The flesh-eating screwworm is back in the United States after being wiped out

New World screwworm, a fly whose larvae eat living tissue, is turning up in American livestock and pets again. USDA confirmed its first detections in Texas in early June 2026, and by late June a count of 20 cases stretched across 13 Texas counties and one dog in New Mexico. Researchers at the University of California San Diego wrote in mid-September that the parasite, once eliminated from the country, is back in a place where it used to live.

The word “back” carries a narrower meaning than it sounds. It describes confirmed infestations in animals and the emergency response around them, and it does not by itself show that the fly has re-established permanent breeding populations across the country.

The 1966 eradication and the sterile-fly method

The claim that the screwworm was wiped out comes from the government’s own record. USDA’s Animal and Plant Health Inspection Service says on its screwworm page that the sterile insect technique eradicated the pest from the United States in 1966. The same page records that a small outbreak in the Florida Keys was eliminated in 2017, so the accurate summary is eradication from the mainland by the mid-1960s, with one brief and contained incursion since.

The campaign worked by releasing huge numbers of sterilized male flies. A female screwworm mates once, so a female that mates with a sterile male lays eggs that never hatch, and repeated releases collapse a local population. In the 1950s, UC San Diego’s team notes in its September summary, infestations inflicted enormous losses on U.S. livestock producers before the eradication campaign eliminated the insect from the country.

That same method is now the front line again. APHIS says the only sterile-fly production plant operating in North America is the joint USDA and Panama facility managed through the Commission for the Eradication and Prevention of Screwworm, and the agency notes that the pest has been moving north through Central America and Mexico over the past several years.

Detections in Texas and New Mexico since June

In a June 8 announcement, USDA confirmed two more cases, a calf in La Salle County and a dog in Andrews County, Texas, with early reports that the dog had recently traveled to Mexico. Dudley Hoskins, the under secretary for marketing and regulatory programs, said the department had identified and confronted four confirmed detections over the previous week. No human cases were reported, and the announcement describes infection in people as rare.

The count rose quickly. A June 9 update from the agency put the national total at six after another calf was confirmed in La Salle County, and the response it listed included case tracing, expanded trapping and surveillance, and the release of sterile insects. By June 25, Fox 7 Austin reported 20 confirmed cases, 19 in Texas and one in New Mexico, in cattle, goats and dogs, with quarantines covering Bandera, Edwards, Gillespie, Kerr, Kimble, La Salle, Medina, Pecos, Sutton, Uvalde, Val Verde, Webb and Zavala counties.

Texas Animal Health Commission Executive Director Lewis Dinges and Agriculture Secretary Brooke Rollins were among the officials quoted as the state activated its emergency operations center and barred livestock movement out of the quarantine zones without permits and inspections.

The UC San Diego assessment of the return

The mid-September summary comes from UC San Diego working with the County of San Diego Health and Human Services Agency. Eliah Aronoff-Spencer of the UC San Diego School of Medicine and Natasha Martin, a mathematical epidemiologist at the university, are among the researchers named, alongside San Diego County public health officials Seema Shah and Mark Beatty. They report that detections increased quickly once the parasite entered Texas, and that it has reached both Texas and New Mexico, describing it as back in a place where it used to live.

The group’s account stresses that the danger is not limited to cattle. Without treatment, severe infestations can badly injure or kill livestock and wildlife, and humans can also be infected, though such cases are uncommon. Infection happens when larvae enter wounds or mucous membranes, where they must be removed quickly, which is why early detection and treatment matter for pets, livestock and wildlife alike. USDA’s own June statement says quick detection, treatment and reporting are vital to safeguarding all three.

No current national case count appears in the research summary, and no September tally from USDA could be read for this report. The most recent figure available in the public record is the 20 cases reported on June 25. Officials said at the time that sterile flies need multiple 21-day cycles to collapse a local population, and that more cases were expected in the days that followed. Whether the Texas and New Mexico detections end in a second eradication or in a lasting presence is not something the sources decide.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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