New World screwworm officially crosses border into . . . Maryland?
Ranchers in Texas, alongside the U.S. Department of Agriculture, have been bracing for the return of the New World screwworm (NWS), a parasitic fly that lays its eggs in the wounds of warm-blooded animals and, once newly hatched, eats living flesh.
As reports of NWS detections have spread up through Central America, reaching as far north as several hundred miles of the U.S.-Mexico border, the U.S. has ramped up investment in various efforts to stem the tide of NWS and protect the border at all costs.
Despite those efforts, the dreaded headline has broken: the NWS has been confirmed in the U.S., but it comes with a large asterisk. The parasite has yet to infest herds of cattle or swarm across the southern border.
Instead, the first new case of NWS myiasis (fly larvae infesting a wound) comes from a single human patient returning from travel in El Salvador to Maryland. This isolated case represents a very low risk to public health in the U.S., according to Andrew Nixon, a Department of Health and Human Services (HHS) spokesperson.
Background: NWSs were once commonplace in the southern U.S., regularly infesting livestock, wildlife, pets, and (less frequently) humans. These infestations, aside from being visually horrific, maimed, killed, and required constant vigilance to protect against.
In 1957, nuclear science came to the rescue when Florida launched a sterile insect technique (SIT) program against the parasite. SIT involves irradiating males so they are unable to reproduce and then releasing them into the wild population. There, they compete with nonirradiated males, leading to a rapid decrease in overall population. In the NWS, SIT is particularly effective, because the females of the species mate only once in their life.
By 1962, the NWS was fully eradicated from the U.S. (aside from a small 2017 outbreak in the Florida Keys), but the eradication project continued. A sterile fly production plant capable of producing 500 million irradiated males a week was set up in Mexico, which was declared free of the parasite in 1991.
NWSs continued to be pushed south through Central America and by the turn of the century were wiped out all the way to the Darién Gap, the narrowest geographical region in southern Panama. There, for more than twenty years, NWS was successfully kept at bay through the continual release of new flies.
That was until 2023, when the barrier was breached. Reports of screwworms began appearing throughout Panama, then Costa Rica, then Nicaragua, and continued north through 2024 and 2025 into Mexico until reaching within 700 miles of the U.S border.
Recent progress: U.S. Secretary of Agriculture Brooke Rollins has been vocal on and active in combatting the northward progress of the NWS. In May, Rollins suspended imports on live cattle, horses, and bison through U.S. ports of entry along the southern border.
In June, she announced the launching of a new $8.5 million SIT facility at the Moore Air Base in South Texas, which is slated for completion by the end of this year. Along with that announcement, she also released a five-step response plan to combat NWS which includes a framework for close collaboration with universities, U.S. states, and Mexico.
Earlier this month, HHS issued a declaration allowing the Food and Drug Administration to issue Emergency Use Authorizations for animal drugs to treat or prevent infestations of NWS. On this move, HHS Secretary Robert F. Kennedy Jr. said, “This authorization equips FDA to act quickly, limit the spread of New World Screwworm, and protect America’s livestock.”
Maryland fly: The recent case of NWS myiasis was an isolated one. According to David McAllister, a Maryland Department of Health spokesperson, “The investigation confirmed there is no indication of transmission to any other individuals or animals.” He went on to add that this case was a “timely reminder for health care providers, livestock owners, and others to maintain vigilance through routine monitoring.”
Still, 10 years ago, someone traveling to El Salvador would have had a near-zero chance of returning as an unwitting host to NWS. The incident also serves as a call to action to “bring the fight to the screwworms,” as Rollins detailed in her response plan.
