By Dr. Blayne Mozisek MS, DVM, MAM, DACPV
Read time: 17 minutes
New World Screwworm: 7 Vital Facts to Protect Your Flock
The New World screwworm, a flesh-eating parasite the United States eradicated decades ago, was first confirmed in Texas on June 3, 2026, and two confirmed cases in Zavala County have changed the conversation for every animal owner in the region (USDA APHIS). As a veterinarian based in Texas with a background in raising cattle, I have been watching this pest work its way north for the past two years with more than professional interest. For most of my career, the screwworm lived only in old veterinary textbooks and the stories of older ranchers who remembered the eradication campaigns of the 1960s. Now it is on our doorstep, and federal and state animal health authorities have been monitoring its advance closely the entire way. As a board-certified poultry veterinarian, I want to give you the science behind the headlines: what this fly actually is, how we wiped it out the first time, why it is back, and the concrete steps that protect your flock, your livestock, and your pets. This is not a reason to panic. It is a reason to understand the threat and act with informed vigilance.
What’s in This Article
What Is the New World Screwworm? Biology of a Flesh-Eating Pest
The Campaign to Eradicate Screwworm From the US
The 2025 to 2026 Resurgence and Sterile Fly Response
Recognizing Screwworm Myiasis in Livestock and Backyard Poultry
How to Protect and Inspect Your Animals
Practical Takeaways for Backyard Flock Owners
What Is the New World Screwworm? Biology of a Flesh-Eating Pest
The New World screwworm is the larval stage of the fly Cochliomyia hominivorax, whose Latin name translates roughly to “man-eater,” and it earns that reputation honestly. Unlike the maggots almost everyone has seen at some point, screwworm larvae feed on the living flesh of warm-blooded animals, not on dead tissue (Cureus). That single trait is what makes this parasite so dangerous and so different from the ordinary nuisance flies in your coop.
How the New World Screwworm Differs From a Common Blow Fly
This is the distinction that matters most, and it is the one most often confused online. Common blow flies and bottle flies are carrion feeders. When you see maggots on a dead bird or in spoiled feed, those are facultative flies that target decaying matter. They are unpleasant, but they are not invading healthy tissue.
The New World screwworm is an obligate parasite, meaning its larvae must consume living tissue to survive (Insects journal). The female fly is drawn to open wounds and the mucous membranes around natural body openings. After her eggs hatch, the larvae burrow head-down into living flesh, screwing deeper as they feed, which is where the name “screwworm” comes from. They release proteolytic enzymes that digest tissue, enlarging the wound and attracting still more flies (Journal of Feline Medicine and Surgery). A wound that should be healing instead grows larger, deeper, and more painful by the day.
The Life Cycle: From Egg Mass to Burrowing Larvae
Understanding the life cycle explains both the danger and, fortunately, the weakness we exploit to control this pest. A female screwworm fly lays her eggs in neat shingled masses of 150 to 500 at the edge of a fresh wound or near a body opening (Merck Veterinary Manual). The eggs hatch within roughly 24 hours, and the newly emerged larvae immediately begin feeding on the host’s living tissue.
Over the next several days the larvae grow through three stages, then drop to the ground to pupate and emerge as adult flies. Here is the biological detail that anchors our entire control strategy: the female screwworm fly typically mates only once in her lifetime (Insects journal). That single trait, which I’ll discuss again shortly, is the weak point we exploited to wipe out this pest once before, and the same weakness is being used to eliminate it today.
The Campaign to Eradicate Screwworm From the US
The story of how we eliminated the screwworm in livestock is one of the great triumphs of veterinary science, and it is worth knowing because the same playbook is being run right now.
A Brief History: Knipling, Bushland, and the Sterile Fly Technique
In the 1950s, two USDA entomologists, Edward Knipling and Raymond Bushland, developed a radical idea built on that single-mating biology I mentioned earlier (USDA Ready Reference Guide). If you could flood an area with sterile male flies, wild females would mate with them, lay unfertilized eggs, and produce no offspring. Repeat that often enough and the population collapses.
The proof came on the Caribbean island of Curaçao in 1954, where aerial release of sterile flies eradicated the local screwworm population in about six months (USDA Ready Reference Guide). The technique then scaled up dramatically. Florida was declared screwworm-free by 1959, and the entire United States achieved eradication by 1966, the first successful continental application of the sterile insect technique (Cureus). Regional partnerships pushed the eradication line south through Mexico and Central America in the decades that followed.
