Screwworm: What Ranchers Need to Know (Updated 9/11/2025)
The New World screwworm has returned to the spotlight in Texas and across the United States. This guide gives ranchers, exotic animal breeders, hunters, and the public clear steps on how to respond to an outbreak, treat affected animals, and apply best practices to stop its spread.
What New World Screwworm Is
New World screwworm is not a worm. It is the larval stage of a fly. The fly lays eggs along the edges of fresh wounds, the umbilicus of newborns, the ear canal, nostrils, mouth, and anywhere skin breaks. The larvae feed on living tissue. Untreated animals deteriorate fast. Death follows in days in severe cases.
Older hands remember the last era. They roped, doctored, and burned time fighting wave after wave. Many of them sat in this workshop. The message they carried forward was simple. This is beatable with discipline and with the sterile insect technique. It is not beatable with hope.
Why We Are Talking About This Now
The resurgence moved north through Central America after crossing the Darién Gap in August 2022. It reached Mexico in November 2024. Official case counts reported by Mexico grew from the thousands in mid-2025 to more than four thousand by late July, with the northernmost known case near Veracruz. That case sat roughly 375 miles from the U.S. border. Case counts increase when surveillance is robust. They can understate reality when surveillance is thin. Wildlife cases in Mexico are not reported in the same way as livestock. That matters because wildlife populations amplify risk across a landscape.
The spread across Central America took roughly 27 months in this cycle. That pace mirrors historical patterns. South Texas will be the first U.S. region to feel pressure if entry occurs. That is not theory. It is the path of least resistance.
What Agencies Are Doing
The Secretary of Agriculture announced 850 million dollars to strengthen the fight. 750 million funds a new sterile fly facility near Mission, Texas at Moore Airfield. One hundred million funds on-the-ground response. The sterile insect technique works. It is the only eradication tool. It overwhelms wild populations by releasing sterile males at scale. Females mate once. A sterile mating means no viable eggs.
A Panama facility at Pacora currently produces about 100 million sterile flies per week. Historic suppression lines consumed up to 700 million per week at peak. Mexico is working to bring a facility back online to add capacity. The new Texas plant will take time to build and bring to full production. Until those lines run, Panama is the only source. When the United States confirms entry, Panama’s weekly output prioritizes U.S. lines. That is helpful. It is not enough by itself to hold an extended frontier. More capacity must come online.
What You Need to Watch in Animals
You will not detect screwworms early by waiting at the house. You will find it by looking. You are looking for behavior and for wounds that do not act normal. Depressed animals. Isolation from the herd. Irritability. Head shaking. Flies clustering at a wound that smells wrong. Eggs laid in shingle layers at the wound edge. Fast expansion of a lesion over a day or two. Bloody discharge that is too wet for too long. Think fresh flesh, not pus.
Calves, lambs, kids, and fawns carry a special risk at the umbilicus. Ear tag holes and injection sites also attract flies. Tick bites do the same. Any cut you make working becomes a target. Treat that site before you turn the animal out.
How Reporting Works and Why Movement Matters
Treat a suspected screwworm case as a foreign animal disease. Pick up a phone. Do not put the animal in a trailer or transport anywhere. Do not drive to the clinic. Movement drops larvae along roads and at pens. Those larvae pupate and emerge. You spread your problem to your neighbors and yourself.
State and federal veterinarians will come to you. Texas Animal Health Commission and USDA coordinate submissions to the Iowa lab for confirmation. They do not want a box full of random maggots. They want credible samples from animals that fit the clinical picture. Local region managers direct inspectors. Your private veterinarian is part of the loop. If you do not have an active veterinarian-client-patient relationship, establish it now.
What A Quarantine Looks Like
A positive case triggers a control zone. The core zone reaches about three kilometers (1.86 miles) around the case. That zone holds tight restrictions. An outer ring of about ten kilometers (6.2 miles) runs high surveillance and inspection. A broader buffer can extend ten to twenty kilometers more depending on spread. The shape depends on terrain, roads, and wind. The goal is simple. Stop movement of infested animals. Treat and inspect clean animals so commerce continues with control.
Auction barns, feedyards, and game ranches inside a control zone can keep operating under inspection protocols. Loads get looked at. Wounds get checked. Any animal that is suspect gets held and treated. The point is not punishment. The point is to keep a bad day from becoming a lost season.
What The Law Allows You to Use
Only two tools are indicated in the United States specifically for New World screwworm at this time. Coumaphos is used in regulatory programs and in immersion systems. Permethrin wound sprays sold for screwworm or wound myiasis treatment are labeled for therapy and short residual protection at the wound site. Use them on every wound you create while working cattle, sheep, goats, and exotics when pressure rises. Use them on suspect natural wounds as you find them.
