Not to Alarm Anyone, but Flesh-Eating Screwworms Have Entered the US

A case of New World screwworm has been confirmed in South Texas, the US Department of Agriculture announced Wednesday night. It marks the first detected breach of the US-Mexico border by the ravenous flesh-eating flies, which have been making their way up through Central America for the past several years.

In a social media post on Wednesday afternoon, the USDA revealed that a sample from Texas had been sent to the National Veterinary Services Laboratories (NVSL) in Ames, Iowa, for confirmatory testing of a screwworm infection. Agriculture Secretary Brooke Rollins later posted that the testing had confirmed the infection, which was found in a three-week-old calf in Zavala County, Texas.

Chatter of a screwworm detection had already been building this week, rattling the US cattle industry.

Although many animals, including humans, can be victims of the parasite, the screwworm is especially dangerous to livestock. Female screwworms lay hundreds of eggs in the wounds and openings of warm-blooded creatures, allowing their larvae to feast on the living animals, causing deep, festering, life-threatening wounds. Although the screwworm was once endemic to the US, it was eradicated amid a yearslong control effort in the 1960s. The USDA estimates that keeping screwworms out of the US has saved the livestock industry $900 million each year.

But the fly has broken through control efforts in Central America and has been inching closer. On May 28, a case was found 25 miles from the border in a five-year-old goat in Coahuila, Mexico, according to the USDA. The case was one of many detected in recent days, including a case in a calf just 39 miles from the border, also in Coahuila.

In a media call on Tuesday, Agriculture secretary Brooke Rollins said, “There is no doubt that this is a very, very serious threat to our livestock.” But she also disputed claims that the fly is closer or even already in the US.

On Monday, state representative Don McLaughlin claimed on social media that a screwworm case was found just one mile from the Texas border, which Rollins and the USDA denied.

“When that false information gets out, it causes significant panic,” Rollins said Tuesday, according to the Texas Tribune. “And rightly so, especially if it’s coming from elected officials and the media.”

On Wednesday, Reuters reported that McLaughlin suspected the fly was now here. He said samples taken Tuesday from two calves on a ranch in La Pryor, Texas—which is in Zavala County, where the screwworm infection was confirmed—were being tested as possible screwworm infections. One infection was said to be in an umbilical cord wound of one of the calves. McLaughlin said he had seen images and videos of the animals and that the larvae seen in them looked like screwworm larvae.

Reuters was shown one of the photos, which it reported as showing “multiple larvae resembling the screwworm inside a bloody circular wound on an animal” but said it “could not immediately verify the photo.”

“At this point, it’s unconfirmed that it’s the New World screwworm,” McLaughlin told the outlet earlier Wednesday. “It ​looks like it, but it’s unconfirmed.”

With the finding now confirmed, the USDA said in a press release Wednesday night that it is setting up a “unified Incident Command Team” with the Texas Animal Health Commission and sending response personnel to the area. It is also setting up a 20-kilometer (12.4-mile) zone around the detected infection for quarantine, movement restrictions, and increased surveillance and fly trapping.

Screwworms were eradicated in the US in the 1960s amid a concerted effort to annihilate their population. This is done with aerial bombings of sterile male flies, which is the most effective weapon against the parasites. The mass release of dud studs elbows out fertile males, preventing them from mating with females, which generally mate only once.

With this method, called Sterile Insect Technique, the flies were eradicated not just from the US but from all of Central America. They were declared eradicated from Panama in 2006.

Until recently, the screwworm population was kept at bay via a biological barrier along the Darién Gap at the border of Panama and Colombia. The USDA partnered with authorities in Panama to build a sterile fly production facility at the gap to regularly release sterile flies and hold the line. But in 2022, the barrier was breached, and the flies have been relentlessly buzzing northward since.

In response, the US has expanded surveillance and trapping efforts in Texas. It is also constructing a $750 million sterile fly production facility in South Texas. The USDA says it is dispersing 100 million sterile insects per week in Mexico and along the US-Mexico border to prevent the flies from advancing further.

In the press release Wednesday night, the USDA said it will be releasing sterile flies via ground-release chambers in the area around the detection. That’s in addition to the 4 million flies already being released in the area by air this week.

“The United States has defeated this pest before, and we will do it again,” Dudley Hoskins, USDA undersecretary for marketing and regulatory programs, wrote in the press release.

This story originally appeared on Ars Technica.