The Screwworm America Eradicated With Sterile Flies Is Back, and This Time Washington Also Reached for an Emergency-Authorized Chemical

To see why an FDA notice about a livestock wound spray matters, go back sixty years, to one of the most complete victories American science ever won over a parasite. In the 1950s two USDA entomologists, Edward Knipling and Raymond Bushland, worked out a kind of biological judo. The female New World screwworm fly mates once in her short life, so if you flood the countryside with millions of sterilized males, she spends that one chance on a partner who leaves her eggs barren. Do it long enough, over a wide enough area, and the population collapses. By 1966 the screwworm was gone from the United States, and the sterile-fly line was pushed steadily south until it sat at the Darien Gap between Panama and Colombia, a living wall the region has held since 2006. It worked because it was patient, deterministic, and left no chemical residue in a single steak.

That wall has been breached, and Washington has answered through two very different doors. One is the old method, scaled back up. The other is an emergency power most Americans first learned to recognize during the pandemic.

The screwworm came back up the isthmus. Cases tracked north through Central America and Mexico after 2023, and in September 2025 Mexico confirmed a detection in Nuevo Leon, less than seventy miles from the Texas border. On June 3, 2026, APHIS found the first animal case on US soil: a three-week-old calf in Zavala County, Texas, larvae working in the raw skin around its navel. The proven method is back in the field. USDA is scaling sterile-fly production, spending $21 million to convert a fruit-fly plant in Metapa, Mexico that is ramping toward 100 million sterile flies a week, with a longer-term target of 300 million. That is the same order of magnitude that cleared the country the first time.

The second door opened on August 7, 2026, when the FDA issued an Emergency Use Authorization for CLiK Extra, a topical suspension made by Elanco, to prevent screwworm infestations in sheep, cattle, goats, swine, camelids and a long list of captive and wild species down to the Sonoran pronghorn. That authority lives in section 564 of the Food, Drug, and Cosmetic Act, the same statute heavily used to put COVID vaccines and tests into arms and nostrils before their full approvals existed. It was written for human medical countermeasures in a security emergency. In August 2025 the HHS Secretary declared that the screwworm justified pointing that machinery at animal drugs, and CLiK Extra now joins a growing series cleared through it, after four animal drugs authorized in April and earlier authorizations to Boehringer Ingelheim and to Elanco itself.

CLiK Extra did not run the full gauntlet of a new-animal-drug approval, with its residue studies, published tolerances and withdrawal times worked out before a bottle ever reached a ranch. Its active ingredient, dicyclanil, is a pyrimidine-derived insect growth regulator that has been sprayed on Australian sheep against blowfly strike for about two decades, sold as CLiK and now CLiK Extra. It works by interfering with the larva’s molting and development, so maggots never mature. On a wound it is prevention, not rescue. Elanco’s technical consultant, Dr. Wayne Ayers, framed the product modestly, saying that “education is a producer’s most powerful tool in this fight.” Fair enough. Elanco is not a neutral party here: it is the sponsor of the emergency authorization, the manufacturer of CLiK Extra, and a company that markets a lineup of other screwworm and livestock parasiticides.

Dicyclanil also carries a toxicology file. European regulators set maximum residue limits and an acceptable daily intake of 0.007 milligrams per kilogram of body weight precisely because residues of the compound settle into the fat, liver and kidney of treated animals. That number is not decoration. In an eighteen-month feeding study, female mice given dicyclanil developed benign liver tumors and hepatocellular carcinomas at the highest doses. None of that makes a wound spray on a Texas heifer a public-health emergency. The question is narrower and harder to wave off: has the FDA made the residue and withdrawal math public before emergency use spreads across a dozen species, and does that math even exist for most of them?

That last point is where the emergency shows. The EUA is not a blank check on residues. It keeps treated dairy animals out of the milk supply and sets an 86-day wait before any treated animal can be slaughtered for food, a long buffer by the standard of the published depletion data. But that published data is almost entirely about sheep. Dicyclanil’s residue studies, the European limits, the entire depletion record were built on wool sheep, the species the chemical was made for. The emergency authorization now green-lights it on cattle, swine, goats, camelids, bison and wild deer, animals whose fat and liver were never part of that record. An 86-day interval set by emergency order is not the same thing as a published tolerance a rancher, a packer or a reporter can check.

That is the trade the country is making, and it should be said plainly. The sterile-fly program is slow, sovereign and leaves nothing behind in the food supply, and it is the thing that actually eradicated this pest once. The chemical door is fast, proprietary, and shifts a residue question onto a compressed timeline and onto species it was never tested in. Both are open at once.

Two numbers will tell you which door is doing the real work. The first is sterile-fly output from Metapa and the border line: if USDA reaches and holds production in the hundreds of millions a week, the 1966 method still has the horsepower to win. The second is what the FDA and USDA publish about dicyclanil residues in cattle and swine, because an emergency can justify skipping a step, but it cannot make the underlying toxicology question disappear. Eradication depends on sterile-fly capacity. Public trust depends on residue rules anyone can read. The next screwworm case count out of Texas will tell us how much time there is to get both right.

Sources

1. FDA – Emergency Use Authorization for CLiK Extra (dicyclanil) to prevent New World screwworm (Aug. 7, 2026)
2. Federal Register – HHS declaration of emergency under FD&C Act §564 for New World screwworm (Aug. 20, 2025)
3. Federal Register – Authorization of emergency use for four animal drugs for New World screwworm (Apr. 17, 2026)
4. FDA – Emergency Use Authorization legal and regulatory framework (§564, FD&C Act)
5. USDA – Mexico confirms New World screwworm in Nuevo Leon, near the Texas border (Sept. 21, 2025)
6. USDA APHIS – Confirmation of New World screwworm in the United States
7. USDA APHIS – New World Screwworm Response (sterile-fly production scale-up)
8. ASM – New World Screwworm: Rise, Fall and Resurgence (sterile insect technique, Darien barrier)
9. EMA – Dicyclanil MRL summary report (acceptable daily intake, food-animal residues, mouse liver tumors)
10. Elanco – CLiK Extra receives FDA Emergency Use Authorization (company statement)