UPDATE: As of June 4th, 2026, the USDA has announced the first confirmed case of New World Screwworm in the United States in decades. This was in the umbilicus of a 3-week old calf in Zavala County, TX. Here is a link to the press release. Wade through the self congratulatory tone that the federal government can seem to muster in any circumstance- and information about the actual case is in there.
UPDATE 2.0: Here is a PDF guide for sheep producers on NWS and treatment released by the American Sheep Industry Association (ASI):
New world screwworm (Cochliomyia hominivorax ) is a fly that lays eggs in open wounds or body orifices of living, warm-blooded mammals. When the larvae emerge, they invade the surrounding tissue, aggressively feeding and burrowing. This behavior creates extensive, excruciating wounds in the host animal, which results in horrifying infections and death within 1-2 weeks in the vast majority of untreated animals.
Through a feat of international cooperation, New Word Screwworm was eradicated in North and Central America as far south as Panama by using the release of sterile male flies (SIT or Sterile Insect Technique). The new world screwworm adult female flies only lay eggs once before dying. If they mate to a sterile male, they lay infertile eggs. Male flies on the other hand , mate several times. So sterile male flies remain competitive with unaltered flies for several matings. The population is quickly depleted if the majority of available males are sterile. This technique was conceptualized in the 30s and put to work in the early 50s against New World Screwworm. The Screwworm was declared eradicated in the United States by the early 1980s.
After its success against the New World Screwworm, SIT had been used against multiple species of flies and mosquitos, raising a public response ranging from awarding heroic honors of the developers, to outcry over the ethical implications of attempting to eradicate entire species because they are economic juggernauts. More resources to explore this debate can be found at the bottom of this article.
The New World Screwworm has remained endemic to many areas of South America and the Caribbean. A biological barrier was maintained using SIT in Panama, effectively until 2023. This barrier likely failed due to steadily increasing temperatures, allowing greater numbers of New World Screwworm to move north, overwhelming the scheduled sterile fly releases. There were also instances of cases coming from uninspected livestock crossing borders. Since 2023, cases have now been reported in every country in Central America. Most recently, reports have been made in Mexico just 60 miles shy of the Texas border.
For the last three years, concern has been mounting. In the last year, the torch has been taken up by the federal government in a series of evasive actions and call to actions, all with an unfortunately tiresome, Nationalist tone as though United States borders existed anywhere but in the human imagination. Information on these federal initiatives, as well as the international agenda regarding New World Screwworm, can also be found at the end of this article.
The remainder of this article will be dedicated to what shepherds (and any warm blooded mammal raising warm blooded mammals really) in areas that have not dealt with this problem in many decades can do at home and on the farm. We will do this in three parts: what you are watching for, what to do if you think you see it, and what to do in the meantime.
What are you watching for?
The flies are just a skosh bigger than houseflies, they have very large, orange-red eyes, and an iridescent, blue-green body with black stripes. The eggs are laid in body orifices or wounds. The initial wound may be as small as another insect bite. The larvae that hatch will be white initially, but will turn pinkish red in adolescence. They burrow down into the flesh so their derrieres are what is visible in the wound. This may be very hard to detect depending on how deep the wound is and what color the larvae are. Other fly larvae like to eat dead necrotic tissue. The outside or oldest portions of the wound may be dying tissue, and other non-screwworm maggots may end up there further, confounding or masking the screwworm problem. Wounds or draining tracts that are spreading or not healing normally should all get a good probing, hopefully with the aid of some local analgesia (pain relief) or anesthesia (numbing).
As always, pay attention to the flock and do a very thorough once-over of any animal who is limping, agitated, scratching or shaking, separating themselves, off feed, or otherwise generally off, even if a wound is not immediately apparent. I have seen pictures of New World Screwworm larvae burrowed between the toes of cloven hoofed animals and imagined how easily this must be overlooked.
What do you do if you think you have spotted a fly or a larval infestation?
Collect the fly or larvae in a sealed container. Then call your state veterinarian, wildlife department, or animal health authority, you can find their office number with a quick internet search. Also, call your flock’s personal veterinarian. Tell them about your suspicions. Do not transport any animals or continue business as usual until the fly, larvae, wound, or animal has been inspected or processed according to whatever instructions you get when you make that call.
What preparations can you make now, before New World Screwworms come calling?
Use biology to your advantage by managing animals as though the New World Screwworm is already here, particularly if you are in Texas, Arizona, New Mexico, or similar locations. The flies are very sensitive to temperature and do not survive or thrive in cold weather. Infestations, therefore, are much more common through warmer and hotter months. Minimize wound-making practices like disbudding and banding during warm times. Consider lambing on the cooler ends of seasons since the fly loves to lay eggs in an open umbilical stump. Work on wound prevention and management, as any good shepherd already is. Up your game on deterring any and all flies from enjoying wound time.
Along with painstaking, manual larvae removal and epic wound care, a cornerstone of treatment is the use of antiparasitics like Ivermectin. These antiparasitics are precious and overused in routine management-- for example, deworming whole herds on a schedule out of habit rather than necessity, has the potential to decrease their effectiveness overall. Overuse has led to resistant populations (those that cannot be killed by our chemical antiparasitics) of internal and external parasites, and the potential exists with New World Screwworm to have similar concerns. Continue to use these medications judiciously.
Keep yourself informed of developments. If you can stomach the USDA website at the moment-- that is a good resource. Just keep your head down and try to see the facts through the rhetoric. If your constitution cannot handle this, then follow the extension services. Texas A+M has a page of quick fact sheets and also regularly posts updates on their website. Their information is available in both Spanish and English. The American Veterinary Medical Association is also regularly updating professionals, but their website and posts are public.
In Conclusion
The human mind can be very tempted to panic in the face of novelty. I experience or witness this regularly in veterinary medicine. I will be standing before a innaetant (I do not know what word this is meant to be) giraffe and feel myself wanting to shut down because, what do I, a sheep veterinarian roped into seeing dogs, cats and hamsters occasionally, know about a giraffe ?!??!? If I take a deep breath and a step back, I know principles-- of health, of management, of nutrition, of ruminant function, and I have powers of observation. All of this is applicable.
This is no different with New World Screwworm. Sure, we have a fast moving, healthy flesh-eating parasite that will kill most infected animals within ten days…and that is not terribly pleasant. But you have skills and principles that will help squelch this threat or minimize the damage. Close observation, good husbandry and practices that keep animals at a baseline of health, both physically and mentally, will go a long way. And, if the worst case happens and New World Screwworm is once again endemic in the United States, we will learn from the past and from the areas that never got the luxury of a break from this bugger; we will adapt, and so will our sheep.
Sheep and humans have been through a lot in the more than 11,000 years that we have been travelling together. The sheep have been incredibly generous all along in helping us through. If we listen and respond, we’ll walk through this too.
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International response:
Fighting the Resurgence of the New World Screwworm
United States current response strategy:
DISEASE RESPONSE STRATEGY NEW WORLD SCREWWORM MYIASIS
https://usaha.org/wp-content/uploads/2026/03/USAHA-NWS-Whitepaper-FINAL-18March2026.pdf
Ethics of SIT and other efforts to eliminate inconvenient species:
The Ethics of Eliminating Harmful Species: The Case of the Tsetse Fly
Do You Hear That? It’s the Buzz About Sterile Insect Technique for Mosquito Management