New World Screwworm Information

New World Screwworm (NWS) is a highly destructive pest that affects livestock, other warm-blooded animals such as wildlife and pets, and occasionally birds and humans. NWS infestations occur when the female fly lays eggs on an animal’s open wounds or other vulnerable areas. The larvae hatch and then feed on the living tissue, causing tissue destruction and disease.

Background

NWS was eradicated in the United States in the 1960s, in Mexico in the 1970s, and in Central America in the early 2000s. Since that time, NWS infestations have been confined largely to South America. In 2023, however, NWS detections exploded in Panama, where the average number of cases of NWS had been 25 cases per year but rose to more than 6,500 cases between 2023 and 2024. The infestation spread northward throughout central America and more recently into parts of Mexico. Since 2023, NWS has been detected in Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador and Mexico. Detected in southern Mexico in December of 2024, new infestations have since been detected further into Mexico, including in the Yucatan Peninsula and north toward central Mexico. The potential for the pest to reach the United States is quite real and alarming.

In response, the USDA has instituted new entry requirements for animals crossing the Mexican border. However, the most effective treatment to prevent the spread of NWS is the Sterile Fly Treatment program. Because the female screwworm fly mates only once in its lifetime, sterile flies are produced and then released into the environment where they mate with the female flies. No offspring are produced, and the females die without reproducing, effectively reducing/eliminating the threat from these flies. The U.S. has maintained a sterile fly treatment facility in Panama for many years, but due to a number of factors the production of sterile flies broke down at that facility, resulting in new spread of the disease. USDA is working to create new sterile fly production facilities in the U.S. to address this emerging threat. USDA has stated that this might take up to five years to reach full capacity.

ASI encourages sheep producers – particularly those in the Southern U.S. and along the Southern border – to familiarize yourselves with this pest in order to quickly identify it should the pest enter the country.