Macrocyclic lactones sit in most parasite control plans. They include ivermectin, doramectin, and moxidectin. South American experience shows mixed screwworm efficacy and clear resistance pressure after decades of frequent dosing under tropical parasite loads. They are tools to discuss with your veterinarian for local conditions. They are not eradication tools. The law treats pesticides and drugs differently. You cannot use an EPA-registered pesticide off label. You can use an FDA-approved drug in an extra-label manner under a valid veterinarian-client-patient relationship, but you cannot mix any of these into feed for food animals under extra-label authority. Follow residue rules. Break them and you invite recalls, penalties, and closed markets. The easiest way to stay clean is simple. Talk to your veterinarian. Get a written plan that fits your operation.
A Practical Risk Framework You Can Run Today
Use four ideas to drive decisions. Exposure risk. Attack rate. Case fatality rate. Impact cost.
Exposure risk asks one question. How likely is it that the fly is here. Distance to the last confirmed case drives this number. So does time since the last confirmation. A case 420 miles away a month ago is not the same risk as a case 420 miles away yesterday. Agency trap lines and official reports move this number up and down. Seasonal weather shifts matter. Screwworm activity drops in cold conditions. It slows in extreme heat. It surges with warmth and moisture.
Attack rate asks a different question. Given the fly is on the landscape, how likely is it to find and infest your animals. Husbandry changes can move this number. Finish spring calving earlier to dodge peak pressure. Avoid elective wounding during the hottest part of the season. Treat every induced wound on the day you make it. Reduce tick burden. Reduce horn fly stress. Both actions lower wounding and reduce attraction. Shorten exposure windows for neonates. Small changes stack.
Case fatality rate looks at what happens after infestation. If you do nothing, young animals with head or navel strikes die at high rates in short order. If you find cases fast and treat them correctly, you convert deaths into recoveries. Your surveillance and your response speed move this number the most. You improve both by tightening pastures, shifting to areas with more visibility during peak risk, and training crews to look for the exact signs that matter.
Impact cost closes the loop. What does each loss cost you. You can use a conservative value for a four to five hundred weight calf. You can use a different number for lambs, kids, or exotics. Multiply exposure risk by attack rate by case fatality rate by impact cost. The product is your expected loss per head. That number tells you how much you can invest to lower the first three numbers and still come out ahead. It also gives you a rational way to stop arguing about a drum price and start arguing about the right plan.
Smear 62
Back in the 1940s, ranchers were desperate for ways to stop screwworms from eating their livestock alive. The USDA’s answer was Smear 62, a thick chemical paste made from diphenylamine, benzene, and sulfonated oils. It was brushed onto wounds after pulling out as many larvae as possible, with the idea that it would kill the remaining worms, coat the tissue, and prevent re-infestation. By 1945, the agency was calling it a breakthrough, and for a while it was the go-to treatment across the South and Southwest. Smear 62 worked well enough to reduce losses, but it was hardly perfect. The strong odor, repeated applications, and toxic ingredients meant it wasn’t ideal for long-term use, and by modern standards it was flat-out hazardous. Benzene is now known to be carcinogenic, so you can imagine why no one brags about keeping a bucket of Smear 62 in the barn today.
Swormlure
Swormlure is basically fly bait for the world’s nastiest flesh-eating insect. USDA scientists started developing it in the 1960s to lure screwworm flies into traps so they could be monitored or poisoned. Think of it as perfume for parasites, except instead of Chanel No. 5 it smells like rotting meat mixed with sweat.
The formula has gone through versions (Swormlure-2, Swormlure-4, Swormlure-5), each tweaked to be more attractive and stable. The mixture contains compounds that mimic the odors of decomposing animal tissue, which screwworm flies seek out to lay eggs. When paired with insecticides or sticky traps, it doesn’t just tell you whether screwworms are in an area, it helps kill them. Agencies like USDA and Texas Ag use Swormlure-5 in monitoring stations along the U.S.–Mexico border to detect any new outbreaks before they spread.
How Weather and Timing Change Your Moves
Flies slow down when average temperatures drop into the high forties. They slow down in extreme heat when average temperatures push into the nineties. They do not vanish. They pulse with conditions. Use that seasonal rhythm. Finish spring calving in late February or mid-March instead of April if your forage and cow condition allow it. Work calves and lambs early in the morning during hot months. Doctor wounds on contact. Stage permethrin sprays at pens and on side-by-sides.
