U.S.’s screwworm fix is still a year away, risking more spread

When the New World screwworm reached the US earlier this month after advancing across Mexico for more than a year, federal officials were prepared to quarantine animals and distribute treatments. But the country’s key tool for suppressing the pest — a facility that breeds sterile flies to halt reproduction of the parasite — isn’t slated to begin operating until November 2027.

The screwworm is actually a fly whose larvae infest the wounds of warm-blooded animals. So far, it has been detected in six cattle in Texas, the country’s top producer. 

That’s raising alarms at a difficult time for the cattle industry, as drought and high production costs have culled the nation’s herd to a 75-year low. The cases are the first in US livestock since an outbreak five decades ago, also in Texas. That was eradicated a decade later only with the help of sterile flies, as the US and Mexico scaled up production to as many as 500 million insects a week.

For now, the US, has only a fraction of the sterile flies needed to mount an effective response. 

A facility in Panama is currently the only operational sterile fly production site in North America, making and dispersing 100 million insects a week, according to the US Department of Agriculture. Another plant in Metapa, Mexico, could as much as double overall output when it comes online as early as this summer. 

But the biggest hopes are centered on a larger production facility under construction at Moore Air Base in Texas. That won’t reach its initial goal of 100 million flies a week until November 2027. Ramping up to full capacity of 300 million flies will take even longer.

Agriculture Secretary Brooke Rollins said on the sidelines of a Senate hearing Wednesday that the US is “not going to be able to eradicate it until we’ve got the couple hundred million more flies coming in, but we will be able to contain it.” She added that she doesn’t “have a good enough sense yet” of how far screwworm might spread in the meantime.

“I want to give it maybe a month and watch and see what happens,” Rollins said.

Read More: What Is Screwworm and Why Is It a Cause for Concern?: Explainer

The facilities sterilize screwworm pupae with radiation to produce sterile flies, and the males are then distributed to mate with wild female flies. The resulting eggs are unfertilized, and because the females typically mate only once, the cycle prevents new screwworm flies from being born. Without intervention, a female fly could lay more than 3,000 eggs over a lifespan of two to four weeks, according to Lee Haines, associate research professor of biological sciences at the University of Notre Dame.

The USDA has already opened a new facility in Texas solely for dispersing flies, and earlier this week said it had developed a way to double production with a new male-only strain of sterile flies. Those preemptive investments are “probably already mitigating some of that risk,” said Glynn Tonsor, a professor of agricultural economics at Kansas State University.

Yet the US will probably “be handicapped for a while in being able to disperse the number of sterile males that we need in order to truly combat this pest problem,” said Arlan Suderman, the chief commodites economist at StoneX Group. “We really need that plant in southern Texas. That takes time.”

Livestock producers, in the meantime, will face an indirect cost burden that comes with monitoring and treating animals, he said. That threatens to send cattle prices even higher and discourage the rebuilding of the US cattle herd. The prolonged supply crunch has already left beef processors operating at losses and sent consumer beef prices soaring to records.

Read More: Record Beef Prices Spark Blame Game in Complex Cattle Economy

Texas Agriculture Commissioner Sid Miller has criticized the USDA’s screwworm response, calling for the use of a targeted bait system that attracts and kills female flies before they can reproduce, in tandem with the release of sterile flies. 

Miller also likened the current output of 100 million flies a week to “squeezing the middle of the balloon,” saying that shifting the quantity of flies toward Texas from the Mexican border only leaves a different swath uncovered.

USDA Under Secretary Scott Hutchins said earlier this month that while it is “so important that we do have a lure-and-kill type of technology,” the agency is not using the bait system that Miller suggested because it uses a “very indiscriminate attractant that brings in literally every fly within an area.”

The last outbreak in Texas affected nearly 1.5 million cattle and cost the state’s economy $375 million, before sterile fly releases helped drive the screwworm out of the US and eventually down to Panama. The production facility there has been operating since then, largely holding the pest at bay until the latest outbreak in Mexico. 

US senators in a Thursday letter asked Rollins to accelerate the production of sterile flies, including by exploring the USDA’s reach under the Defense Production Act. They called for additional hiring and a commitment that the agency’s plans to relocate much of its workforce out of Washington won’t disrupt the screwworm response.

Read More: Rollins Defends Screwworm Response as USDA Staff Cuts Draw Fire

The US is also trying to manage the screwworm’s spread through quarantines and the distribution of medications. The US Food and Drug Administration had already issued conditional approval for several drugs, and Rollins has said some supplies from the USDA’s National Veterinary Stockpile have been flown to Texas. 

Justin Welsh, Merck Animal Health’s executive director of US livestock technical services, said the availability of the company’s product is “very complete” and that it has been replenishing inventories for its distributor partners “literally daily.”

Still, applying treatments across entire herds “gets really challenging from an expense approach,” said Derek Foster, an associate professor of ruminant medicine at North Carolina State University. The burden includes not only the cost of medications but also the labor needed to apply them, especially “over what could ultimately be a really prolonged period of time,” he said.

Meanwhile, the USDA is preparing its strategy. In an X post on Friday, the agency presented it as a “Main Event” battle pitting the sterile flies against the screwworm: “One enters to reproduce. One enters to end the bloodline … One mission. One goal. Knockout New World screwworm!”