The US Department of Agriculture confirmed an additional three cases of New World screwworm, bringing the total number of detections in the US to five and raising concerns over a broader spread of the parasite.
The fly was detected in a calf in La Salle County, Texas, a goat in Gillespie County, Texas, and a dog in New Mexico, the agency said on June 8.
That comes after two cases were confirmed last week in calves in Zavala County in South Texas, marking the first US detection of the fly in over a decade, and the first in cattle in about five decades.
The detection in La Salle County is close to Zavala, where the past two cattle cases were confirmed.
But the confirmation of screwworm in the dog places the pest far outside that region and across state lines.
The animal had initially been linked to Andrews County in Texas because it was reported by a veterinarian in the state, but the dog’s home is in Lea County, New Mexico, the USDA clarified later on June 8.
While that is “believed to be an isolated case”, the USDA and state partners are inspecting additional animals in the dog’s home and increasing outreach in the area “because the dog’s recent travel and exposure history remain unknown”, the agency said.
The USDA said earlier that the dog was believed to be in Mexico recently. The agency is also gathering more details of the case involving the goat.
Agriculture Secretary Brooke Rollins, speaking on CNBC on June 8, said the US is “going to do everything we can, investing over US$1 billion (S$1.29 billion)” to push the New World screwworm out of the country.
The spread risks more cautionary measures to limit cattle movement. Georgia’s Agriculture Department said on June 5 that it would restrict the movement of both livestock and pets into the state from a dozen Texas counties, while Canada is blocking the transit of livestock that has been in Texas within a three-week window.
That adds more worries for a US beef industry already struggling with supply tightness as the domestic herd has dwindled to a 75-year low.
The detection of screwworm is probably going to further delay any efforts to expand the US herd, said Glynn Tonsor, a professor of agricultural economics at Kansas State University. Tonsor said he was “pretty vocal” late in 2025 that more ranchers would start the process of expansion this summer, but increased dryness – and now screwworm – are creating more uncertainty, he added.
Tighter cattle supplies, even if limited to Texas alone, would also continue to pressure beef processing plants that have already been operating at losses due to the cattle shortage. Companies including Tyson Foods and JBS NV have resorted to closing plants amid high cattle prices.
The screwworm is a fly whose larvae burrow into the flesh of animals, leading in some cases to death. Infestations are treatable, including through a spate of medications the US Food and Drug Administration has already approved under emergency use, and the USDA has said the food supply is safe.
The 1976 outbreak in Texas affected over 1.5 million head of cattle and was ultimately eradicated through the dispersal of sterile flies that limited the pest’s ability to reproduce. The USDA plans to release eight million sterile flies each week to the current control zone, which spans 20km around the initial detection in La Pryor, in Zavala County.
The dispersal of such flies will need to reach 400 million a week, Rollins said on CNBC. A facility in Panama produces and disperses 100 million flies a week, “but we need a lot more than 100 million flies a week to eradicate”, she said.
The US will be at the point “where we’re going to start being able to push back, hopefully within a couple of months,” Rollins said.
A production facility under construction in Texas – the first of its kind in the US – is expected to produce 100 million flies a week once it begins operating in November 2027, with further growth to full capacity of 300 million flies a week.
The agency has developed a way to produce only male sterile flies, which will “instantly double our production” as sterile females “do us no good in this particular fight”, Scott Hutchins, the undersecretary for research, education and economics, said in a press conference on June 8.
The agency is working to determine how to specifically attract female screwworm flies, instead of a more indiscriminate approach, Hutchins said.
The USDA is also starting to engage in a trial to look at ivermectin in feed to control the spread of screwworm in wildlife, Michael Schmoyer, the head of the USDA’s screwworm directorate, said in the press conference. BLOOMBERG
