The resurgence of the New World screwworm (NWS) in Mexico and Central America has raised alarms among U.S. animal health officials. While Wyoming benefits from its distance from the southern border, the state's vast grazing lands and limited surveillance infrastructure make the state particularly vulnerable to such outbreaks. 

In response to the escalating threat, U.S. Secretary of Agriculture Brooke Rollins announced on May 11, 2025, a temporary suspension of live animal imports—including cattle, horses, and bison—through ports of entry along the southern border. “The potential for rapid spread of this dangerous parasite into our livestock systems cannot be taken lightly,” Rollins said in the announcement. The decision, which is being evaluated on a monthly basis, is part of a broader federal effort to prevent the introduction of NWS into the United States. 

The USDA emphasized the move is a precautionary measure to protect both animal health and the rural economy, citing the parasite’s destructive nature and the speed with which it can move through herds. The NWS is  a parasitic fly whose larvae feed on the living tissue of warm-blooded animals.  

Infestations can lead to severe tissue damage, secondary infections, and, if untreated, death. The economic implications are substantial, with historical estimates indicating losses exceeding $100 million annually during previous U.S. outbreaks. 

Dr. Hallie Hasel, Wyoming's State Veterinarian, expressed concern over the state's preparedness to handle a potential NWS incursion. “Wyoming is currently unprepared for a NWS detection,” Hasel stated. “The Wyoming Livestock Board has limited staffing, and the state does not have a designated agriculture emergency coordinator.” 

The challenge is compounded by the nature of Wyoming's livestock operations. “With Wyoming livestock grazing on large pastures and allotments, undetected NWS could spread rapidly during the summer months,” Hasel noted. “The state's expansive and remote grazing areas make early detection and rapid response particularly challenging.” 

Originally eradicated from the U.S. in 1966 through an aggressive sterile insect technique program, the parasite has remained largely at bay due to a long-standing biological barrier maintained in Panama. That barrier, established through international cooperation, has kept the pest from reentering North America.  

However, in late 2024, Mexico confirmed the presence of NWS in Chiapas, and by early 2025, the pest had been detected as far north as Veracruz—putting it roughly 700 miles from the U.S. border. 

In Wyoming, Hasel said the impact of the import suspension is likely to be minimal for most livestock operators. However, some sectors—such as rodeo livestock contractors—could experience disruptions if the closure remains in place long-term. “Rodeo contractors may have difficulty filling contractual obligations should the closure continue for an extended period,” she said. 

Still, Hasel emphasized that the bigger issue lies in the state’s ability to respond quickly should the parasite reach U.S. soil. To that end, the Wyoming Livestock Board is urging everyone who works with animals to closely inspect wounds that are slow to heal or appear unusual. “We’re asking everyone—whether it’s ranchers, shelter staff, veterinarians, or animal control officers—to take a second look at anything suspicious,” Hasel said. 

The USDA has issued similar guidance, encouraging producers to look for maggots in wounds, foul-smelling discharges, behavioral changes, and signs of discomfort in their animals. The parasite’s larvae feed exclusively on living tissue, making them particularly dangerous in comparison to common flystrike or surface infections. Without swift treatment, an infected animal can succumb within days. 

The implications for Wyoming’s livestock industry extend beyond animal health. A large-scale outbreak could disrupt markets, prompt regulatory restrictions on movement, and lead to increased costs for surveillance and treatment. Hasel said it would also strain already limited veterinary resources in rural parts of the state. 

Hasel stressed that this situation is rapidly changing on a near daily basis, and ranchers should stay informed. Education, vigilance, and communication will be critical to preventing an outbreak or responding effectively if one occurs. Hasel said the Wyoming Livestock Board will continue working closely with USDA officials, veterinarians, and industry groups to raise awareness and share the most up-to-date guidance. 

How the U.S. Eradicated Screwworms in the 1960s 

The New World screwworm, once a devastating pest across the southern United States, was officially eradicated from the country in 1966—thanks to one of the most successful insect control programs in history. 

Screwworms are the larvae of the Cochliomyia hominivorax fly. Unlike other maggots that feed on decaying flesh, screwworm larvae consume healthy, living tissue, causing deep, festering wounds in livestock and wildlife. Infestations often proved fatal if untreated, and the economic impact on ranchers was severe. 

The breakthrough in eradication came with the development of the sterile insect technique (SIT). Pioneered by USDA researchers, the method involved mass-rearing millions of male screwworm flies, sterilizing them with radiation, and releasing them into the wild. Since female flies only mate once in their lifetime, mating with a sterile male meant no offspring—and over time, the population crashed. 

The SIT campaign began in Florida in the 1950s and gradually expanded westward. By 1966, screwworms had been eliminated from the U.S., and the program’s success led to continued efforts to push the pest further south. Today, the United States and Panama jointly maintain a biological barrier in the Darién Gap to prevent reinfestation.