How the Sterile Fly Technique Works
The sterile fly technique, more formally the sterile insect technique or SIT, remains the cornerstone of screwworm control. The process starts in a mass-rearing facility where millions of screwworm pupae are produced on artificial diet. At five to six days old, the pupae are exposed to gamma radiation, which sterilizes the developing flies without significantly harming their longevity, mating ability, or searching behavior (USDA Ready Reference Guide).
The sterile flies are then released over the target area, usually from aircraft but also by ground dispersal chambers in tighter zones. Because wild females mate only once, a mating with a sterile male yields no viable eggs, and each successive generation shrinks until the population is gone (Insects journal). It is biological control at its most elegant, using the insect’s own reproduction against it without broadcasting insecticides across the landscape. This sterile fly technique is the same tool driving the current response.
Maintaining the Barrier: COPEG and the Panama Shield
Eradication is not permanent on its own, because the screwworm remains endemic in parts of South America and the Caribbean and can re-invade. To hold the line, the United States and Panama jointly operate COPEG, the Panama-US Commission for the Eradication and Prevention of Screwworm (USDA Ready Reference Guide). For years, the Pacora facility near the Darién Gap released tens of millions of sterile flies weekly, forming a living barrier at the narrow neck of land between the Americas. That barrier kept screwworm in livestock out of North and Central America for decades, until recent events overwhelmed it.
The 2025 to 2026 Resurgence and Sterile Fly Response
Here is where the historical triumph meets the present crisis, and why a Texas detection in June 2026 is front-page news for animal owners.
Northward Spread Through Central America and Mexico
The current wave began in 2023, when Panama and Costa Rica identified a screwworm outbreak that broke through the long-standing barrier (CDC). Since then, the pest has marched north through every country in Central America and into Mexico. As of May 20, 2026, affected countries had reported more than 171,700 screwworm cases in animals and more than 1,960 cases in people (CDC).
The economic and trade consequences arrived quickly. As cases advanced toward the border, the US suspended live cattle, bison, and horse imports through southern ports of entry (USDA). This northward spread of screwworm in livestock has been progressing for more than two years, and its march across Mexico set the stage for exactly what veterinarians and agricultural officials had been bracing for the entire time.
Screwworm Confirmed in Texas: What the June 2026 Detection Means
On June 3, 2026, USDA confirmed the first New World screwworm case in a calf in Zavala County, Texas. On June 5, 2026, officials confirmed a second affected calf in the same county, about 5–6 miles from the initial case. As of early June 2026, no additional animal cases have been confirmed beyond these two calves, and other suspect samples from the area have tested negative. This detection did not catch anyone flat-footed. USDA and its partners had been actively responding to the pest’s northward advance for more than a year, suspending livestock imports, expanding sterile fly dispersal along the border, and standing up surveillance and infrastructure in South Texas. So when the calf in Zavala County was confirmed, officials immediately escalated an already-running NWS Response Playbook with several coordinated actions.
The response so far includes:
Forming a unified Incident Command Team with the Texas Animal Health Commission and deploying personnel to the area.
Establishing a 20 kilometer infested zone around the detection, with quarantines, animal movement controls, and surveillance.
Expediting targeted release of sterile flies using ground release chambers, on top of the 4 million sterile flies already released aerially each week in the area.
Increasing trapping along the border and stepping up wildlife surveillance (USDA APHIS).
This is exactly the rapid, science-based containment the eradication history predicts. A single detection triggers an overwhelming local response designed to stop establishment before it starts.
Scaling the Response: 500 Million Sterile Flies a Week
Behind the local response is a national mobilization. USDA continues to disperse 100 million sterile insects per week in Mexico and along the US-Mexico border, with the release polygon adjusted by modeling to maximize protection (USDA APHIS Current Status). The agency is investing heavily to push domestic production toward roughly 500 million sterile flies per week, the same scale used to eradicate the pest the first time (USDA APHIS).
