Preparing Livestock Producers for New World Screwworm Reemergence in the United States

Abstract

New World Screwworm (Cochliomyia hominivorax) is a fly larva, or maggot, that can burrow into the flesh of a living animal, unlike other fly larvae that only feed on dead flesh. New World Screwworm (NWS) can affect any livestock species, as well as pets, wildlife, birds, humans, and essentially any warm-blooded animal. In the 1950’s and early 1960’s, NWS infestations resulted in devastating losses to cattle and wildlife in the southern United States. Economic impacts reached $50-100 million (USDA, 2025). In the 1960’s NWS was eradicated in the United States (US). Since then, the US and Panama have collaborated through the Commission for the Eradication and Prevention of New World Screwworm Infestation in Livestock (COPEG) to prevent NWS from spreading north of the Darién Gap by releasing sterile male flies. Since NWS female flies tend to breed once in a lifetime, this process decreases the NWS population. In 2023, NWS was able to break this barrier and spread northward. As of late September 2025, NWS was detected as far north as Nuevo León, Mexico (Figure 1). COPEG continues to release sterile flies in Central America and Mexico, however sterile fly production is at maximum capacity at the current facilities. US livestock producers should be prepared to implement proactive management and husbandry practices in the near term as well as preventative measures to limit the potential impact an infestation will have on the livestock industry if and when a regional detection happens.