitor and a narrower risk window.
- Update your parasite control program now. Sit down with your veterinarian to identify which products are currently available and labeled under Emergency Use Authorization.
- Build your response plan now. Know which veterinarian you'll call, how herd health checks might need to evolve, which products you'll use and what your movement plan is if a quarantine zone is established near you.
- "Eyes on animals" is your most powerful tool. USDA officials are clear: fly traps help, but animal inspections are more sensitive. Nothing replaces a producer who knows their herd and checks it regularly.
- Know the signs — sight, smell and behavior. Look for unusual irritation or head-shaking; wounds that swell overnight or ooze cloudy fluid; and the unmistakable smell of decay. If you peel back skin edges and see rice-grain-sized maggots with dark spines — act immediately.
- Know what the fly looks like. Adult NWS flies have orange eyes, a metallic blue-green body and three dark stripes along their backs — the center stripe is shorter than the outer two. Larvae burrow into living tissue and spiral deeper like a screw driving into wood.
- Prioritize high-risk sites every check. Navels on newborns, vulvas on fresh cows, sheaths on bulls and recent wound or surgical sites. These are NWS entry points — inspect them first, every time.
- Use technology to add "eyeballs" when labor is short. Game cameras, virtual fencing and behavior tracking tags can help flag problems sooner. NWS is an infestation of individual animals — finding one early means the rest of the herd might be clear.
- Watch wildlife, too. Deer, wild pigs and other warm-blooded wildlife are all susceptible. In past outbreaks, wildlife were a significant driver of spread. Hunters, wildlife managers and taxidermists should all be watching for signs of myiasis.
NWS is a federally reportable foreign animal disease. If you suspect an infestation, you are legally required to immediately report. Officials would rather investigate 1,000 false alarms than miss a single real case.
- Call your veterinarian or USDA APHIS immediately. Contact your USDA area veterinarian in charge or your state animal health official. Do not wait. Every generation of unreported flies dramatically expands the population and risk to your neighbors.
- Hold the animal — don't move it. Premises with a suspected infestation will be placed on a hold order while under investigation. Restricting movement until inspection is completed is required and the right call for the industry.
- A quarantine is not a shutdown. A structured process of inspection, treatment and health certification allows animal movement to continue. You will need an animal health certificate (iCVI for interstate, state-issued for intrastate) to move animals out of an infested zone — but movement is not halted.
- NWS is not a food safety issue. Animals presented to slaughter are processed under FSIS regulations. There are no movement requirements for animal products. This is an animal welfare and trade concern, not a meat safety concern.
- Think about your neighbors. Failure to report results in multiple generations of additional flies, expanding the risk for every operation around you. Reporting fast protects the whole community, not just your ranch.
New World Screwworm Management
Crisis Management
The Screwworm Battle Begins on U.S. Soil: Now What?
Understanding the immediate biosecurity implications, quarantine zone, and next steps now that cases have been detected.
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Identification & Detection
Protect Your Livestock: Signs of New World Screwworm
Learn how to spot the early physical indicators and clinical signs of an active larval infestation in your herd.
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Identification & Detection
A Smell You’ll Never Forget: Calf Infested with New World Screwworm
A deep dive into sensory detection and a real-world case study of recognizing infested wounds in young calves.
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Prevention & Treatment
What Products are Available to Prevent and Treat New World Screwworm?
An overview of chemical controls, approved livestock topical, and emergency use authorization treatments.
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Prevention & Treatment
New World Screwworm: Prevention and Reporting Guidelines
Step-by-step protocols for safeguarding your property and the correct channels for official regulatory reporting.
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Crisis Management
The Importance of Wildlife Monitoring in the New World Screwworm Fight
Why tracking local wild populations is a critical piece of preventing widespread transmission across state boundaries.
Read Story →
“Most screwworm migration does not come from adult flies making heroic flights northward. Instead, it comes from the movement of infested animals — cattle, deer, wildlife — and yes, even in rare cases, people. Wherever the animal goes, the parasite goes with them,” says Dr. Christopher Lee.
Historically, colder temperatures have played a crucial role in controlling New World screwworm’s geographical spread.
U.S. Department of Energy’s National Nuclear Security Administration’s is partnering with Texas A&M University to advance the sterile fly technique to halt NWS reproduction.
With New World screwworm within 70 miles of the U.S.-Mexico border, the livestock industry is on high alert. USDA continues to fight the northward spread of the parasite while debate continues on the border closure.
USDA says this is now the northernmost detection of NWS during this outbreak, and the one most threatening to the American cattle and livestock industry.
New World screwworm was essentially eradicated in the U.S. in 1966, but the persistent pest has rebounded through considerable geography in the past year, prompting the closure of the U.S./Mexico border to cattle in recent months.
The next step to battling NWS is using swormlure, a synthetic bait designed to attract adult screwworm flies, combined with an insecticide to combat the pest.
The image, the smell, the emotions: five cattle producers and veterinarians from around the world share their first-hand experience with New World screwworm.
Secretary Rollins takes decisive action and shuts down cattle, bison and equine trade due to further northward spread of the devastating pest in Mexico.
As the fight to keep the flesh-eating parasite out of the U.S. intensifies, the economic impact on ranchers and the industry is top of mind. “This pest will be one that leaves quite a mark on our economy,” says one veterinarian.
A dangerous livestock pest, the New World Screwworm, has resurfaced in Central America, prompting a U.S. livestock import ban and renewed warnings for producers to watch for signs of infestation.
Secretary of Agriculture Brooke Rollins announces plans to reopen Moore Air Base in Texas as a New World screwworm sterile fly distribution facility. Long-term production is anticipated to be 300 million sterile flies per week.
NCBA applauds Secretary of Agriculture Brooke Rollins’ aggressive efforts to suspend Mexican cattle, horse and bison imports, saying Mexico’s corruption and mismanagement has caused the pest to spread closer to the U.S.