New World screwworm, a parasitic fly whose larvae eat living flesh, has reached Texas and New Mexico, according to a University of California San Diego account of a new risk-mapping effort. The insect was driven out of the United States in the last century, and its return is the reason county health officials in California are already building tools to watch for it.
What the larvae do and what the old campaign cost
Most fly larvae feed on dead tissue. Screwworm larvae do not. According to the release, females lay eggs in open wounds or natural body openings, and the maggots that hatch eat the living flesh around them. Untreated infestations can badly injure or kill livestock and wildlife, and the release adds that the fly can affect any warm-blooded animal. Adult flies travel miles on their own, and infested animals moved between places carry the parasite with them, so two separate routes of spread feed one another.
The economic history explains why officials treat a return seriously. In the 1950s the screwworm caused an estimated $50 million to $100 million in damage each year in the United States, the release says. A large eradication campaign then removed it from the country and pushed its range south as far as Panama. The technique behind that result was the sterile insect method: males sterilized by radiation were released in large numbers, and wild females that mated with them produced no viable offspring. These details come from a University of California San Diego release published through ScienceDaily in mid-September 2026, which describes the pest as a threat to livestock, wildlife and pets and, rarely, to people. The same release reports that the parasite is moving faster than local officials expected, which is why a research team and a county health agency joined forces to estimate where it might surface next.
Detections in Texas and the USDA response
San Diego County’s own New World screwworm page records the northward creep. It states that a calf less than 70 miles from the U.S.-Mexico border tested positive in September 2025, the northernmost case of the outbreak at that point, and that by June 2026 the parasite had been detected in a calf’s umbilical wound in Texas. The county also notes that the few human cases in the United States over the past decade were all linked to travel.
The U.S. Department of Agriculture now leads the response. Its Stop Screwworm hub describes intensive surveillance along the border, screening of cattle before they enter the country, and a New World Screwworm Directorate inside the Animal and Plant Health Inspection Service. The page, last modified October 6, 2026, carried Texas notices dated September 21, October 6 and October 10 about infested zones being released or modified. It did not describe a New Mexico detection, so that part of the story rests on the university release.
Sterile flies remain the main weapon. The hub says USDA is investing in new and existing production infrastructure with a target of about 500 million sterile flies per week, the amount used to eradicate the pest decades ago, and that a federal Grand Challenge announced January 21, 2026 offers up to $100 million for projects that improve sterile fly production and response. The university release describes the current campaign as one that combines sterile insect releases with pesticides and quarantines that limit animal movement.
A newer tool is also in the works. The release says authorities are “seeking U.S. Environmental Protection Agency approval for a genetically modified all-male strain” that is now being field-tested, and that the San Diego researchers are developing next-generation genetic technologies for future control efforts. The release does not say when a decision from the EPA is expected.
The San Diego risk dashboard and its limits
The dashboard at the center of the release was built by UC San Diego researchers together with the County of San Diego Health and Human Services Agency’s Public Health Services. It estimates screwworm risk county by county across the country by combining confirmed detections, environmental conditions that favor the fly, and news media reports. Eliah Aronoff-Spencer, a professor of medicine at the UC San Diego School of Medicine, said the team “built a dashboard to generate regional risk profiles,” and pointed to the county’s earlier Tijuana River Crisis Environmental Dashboard as a similar project.
Including news reports is a deliberate choice. Seema Shah, medical director of the Epidemiology and Immunization Services Branch at County of San Diego Public Health Services, said “Lab confirmation can take days or weeks,” so media coverage can flag a threat sooner than formal results. Mark Beatty, the branch’s assistant medical director, said the county is “in a very concerning phase right now,” and that officials had expected the fly to move north in 2023 but did not expect it to move this fast.
The tool is not public. Access is currently limited to public health officials and partners, and the researchers say they hope to release a public version eventually. It was developed by Resilient Shield, a CDC-funded UC San Diego center led by Aronoff-Spencer that the release describes as one of 13 core facilities in the CDC’s new Insight Net, a national network for modeling outbreaks. Emily Trumbull, a veterinarian who leads the county’s One Health Program, is working with the California Department of Food and Agriculture to connect local veterinarians with surveillance resources.
For people, the release says infections are uncommon and do not spread from one person to another. Practical guidance on the county page includes seeing a health provider right away on finding maggots in a wound, keeping pet and livestock wounds clean, and reporting suspected animal cases to the state agriculture department within 24 hours. The unanswered question in the release is timing: how quickly the EPA will act on the modified strain, and whether the dashboard will ever reach the public.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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