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.
Across Central America and Mexico, there have been 1,190 human cases of NWS reported and seven deaths. More than 148,000 animals have been affected.
Close calls
In September, the USDA warned that an 8-month-old cow with an active NWS infection was found in a feedlot in the Mexican state of Nuevo León, just 70 miles from the border. The finding prompted Texas Agriculture Commissioner Sid Miller to step up warnings about the threat.
“The screwworm is dangerously close,” Miller said at the time. “It nearly wiped out our cattle industry before; we need to act forcefully now.”
According to the USDA’s latest data, Nuevo León has seen three cases in the outbreak, with none that are currently active. But, its neighboring state, Tamaulipas, is having a flare-up, with eight animal cases considered active. The Mexican state shares a border with the southern-most portion of Texas. Mexico overall has reported 24 hospitalizations among people and 601 animal cases.
For now, the NWS has not been detected in the US, and the CDC considers the risk to people to be low.
“However, given the potential for geographic spread, CDC is issuing this Health Advisory to increase awareness of the outbreak and to summarize CDC recommendations for clinicians and health departments in the United States on case identification and reporting, specimen collection, diagnosis, and treatment of NWS, as well as guidance for the public,” the agency said.
Generally, the agency advises being on the lookout for egg masses or fly larvae in wounds or infection sites, especially if there’s destruction of living tissue or feelings of movement. Once discovered, health care workers should report the case and promptly remove and kill all larvae and eggs, preferably by drowning in a sealed, leak-proof container of 70 percent ethanol. “Failure to kill and properly dispose of all larvae or eggs could result in the new introduction and spread of NWS in the local environment,” the CDC warns in bold. At least 10 dead larvae should then be sent to the CDC for confirmation.
The USDA is currently releasing 100 million sterile male flies per week in Mexico to try to establish a new biological barrier.
This isn’t the fly’s first attempt at a US comeback since the 1960s. In 2016, the flies were somehow reintroduced to the Florida Keys, where they viciously attacked Key Deer, an endangered species and the smallest of North America’s white-tailed deer. The flies were eliminated again in 2017 using the sterile fly method.
Originally published at Ars Technica
