On September 21, 2025, USDA reported 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.
Surveillance Information
USDA provides guidance for detection and surveillance of NWS. Watch for:
If suspected clinical signs are noted, contact your herd veterinarian immediately as NWS is a reportable disease to the State Animal Health Official and the USDA federal veterinary authorities. The process for reporting is provided by USDA in their publication, Foreign Animal Disease Investigation Guide: New World Screwworm. Additional information is included in the USDA publication, Standard Operating Procedure for Possible Detections of New World Screwworm in Animals.
Historic Perspective and Control Strategies
While NWS is currently not in the US, other than a single human case in August 2025 due to travel to affected areas, NWS has been an issue for US livestock producers since at least 1842 according to a USDA publication, New World Screwworm Ready Reference Guide – Historical Economic Impact. The need to prevent NWS infestation in the US and its cost to the livestock sector and overall US economy is clear.
During a SHIC/AASV webinar on NWS conducted on June 13, 2025, Dr. Cody Egnor, veterinary medical officer at USDA, addressed expected control strategies. USDA has actively focused on the use of Sterile Insect Technique to reduce and eliminate the flies in affected locations. This method involves the release of sterile flies into the wild population to reduce the reproductive capacity of NWS flies and eventually eliminate them from the environment. Prevention activities include robust regulatory controls, active field surveillance for myiasis, sterile fly release, and stakeholder engagement. USDA’s preparedness and response activities are contained in the Disease Response Strategy – New World Screwworm Myiasis.
Currently, there are no labeled products approved for treatment of NWS myiasis in swine. However, USDA and FDA are working together to evaluate emergency and conditional use of specific products in livestock species. Additional information from the FDA regarding NWS and treatment guidance for veterinarians can be found in the document, Animal Drugs for New World Screwworm.
The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at [email protected] or Dr. Lisa Becton at [email protected].
