Zoetis (NYSE: ZTS) has added its flagship dog parasite chew to the list of drugs U.S. veterinarians can use against New World screwworm.

On August 13, 2026, the FDA issued an Emergency Use Authorization, or EUA, for Simparica Trio to treat screwworm infestations in dogs and puppies. Zoetis announced it in a press release, and the FDA put out its own notice the same day. The chew was already FDA-approved in 2020 for heartworm prevention and for flea, tick, and intestinal worm control.

Two details matter more than the headline. The authorization covers treatment, not prevention. And Simparica Trio is late to this market.

What Screwworm Is

New World screwworm is a parasitic fly. It lays eggs in open wounds and body openings of warm-blooded animals, and the larvae, or maggots, eat living tissue. The infestation is called myiasis. Left untreated, it can kill.

The pest was eradicated from North and Central America decades ago but has been spreading north again since 2022, according to the FDA. In its August announcement, the agency said screwworm had only been detected in specific areas of Texas and New Mexico, and it described most U.S. dogs as low risk.

Authorized Is Not Approved

An EUA is not an approval. In August 2025, the Secretary of Health and Human Services declared that screwworm posed a significant potential public health emergency. That declaration lets the FDA authorize animal drugs when it’s reasonable to believe they may work and the known and potential benefits outweigh the risks. The bar is lower than full approval, and the authorization lasts only as long as the emergency declaration does.

Zoetis’s release says as much: Simparica Trio is not approved for screwworm. The FDA adds that it isn’t authorized for prevention either. One quote in the release describes the chew as supporting both routine control and screwworm response needs. Readers shouldn’t take that as a prevention claim. The EUA doesn’t cover one.

The Evidence Behind It

Zoetis’s release includes no efficacy data. The FDA’s public summary of the authorization does.

The case rests on one laboratory study run in Rio de Janeiro, Brazil, in 2025. Twelve beagles each received a small surgical wound on the neck, which was then seeded with about 50 young screwworm larvae. Six dogs got Simparica Trio. Six got a plain food pellet.

After 24 hours, no live larvae were recovered from the treated dogs. The untreated dogs averaged 30.7. By the FDA’s measure, that works out to 100% effectiveness.

Expelled, Not Killed

There’s a nuance. The drug worked mostly by driving larvae out of the wound, not by killing them in place. Treated dogs expelled about 85% of their larvae on average, compared with about 9% in the control group. Average larvicidal effectiveness, the share of larvae found dead, was about 26%. One treated dog vomited. The FDA did not require new animal safety studies for this authorization.

Six treated dogs is a small sample. It meets the emergency standard. It is not the kind of evidence behind a full approval.

Late to a Crowded Shelf

Zoetis isn’t first here. Not close.

Elanco’s Credelio (lotilaner) received the first EUA ever issued for an animal drug in October 2025, to treat screwworm in dogs. Elanco (NYSE: ELAN) then won conditional approval for Credelio Quattro-CA1 against screwworm in dogs that December. Boehringer Ingelheim, a privately held German company with no publicly traded stock, received EUAs for NexGard and NexGard COMBO in February 2026. Zoetis’s release mentions none of them.

Why the FDA Authorized It Anyway

So why authorize another? The FDA’s summary explains it. The conditionally approved dog product is limited to dogs at least 8 weeks old and 3.3 pounds, and it must be given with food. Simparica Trio’s authorization covers dogs of all ages and weights and can be given with or without food, which matters for a sick dog that won’t eat.

As of September 3, the FDA had issued 15 EUAs and three conditional approvals for animal drugs against screwworm across all species.

Safety

Sarolaner, one of Simparica Trio’s three active ingredients, belongs to the isoxazoline drug class. The FDA notes that isoxazolines have been linked to neurologic reactions in dogs, including muscle tremors, loss of coordination, and seizures. That’s why the product is prescription-only.

The Investor Angle

For Zoetis, the dog authorization looks more defensive than growth-driving. Simparica Trio is under pressure. In its second-quarter filing, Zoetis said the brand faced heightened competition and price sensitivity in the U.S., and on its earnings call it reported U.S. Simparica franchise revenue down 6% to $308 million.

Screwworm has helped Zoetis elsewhere. The company said U.S. cattle product sales grew partly on elevated demand tied to the outbreak. Its cattle injectable Dectomax-CA1 holds a conditional approval for screwworm, and Zoetis describes it as the only nonprescription conditionally approved product of its kind for cattle.

Zoetis has not said how much revenue, if any, it expects from the dog authorization. With the FDA describing most U.S. dogs as low risk, the main value may be keeping Simparica Trio on equal footing with Credelio and NexGard when veterinarians compare parasite products.

Sources

• Zoetis: Zoetis Receives Emergency Use Authorization for Simparica Trio for Treatment of New World Screwworm Infestations in Dogs and Puppies, Business Wire, August 13, 2026. Primary source release.
• FDA: FDA Issues Emergency Use Authorization for Drug to Treat New World Screwworm in Dogs and Puppies, cited for the treatment-only scope, U.S. detection status, and isoxazoline safety information.
• FDA: Freedom of Information Summary, Simparica TRIO (EUA 006691), cited for the laboratory study design and results and FDA’s rationale for authorization, none of which appeared in the Zoetis release.
• FDA: Animal Drugs for New World Screwworm, cited for the count of EUAs and conditional approvals and for the NexGard, NexGard COMBO, and Credelio Quattro-CA1 actions.
• FDA: FDA Issues Emergency Use Authorization for New World Screwworm Drug for Dogs (Credelio), cited for the first-ever animal drug EUA and the 2022 northward spread.
• Zoetis Inc. Form 10-Q for the quarter ended June 30, 2026 (SEC EDGAR), cited for Simparica Trio competitive pressure and screwworm-related cattle demand.
• MarketBeat: Zoetis Q2 Earnings Call Highlights, cited for U.S. Simparica franchise revenue.

Editorial Disclosure

This article reports on the FDA Emergency Use Authorization of Simparica Trio for New World screwworm treatment in dogs, based on a press release issued by Zoetis Inc. on August 13, 2026, with study data drawn from the FDA’s Freedom of Information Summary and competitive and financial context drawn from FDA announcements and Zoetis securities filings cited above, since the original release did not address them. Securities discussed: Zoetis Inc. (NYSE: ZTS) and Elanco Animal Health Incorporated (NYSE: ELAN). Boehringer Ingelheim is privately held. BioTech Stocks Daily has not received, nor will it accept, any compensation, direct or indirect, from Zoetis, Elanco Animal Health, Boehringer Ingelheim, or any other corporate or third-party sponsor for the preparation or publication of this coverage. No staff member, principal, or affiliate of BioTech Stocks Daily holds a position, option, or financial interest in any of the securities mentioned as of the date of publication. Statements regarding the duration of the emergency declaration, the spread of New World screwworm, future sales, and competitive positioning are forward-looking statements. They involve substantial risks, regulatory uncertainties, and scientific variables; actual regulatory decisions, disease spread, and commercial results may differ materially. Animal health investments carry financial risk. Emergency authorizations can be revised or revoked, and product sales are subject to regulatory action and market competition, which can result in the loss of invested capital. This article is published strictly for independent news reporting, market context, and educational purposes only. It does not constitute an investment recommendation, endorsement, or professional financial, legal, or tax advice. Furthermore, this content is not veterinary or medical advice; pet owners should consult a licensed veterinarian about screwworm risk and treatment.

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