Biosecurity That Fits Ranch Reality
Do not move suspected animals. That rule matters more than any other. Most spread mistakes start with good intentions and a trailer. Clean blood and fluid off trailers and chutes. Wash down areas with standing fluids after working. Do not stack carcasses. Dispose of them in ways that do not feed flies. Pay attention to personal gear. Maggots can hitch rides in cuffs, pockets, and boot treads after a hard doctoring job. Your habits move risk. Fix the ones that add risk.
Coordinate with neighbors. Share trap data and what you see. If you run an auction barn, pen exotics, or ship livestock, stay plugged into region managers. Build a routine for inspections and pre-shipment checks. Tell buyers what you are doing and why. Clear messages keep commerce moving even when nerves are hot.
What Hunters, Exotic Breeders, Outfitters Need to Hear
You will get calls from bowhunters, hunters, and outfitters. Give them three rules. Watch for head shaking, isolation, and fresh wounds that smell bad. Report suspected wild animals and exotics to your game warden and to the Texas Parks and Wildlife Department (TPWD). Follow carcass handling instructions during any control effort. If you host hunters, add a flyer at check-in with the reporting number and a short description of signs.
What As Soon As Possible
Make a list. The first item is a call to your veterinarian. Put a plan in writing. Confirm which drugs and sprays you will use and how. Confirm legal use for each species on your place. Build a short SOP for crews. Write it in simple language. Put it in the barn room and the glove box. Store permethrin wound spray in every truck and in a sealed bin at each set of pens. Replace it as it empties. Build a monitoring loop that gives you eyes on calves, lambs, kids, and fawns each day during peak pressure. If you cannot ride every pasture, ride the ones with recent working or new births. Keep a log. It does not need to be fancy. Date, pasture, head count at water, suspect animals seen, actions taken.
Schedule working to avoid elective wounds during high risk periods. If you must work, build post-working checks into your plan for three days. If you ear tag, inspect tag holes the next morning. If you brand, recheck those calves and apply residual wound protection. If you trim horns, walk those cattle twice in the following days. If you cut bulls, treat the wound and inspect often. A few minutes at the right time saves a life.
Where This Goes from Here
Agencies are moving on production capacity for sterile flies. The Panama plant runs today at about 100 million per week. Mexico is repositioning capacity. The Texas build will take time to deliver and scale. When the United States confirms entry, those sterile flies will route to our frontier. That will help hold lines and push south. It will not erase your responsibility. It does not change the need for early detection, clean movement, and fast treatment.
History says eradication from a large frontier takes years. The push from the United States through Mexico and Central America to the Darién barrier took decades last time. The reverse flow took months to return to the Mexican frontier from South America in this cycle. That is why your plan matters. It bridges the time between the first bad day and the sustained response. It keeps your herd alive and your property value intact while the larger machine spins up.
Frequently Asked Questions Answered Straight
Will there be a hunting season in Texas? There is no plan to suspend hunting. If any landscape-scale treatments come into play, they will be structured with food safety rules. Those rules pull products with long withdrawal timelines ahead of the season. Parks and Wildlife and the Animal Health Commission will publish guidance. Stay in contact with your game warden.
How far away is the nearest confirmed case? The workshop reported the northernmost known case near Veracruz at roughly 375 miles from the U.S. border by late July. That distance changes with time. Distance is not the only factor. Illegal movement and weather can change risk fast.
What if a load arriving at a barn has an infested animal? Inspectors will isolate and treat the animals. The goal is to keep commerce clean, moving under inspection, not to shut barns down.
Do macrocyclic lactones stop screwworm? They are part of parasite control programs. They show mixed results against screwworm overseas, and resistance is real after heavy use. They do not replace sterile fly releases. Use them with a veterinarian’s guidance and stay inside the law.
What if I find a suspect deer? Call your game warden and the Texas Parks and Wildlife Department (TPWD). They will direct action. Do not attempt to transport sick animals for display or proof.
Is there money to reimburse ranch treatments today? The workshop did not promise program funds for private treatments. The federal investment goes to eradication infrastructure and response capacity. Plan for your own operating costs. Keep receipts. Track time. If programs open later, you will want records.
The Mindset That Wins
Preparation beats panic. You have seen hard things before. This is one more. You protect your operation by tightening the basics. You look at your animals. You doctor what you see. You hold suspect stock in place. You work with your veterinarian. You coordinate with your neighbors and your market. You talk to your hunters through your rules, not rumors. You update your plan as new information comes out. You document. You stay in motion.