New infrastructure is central to that goal, and it is moving fast. A sterile fly dispersal facility at Moore Air Base in Edinburg, Texas, is already complete and operational, with its grand opening held on February 9, 2026; it can disperse up to 100 million flies per week along the border and into the United States if needed (USDA). At the same site, USDA and the U.S. Army Corps of Engineers broke ground on April 17, 2026 on a much larger sterile fly production facility, the only one of its kind in the United States, which is targeted to reach initial production of 100 million flies per week by November 2027 and scale toward 300 million per week, complementing existing production in Panama and Mexico (USDA). A 100 million dollar NWS Grand Challenge is funding innovation in sterile fly production, traps, lures, and therapeutics, while researchers evaluate e-beam and x-ray sterilization and genetically engineered strains (USDA). The FDA has also moved quickly on the drug front, conditionally approving and authorizing several products to prevent and treat screwworm in livestock. Among them, Exzolt Cattle-CA1 (fluralaner), a topical pour-on for beef cattle and replacement dairy heifers, received FDA conditional approval on December 4, 2025 for the prevention and treatment of screwworm larval infestations, giving cattle producers a practical new tool (FDA).
Recognizing Screwworm Myiasis in Livestock and Backyard Poultry
Early recognition is the single most valuable skill an animal owner can have right now, because a screwworm infestation caught early is far easier to treat and report than one that has progressed.
Clinical Signs: Wounds, Odor, and Visible Larvae
Screwworm infestation is a form of myiasis, the medical term for fly larvae infesting living tissue. The classic signs are hard to miss once you know them. Look for wounds that are draining or enlarging rather than healing, along with signs of discomfort in the animal (USDA APHIS).
The hallmark is a serohemorrhagic discharge, a bloody, serous fluid, paired with a distinctly foul odor that is often described as unmistakable once encountered (Journal of Feline Medicine and Surgery). It is worth being honest here: because the screwworm was eradicated from the United States in 1966, very few practicing veterinarians or owners in this country have ever seen a live case, which is exactly why learning the signs now matters so much. You may also see the larvae themselves, packed deep in the wound or around body openings such as the nose, ears, navel of newborns, and genitalia (USDA APHIS). Because secondary maggots from other flies often sit near the surface, the true screwworm larvae are typically deepest in the wound.
Risks to Animals and Humans
Now that you know what the signs look like, it helps to understand who is actually at risk. The screwworm is primarily a parasite of livestock, but it does not stop there. It can infest pets, wildlife, and, less commonly, birds and people (USDA APHIS). Any warm-blooded animal with an open wound is a potential host. In livestock, untreated infestations can be fatal, and historically this pest cost the US cattle industry more than 200 million dollars a year before eradication (Cureus).
Human cases are rare but real. On August 4, 2025, the CDC confirmed the first US human screwworm case in decades, in a Maryland resident who had recently returned from El Salvador (Cureus). Infestations in people occur in open wounds or mucous membranes and require prompt medical care. The reassuring news for consumers is that screwworms do not infest meat, fruits, vegetables, or other foods, and the US food supply remains safe (USDA APHIS).
Special Concerns for Backyard Flocks
Screwworm infestation in birds is uncommon, but it is not impossible, and during an active outbreak vigilance is warranted (CDC situation summary). For backyard poultry keepers, the priority is checking any bird with an open wound. Pecking-order injuries, predator bites, broken blood feathers, scaly leg lesions, and vent damage are all potential sites where a screwworm fly could lay eggs.
Pay particular attention to the vent area, fresh injuries from flockmates, and brooding hens that may have skin abrasions. In other young livestock the navel of newborns is a classic site, so the principle translates to any fresh opening on a young animal. The realistic risk to a healthy, wound-free flock is low, but a bird with an untended injury during fly season deserves a close daily look. For more on routine flock health checks, see our Poultry Knowledge Hub.
Social Media Myths, Clearly Labeled
A great deal of misinformation is circulating on social media, and as a veterinarian I want to correct a few specific claims I have seen shared widely.
Myth one, often seen on social media, is that any maggots on an animal mean screwworm. This is false. USDA has published an official myth-busters factsheet confirming this exact point: myiasis can be caused by botflies, blowflies, and other species, and only an entomologist can confirm whether larvae are truly NWS (USDA). Most of the maggot cases that backyard chicken keepers encounter are what the community commonly calls “fly strike,” common blow fly larvae infesting a dirty, soiled, or already-injured area, especially around a manure-caked vent in warm weather. Fly strike is a genuine and serious poultry problem in its own right and needs prompt treatment, but it is not the same thing as New World screwworm invading healthy living flesh. The distinction is real and important, and only proper identification can confirm screwworm.
