Screwworm Is Back in U.S. Cattle. The CRISPR Response Is Still on the Drawing Board.

A Zavala County, Texas rancher pulled back the hair on a calf on June 3, 2026, and found the first USDA-confirmed New World screwworm in a U.S. animal in more than sixty years. The larvae were burrowing into living tissue, which is what screwworms do. They are parasitic fly maggots that feed only on the living flesh of warm-blooded animals, a category of infestation called obligate myiasis that kills untreated livestock and pets within days.

Since that first case in a small cattle-herding county southwest of San Antonio, at least 45 screwworm cases have been confirmed across Texas and New Mexico, and the outbreak has now jumped species and state lines into dogs in neighboring New Mexico, according to USDA's confirmed detections page and the GenomicEpi outbreak tracker. The U.S. eradicated the species once before, in the 1960s, by saturating the fly population with lab-sterilized males released from a dedicated fly factory in Mexico. The current outbreak is the first to test whether that legacy program can still hold the line, and whether a new generation of tools can do better.

The tool getting the most attention is a CRISPR-based sterile insect technique, or SIT. In the original 1960s program, lab technicians sterilized male screwworm flies with low-dose radiation before air-dropping them by the millions. Wild females that mated with the sterile males laid eggs that never hatched, and the population collapsed. The newer version uses genetic engineering rather than radiation to produce sterile males, and adds a sex-sorting step so only males are released. The mechanism is the same: swamp wild mating with sterile males until the local population dies out. CRISPR-built sterility is supposed to be cleaner, more reliable, and cheaper to scale than the radiation approach (Core Memory).

Cleaner is not the same as deployed. As of early August 2026, the U.S. does not have a mass-release CRISPR-SIT program running in the field. What it has is peer-reviewed cage-trial work showing that engineered screwworm sterility works in laboratory populations, and a research pipeline that is years from a factory that can produce the volume of flies an active outbreak requires. The legacy fly factory at Pacora, Panama, the southern anchor of the original eradication barrier, is still operating. It is the only line of defense with proven production scale, and it is still using radiation, not CRISPR.

That is the gap the outbreak is exposing. The U.S. eradicated New World screwworm in 1966 by running a sterile-fly barrier across the Mexican–Guatemalan border for decades, at the equivalent of billions of dollars in today's spending. When the barrier slipped, most recently because of cattle movements through Central America and the Caribbean, the fly walked back north into Mexico in 2023, into Nicaragua and Costa Rica in 2024, and across the Rio Grande into Texas in June 2026. Forty-five confirmed cases in a cattle state is not a 1935-scale disaster. A single localized Texas outbreak in 1935 killed an estimated 180,000 cattle before the program existed to contain it. But it is the first reappearance in the U.S. in over sixty years, and the response is being run, in effect, with one tool from the 1960s and a research-stage replacement that is not yet at production scale.

The question Core Memory's panel of entomologists and synthetic biologists is whether the modern biotech kit, including CRISPR-SIT, gene-drive-adjacent concepts, sex-sorting, and Wolbachia-based sterility borrowed from mosquito programs, is enough to contain the re-emergence, or whether legacy sterile-fly release plus animal-health measures will still have to do the work. The honest answer in August 2026 is that both are running, and neither has closed the case.

Two things to watch. First, whether USDA APHIS confirms a sustained U.S. breeding population; the agency's current status page tracks this in near real time. Second, whether the CRISPR-SIT program moves from paper to production in time to matter for this outbreak, or only for the next one. The screwworm life cycle runs in weeks. A new sterile-fly production line takes years to bring to scale.