In January 2026, on a small backyard farm near Mazunte, Oaxaca, a case showed how New World screwworm can start with something as simple as a rope rubbing a pig’s neck. The pig, a one-year-old male used as a teaser boar, was kept outside and tied by the neck. Over time, the rope caused a friction burn that turned into an open wound. That was all it took. Flies found the sore, laid eggs along the edges, and soon the wound was full of larvae.
A local veterinarian acted quickly and took steps to keep the problem from getting worse. After checking the pig and recognizing screwworm based on the larvae and the pest’s known presence in the area, the veterinarian thoroughly cleaned the wound, applied a blue antiseptic spray commonly used in Mexico that contains tannic acid and gentian violet, and then applied a maggot-killing wound powder containing coumaphos, propoxur and prontalbin. Within minutes of treatment, the larvae began to die and fall off. The next day, the wound was cleaned again and the treatment was repeated. Larvae were collected and submitted to the appropriate authorities so the case could be documented and tracked.
You might wonder what a backyard pig in southern Mexico has to do with hog production in the U.S. The answer is, quite a bit. NWS cases, such as the ones we saw in Mexico, primarily affects livestock (such as pigs, but also cattle, sheep, goats and horses) and it can also affect pets (e.g. dogs and cats), wildlife (e.g. deer, rabbits, opossums), and, most importantly, humans. Although there are no confirmed NWS cases in the U.S. yet, the risk is growing as outbreaks move north in Mexico. There have already been over 700 swine cases reported in Mexico since screwworm reappeared in 2024, including recent cases in Tamaulipas, just across the border from Texas. This closeness has raised the risk for the U.S, since screwworm only needs two things to get started: a fly nearby and a wound on an animal or person, that the flies can infest.
Commercial barns lower the risk because workers check pigs every day, spot injuries early, and keep flies under control. Outdoor pigs, like those in small herds, on pasture, show pigs, and even feral pigs, are more at risk. These pigs run into more flies, get more small cuts, and, since they are spread out, wounds can go unnoticed long enough for larvae to take hold.
If you raise pigs outdoors, you don’t need to change your whole system. The main goal is to close the easy gaps that screwworm uses. Do this by watching pigs closely every day, preventing injuries when you can, and treating every wound as important. It is also important to alert animal workers and those living in close relationships with pets and livestock to remain alert of these signs both in animals, themselves, and their families.
The prevention message is clear and practical. Prevent wounds by fixing sharp edges, rough panels, sticking-out wires, and anything else that can cause cuts. Check your pigs often, especially those with injuries from fighting, lameness or any skin damage. Treat wounds right away and keep them covered. Work with your vet to choose the best products and steps for your farm.
If you are near the U.S.–Mexico border or in a higher-risk area, be extra careful with timing, since the risk is higher and quick action matters. If a procedure will cause a wound, only do it when you can watch the pig closely and protect the wound right after. If you ever think you see screwworm, do not wait. Call your veterinarian and report it right away. The Mexico case started with a rope burn, and the main lesson is that prevention is about noticing the small things before the flies do.
This article was written by Guillermo Arcega Castillo of the University of Minnesota for the National Hog Farmer.