Myth two is that you should dig the larvae out yourself and handle it at home. This is dangerous advice. Improper removal can spread the pest and worsen the wound, and screwworm is a reportable pest that requires official involvement. Myth three is panic about the food supply. As USDA has stated plainly, screwworms do not infest meat or produce, and federal inspection keeps affected animals out of the food chain (USDA APHIS).
How to Protect and Inspect Your Animals
Screwworm prevention comes down to a few disciplined habits, and they are well within reach of any backyard keeper or smallholder.
Wound Management Is the First Line of Defense
Because the screwworm fly needs an open wound or mucous membrane to lay eggs, wound management is your most powerful defense. The CDC and USDA guidance is consistent: clean and cover all wounds, no matter how small or where they are on the body (CDC clinical overview).
For animals, that means inspecting your flock and livestock daily during fly season, treating even minor injuries promptly, and keeping them covered or protected while they heal. It is also wise to avoid elective procedures that create wounds, such as non-urgent surgeries, dehorning, or castration, during peak fly activity in an at-risk area (Merck Veterinary Manual). The navel of newborn animals deserves special attention, since it is a classic infestation site. Right now, the highest-priority audience is South Texas residents living near the active detection zone, where daily wound checks matter most. If you are in that region, watch for updates from the Texas Animal Health Commission, USDA APHIS, and here at The Poultry Doc as we track any northward New World screwworm migration.
Prevention Tools: Repellents, Doramectin, and Ivermectin
Beyond wound care, several pharmaceutical tools support screwworm prevention and treatment, though I want to be clear about where the evidence is strong and where it is limited. In cattle, the macrocyclic lactones doramectin and ivermectin have documented prophylactic value. One controlled study found doramectin at 200 micrograms per kilogram was 100 percent effective at preventing experimental screwworm infestation, with protection lasting up to 21 days, outperforming ivermectin in the same trial (Veterinary Parasitology, PubMed).
In dogs and cats, oral nitenpyram and spinosad-based products used for flea control promote rapid expulsion of screwworm larvae, and subcutaneous ivermectin with antibiotic support is another documented option (Journal of Feline Medicine and Surgery). For people, EPA-registered insect repellents and covering exposed skin reduce risk (CDC clinical overview).
Here is the honest scientific caveat: most of this drug data comes from cattle, dogs, and cats, not poultry. There is limited published guidance on screwworm prophylaxis specifically in chickens, and the FDA emergency use authorizations to date have centered on cattle, horses, swine, sheep, and deer (USDA APHIS Current Status). For your flock, that makes wound prevention and rapid reporting the most reliable strategy, and any drug use in birds should be directed by your veterinarian. If you need diagnostic support, our diagnostic and lab services can help confirm what you are dealing with. On the drug front, the FDA has now conditionally approved two products for use against New World screwworm in cattle. The first was Dectomax-CA1 (doramectin injection, Zoetis), conditionally approved on September 30, 2025 to prevent and treat screwworm larval infestations and block reinfestation for up to 21 days. The second was Exzolt Cattle-CA1 (fluralaner topical pour-on, Merck Animal Health), conditionally approved on December 4, 2025 for the prevention and treatment of screwworm infestations in beef cattle and replacement dairy heifers. Both approvals are limited to cattle, not poultry, so for birds the strategy still centers on wound prevention, rapid reporting, and veterinarian-directed care.
What to Do If You Suspect Screwworm: Report Immediately
If you suspect screwworm in any animal, do not attempt to manage it alone. Reporting is mandatory and time-sensitive. Contact your state animal health official or your USDA APHIS Area Veterinarian in Charge immediately (USDA APHIS). Veterinarians are required to report suspicious animal cases to state animal health officials and the APHIS office.
If larvae must be collected for identification, they should be taken from the deepest part of the wound and preserved in 70 percent alcohol, but in an active response zone the right first step is almost always a phone call to officials rather than home handling (CDC clinical overview). For suspected human infestations, seek medical attention immediately and do not try to remove the larvae yourself (CDC situation summary). Quick reporting is not just protecting your animals; it is protecting the entire eradication effort.
