New World Screwworm Warning What Every Cattle Farmer Must Know
There is a threat moving toward American cattle farms right now that every rancher and farmer needs to take seriously. New World Screwworm, one of the most destructive livestock parasites ever known, has been detected dangerously close to the United States border. If it crosses into the US, the damage to the cattle industry could be catastrophic.
This is not a distant or theoretical risk. In 2025, the USDA reinstated a suspension on cattle imports from Mexico after New World Screwworm was detected as far north as the states of Oaxaca and Veracruz, less than 700 miles from the Texas border. Since then, detections have continued to move northward.
Every cattle farmer in the United States, and indeed across the UK, Australia, and Europe, needs to understand what this parasite is, what it does, and how to protect their animals.
What Is New World Screwworm
New World Screwworm, known scientifically as Cochliomyia hominivorax, is a parasitic fly native to tropical and subtropical regions of the Americas. Unlike ordinary blowflies that feed only on dead tissue, the screwworm fly lays its eggs in the living flesh of warm-blooded animals. When the eggs hatch, the larvae, which are the screwworms themselves, burrow into healthy living tissue and feed from the inside out.
The name screwworm comes from the corkscrew shape of the larvae as they drill into flesh. A single infestation can involve hundreds or even thousands of maggots feeding simultaneously. Left untreated, screwworm infestations are fatal. The larvae destroy tissue so rapidly that an animal can die within days.
Any warm-blooded animal can be affected. Cattle, sheep, goats, horses, deer, and even humans are vulnerable. The fly is attracted to any open wound, no matter how small. A minor scratch, a fresh branding wound, a tick bite, or the navel of a newborn calf are all potential entry points.
The History of New World Screwworm in the USA
New World Screwworm was once a devastating presence across the southern United States. In the 1950s and 1960s, it caused hundreds of millions of dollars in livestock losses every year. The USDA launched one of the most ambitious eradication programs in agricultural history, releasing hundreds of millions of sterile male screwworm flies to disrupt the breeding cycle.
By 1966, screwworm had been eradicated from the United States. By 1991, it had been pushed all the way down through Central America. A biological barrier maintained in Panama has kept it out of North America for decades.
But the parasite never went away. It remains widespread across South America and parts of the Caribbean. And now, in 2025 and into 2026, it is pushing north again.
Why This Threat Is So Serious Right Now
The current situation is unlike anything the US cattle industry has faced in decades. Detections in Mexico have been moving steadily northward. The USDA has been expanding its sterile fly dispersal program and increasing surveillance, but the parasite continues to advance.
Texas Governor Greg Abbott declared New World Screwworm a threat to national security in 2025. The US has banned live cattle imports from Mexico on a month-by-month basis as the situation develops. Cattle producers across Texas and the southern states are on high alert.
If screwworm establishes itself in the United States again, the economic damage would be enormous. The USDA estimates it cost the industry the equivalent of billions of dollars in today's money before eradication. Re-establishment would devastate the cow-calf sector, drive up beef prices, and require a massive and expensive eradication response.
How To Identify a Screwworm Infestation
Early identification is critical. The sooner an infestation is found and reported, the better the chance of containing it.
Watch for any animal that is behaving abnormally, particularly if it is showing signs of pain or discomfort around a wound. An infested animal will often be restless, shake its head, or constantly move to avoid flies. It may go off feed and become lethargic as the infestation progresses.
Look closely at any wound on your animals. A screwworm infestation will show a foul-smelling wound that appears to be getting larger rather than healing. You will see a mass of cream-colored larvae actively burrowing into the tissue. The wound may have a bloody discharge and will typically be surrounded by swarms of flies.
Common wound sites include the navel of newborn calves, ear tags and piercing sites, dehorning wounds, castration sites, branding wounds, tick bite sites, and any accidental cuts or injuries.
What To Do If You Suspect Screwworm
If you find an animal with a wound that looks suspicious, act immediately. Do not wait.
Do not treat the wound yourself and release the animal. This is critically important. If you treat and release, you may kill the larvae but the animal will mix with others and you will lose track of a potentially confirmed case.
Isolate the animal immediately. Keep it separate from the rest of your herd.
Contact your state veterinarian or the USDA Animal and Plant Health Inspection Service (APHIS) right away. In the US you can call 1-800-USDA-AID. Report your suspicion clearly and follow their instructions precisely.
If possible, collect a sample of the larvae using gloves and a clean container and keep them for laboratory identification. Do not destroy potential evidence.
Your vet or a USDA inspector will come to examine the animal and collect official samples. If screwworm is confirmed, USDA will take over the response and guide you through treatment and containment.
How To Protect Your Herd
While the immediate threat is focused on the southern border states, every cattle farmer should be taking precautions now.
Inspect your animals regularly, at least once a day during warmer months when fly activity is highest. Pay special attention to any animal with a fresh wound. Check newborn calves daily and treat the navel cord with an appropriate antiseptic immediately after birth.
Minimize the number of wounds on your animals. Where possible, delay elective procedures like castration, dehorning, and branding during peak fly season. When wounds are unavoidable, treat them immediately with a wound dressing that deters flies.
Use fly control measures consistently across your operation. Fly traps, insecticide ear tags, pour-on treatments, and sprays all help reduce the fly population around your animals.
Keep your premises clean. Screwworm flies are attracted to decaying organic matter. Remove dead animals promptly and dispose of them properly. Keep manure areas clean and well managed.
Stay informed. Monitor USDA APHIS updates regularly and sign up for alerts from your state department of agriculture. If you are in a southern border state, attend any local briefings or extension meetings about screwworm preparedness.
Key Takeaways
New World Screwworm is one of the most destructive cattle parasites in history and it is currently moving toward the United States border. The threat is real and every cattle farmer needs to be prepared.
Know what to look for. Inspect your animals daily. Minimize and treat wounds promptly. Report any suspicious infestation immediately to USDA APHIS. Do not try to handle a confirmed or suspected case on your own.
The screwworm was defeated once before through science, cooperation, and vigilance. If every farmer stays alert and reports suspected cases immediately, we can stop it again.
Frequently Asked Questions
How do I report a suspected screwworm infestation in the USA? Call USDA APHIS at 1-800-USDA-AID immediately. Your state veterinarian's office can also assist.
Is New World Screwworm in the USA right now? As of 2025 and into 2026, screwworm has not been confirmed in the continental United States but has been detected very close to the southern border in Mexico. The situation is being actively monitored.
Can screwworm affect humans? Yes. Screwworm can infest any warm-blooded animal including humans. Cases in humans are rare but do occur, particularly in tropical regions.
How fast can screwworm kill an animal? An untreated infestation can kill a large animal like a cow within seven to fourteen days. Smaller animals and young calves can die faster.
What states are at highest risk? Texas and other southern border states are at the highest current risk. All cattle farmers should remain vigilant.
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