34 NEBRASKA CATTLEMAN February 2025 Protecting the U.S. Cattle Herd from NWS KATHY SIMMONS, DVM | CHIEF VETERINARIAN, NATIONAL CATTLEMEN’S BEEF ASSOCIATION It has been nearly 60 years since the U.S. cattle industry faced the threat of New World screwworm (NWS), but this dangerous pest is now approaching our doorstep once again. NWS gets its name from the way larvae (maggots) burrow into the flesh of living animals, similar to how a screw drives into wood. NWS is about the size of a common housefly and has orange eyes, a metallic blue or green body and three dark stripes along its back. NWS can cause immense harm to livestock. The female flies are attracted to open wounds or body orifices and lay their eggs in those cavities. When the eggs turn to larvae, those maggots feed on the living flesh, which expands the wound, causing possible infection and pain. Female flies can lay 200 to 300 eggs at a time, and if left untreated, a small infestation can turn into a massive animal health problem. In 1966, NWS was officially eradicated in the United States through the sterile insect technique. Large numbers of sterile male flies were released into the wild to breed with female flies, leading to no offspring. Over time, all NWS flies in North America died out. Through the use of sterile insect technique, NWS was pushed south all the way to the border of Panama and Columbia. This region – called the Darién Gap – served as a barrier zone for keeping NWS far away from our border. Since 2022, NWS has pushed north past the Darién Gap. In 2023, NWS detections in Panama increased from 25 cases annually to more than 6,500 cases a year. NWS was also found in Costa Rica, Nicaragua, Honduras, El Salvador and Guatemala. In November 2024, the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) discovered NWS in a cow in the southern Mexican state of PRODUCTION
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