Practical Takeaways for Backyard Flock Owners
Inspect your flock and livestock daily during fly season, and clean and cover every wound no matter how small, because the New World screwworm fly needs an open wound or body opening to lay eggs.
Learn the three red flags of screwworm myiasis: wounds that enlarge instead of heal, a foul odor with bloody discharge, and larvae burrowed deep into living tissue or around the nose, ears, vent, or navel.
Do not fall for the viral social media claim that all maggots are screwworm; USDA’s own myth-busters factsheet confirms most are harmless blow fly larvae on dead tissue, and only proper identification confirms screwworm.
Avoid elective wound-creating procedures during peak fly activity, and give newborn navels and fresh injuries extra protection.
Rely on wound prevention for poultry, since drug prophylaxis data largely comes from cattle, dogs, and cats, and direct any medication in birds through your veterinarian.
If you suspect screwworm, do not remove larvae yourself; report it immediately to your state animal health official or the USDA APHIS Area Veterinarian in Charge.
Stay calm and informed: the rapid sterile fly response that eradicated this pest once is already deployed, and your vigilance is a real part of keeping the US screwworm-free.
Conclusion
The return of the New World screwworm to Texas is a serious development, but it is not a cause for fear. It is a call for the kind of informed, practical vigilance that backyard keepers excel at. We have eradicated this pest from the United States before using the sterile fly technique, and that same proven strategy is being scaled up across Texas, the border, and Mexico right now. Your part is simple and powerful: keep wounds clean and covered, know the signs of screwworm myiasis, ignore the social media noise, and report anything suspicious without delay. At The Poultry Doc, our mission is evidence-based poultry health education, and few topics show the value of that mission better than this one. Watch your flock, trust the science, and make the call when it counts.
References
Centers for Disease Control and Prevention. (2026). New World screwworm outbreak: Situation summary. CDC. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html
Centers for Disease Control and Prevention. (2026). Clinical overview of New World screwworm. CDC. https://www.cdc.gov/new-world-screwworm/hcp/clinical-overview/index.html
Mastrangelo, T., & Welch, J. B. (2012). An overview of the components of AW-IPM campaigns against the New World screwworm. Insects, 3(4), 930 to 955. https://doi.org/10.3390/insects3040930
Moya-Borja, G. E., et al. (1997). Protective efficacy of doramectin and ivermectin against Cochliomyia hominivorax. Veterinary Parasitology, 72(1), 69 to 77. https://pubmed.ncbi.nlm.nih.gov/9403981/
Anderson, A. L., et al. (2010). Myiasis caused by the New World screwworm fly Cochliomyia hominivorax in cats. Journal of Feline Medicine and Surgery. https://pmc.ncbi.nlm.nih.gov/articles/PMC10911446/
Narrative review authors. (2025). The New World screwworm in the United States: A narrative review. Cureus. https://pmc.ncbi.nlm.nih.gov/articles/PMC12591281/
US Department of Agriculture, APHIS. (2026). USDA confirms presence of New World screwworm in the United States. https://www.aphis.usda.gov/news/agency-announcements/usda-confirms-presence-new-world-screwworm-united-states
US Department of Agriculture, APHIS. (2026). Stop screwworm: Unified government response. https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm
US Department of Agriculture, APHIS. (2017). New World screwworm ready reference guide: Sterile insect response. https://www.aphis.usda.gov/sites/default/files/nws_rrg_sterileinsectresponse.pdf
US Department of Agriculture. (2025). Secretary Rollins announces bold plan to combat New World screwworm’s northward spread. https://www.usda.gov/about-usda/news/press-releases/2025/06/18/secretary-rollins-announces-bold-plan-combat-new-world-screwworms-northward-spread
US Department of Agriculture. (2025). Mexico confirms case of New World screwworm in Nuevo Leon. https://www.usda.gov/about-usda/news/press-releases/2025/09/21/mexico-confirms-case-new-world-screwworm-nuevo-leon
Merck Veterinary Manual. (2025). Obligatory myiasis-producing flies of animals. https://www.merckvetmanual.com/integumentary-system/flies/obligatory-myiasis-producing-flies-of-animals