On September 21, 2025, USDA confirmed the presence of New World Screwworm in Sabinas Hidalgo, located in the state of Nuevo León, Mexico, less than 70 miles from the US-Mexico border. USDA said this is the northernmost detection of NWS during the current outbreak and, consequently, the most threatening to the US livestock industry. The previous northernmost case was reported on July 9, 2025, in Veracruz, approximately 370 miles further south of Sabinas Hidalgo. On September 21, 2025, NWS was detected in an eight-month-old heifer that had been transported to Sabinas Hidalgo from a region in southern Mexico with known active NWS cases. Identifying and responding to NWS becomes essential as the pest nears the US border.
NWS larvae, commonly known as maggots, can infest livestock and other warm-blooded animals, including humans. They most often enter through an open wound, some as small as the size of a tick bite, and feed on the animal’s living flesh. They can also gain access to the host through body openings including the nose and mouth. The name screwworm refers to the maggots’ feeding behavior as they burrow into the wound, feeding as they go, like a screw going into wood. NWS maggots cause damage by tearing at the host animals’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly, damage to the animal, as well as a great deal of pain, becoming an animal welfare issue. Adult screwworm flies are about the size of a common housefly or slightly larger. They have orange eyes, a metallic blue or green body, and three dark stripes along their backs.
- USDA provides guidance for detection and surveillance of NWS. Watch for:
- Maggots in wounds or other body openings, such as the nose, ears, genitalia, or umbilicus of newborn animals
- Wounds that have bloody discharge and foul odor
- Wounds that become deeper and larger as the maggots grow and feed on living tissue
- Animals showing signs of pain including depression, irritability, not eating, and isolating themselves from other animals or people