The US Centers for Disease Control and Prevention issued a health alert to clinicians Tuesday, warning that the savage, flesh-eating parasitic fly—the New World Screwworm—is not only approaching the Texas border, but also felling an increasing number of animals in the bordering Mexican state of Tamaulipas.
The advisory, released through the agency’s Health Alert Network, directs doctors, veterinarians, and other health workers to be on the lookout for patients with wounds teeming with ferocious maggots burrowing into their living flesh. The alert also provides guidance on what to do if any such festering wounds are encountered—namely, remove each and every maggot to prevent the patient from dying, and, under no circumstance allow any of the parasites to survive and escape.
The New World Screwworm (NWS) is a fly that lays its eggs—up to 400 at a time—in the wounds, orifices, and mucus membranes of any warm-blooded animal. The eggs hatch into flesh-eating maggots, which look and act much like screws, twisting and boring into their victims while eating them alive.
While the flies are indiscriminate killers, they’re particularly devastating to livestock and are estimated to cause hundreds of millions of dollars in damage each year. As such, the US Department of Agriculture has been leading the response to the threat, which had been held far at bay until recently. NWS used to be endemic in the US, causing heavy losses to cattle in the country. But, in the 1950s, US scientists developed a strategy to eliminate the flies by raising and releasing sterile male flies. The method exploits the fact that female flies mate only once in their 21-day lifespan. Thus, unproductive mating with a flood of sterile males can cause the population to crash.
With this strategy, screwworms were eradicated from the US around 1966 and, through coordinated efforts, cleared out from Central America in the subsequent decades. They were declared eradicated from Panama in 2006 and actively held at bay with continual sterile-fly releases along the Darién Gap, creating a biological barrier. But the barrier was breached in 2022 amid development and movement through and around the gap. Since then, the flies have quickly been marching north with the help of unregulated cattle movements.