FDA authorizes Merck's Bravecto to treat screwworm in dogs

Merck's Bravecto, the flea and tick chew that earned $264M in the third quarter, can now be prescribed to treat New World screwworm in dogs and puppies. The FDA issued an emergency use authorization to Intervet, Merck's U.S. animal health unit, for a single oral dose of the chewable tablet. Bravecto is not approved for this use.

FDA's summary of the authorization sets the dose at a minimum of 11.4 mg/lb (25 mg/kg), given with food. A dog of 4.4 to 9.9 lb gets one 112.5 mg chew, a dog of 88 to 123 lb gets one 1400 mg chew, and larger dogs get a combination of chews. A licensed veterinarian has to dispense it, and the authorization runs for as long as the federal emergency declaration stays in effect.

FDA built its decision on 15 dogs in Brazil

The evidence is Study S25220-00, a field study in client-owned dogs run from January 14th to April 20th, 2026, in Brazil. It enrolled 15 dogs aged 4 months to 17 years and weighing 5.8 to 32.9 kg, in a single treatment group with no control group. FDA reports no live larvae in the 15 dogs at 24 hours, a larvicidal effectiveness of 100%, and no adverse reactions in the study.

FDA states the limits itself. The sample was small, the study had no control group, and the data cannot say whether the drug killed the larvae or caused them to leave the wound. Puppies aged 8 to 9 weeks showed lower peak drug concentrations than adult dogs, which FDA says may reduce effectiveness.

Merck's own Mexico release points to the same species and country. MSD Animal Health, a division of Merck & Co., said on August 24th, 2026 that Mexico's agriculture ministry approved Bravecto Chew for screwworm in dogs. It cited a Brazil study of 15 naturally infested dogs with 94.36% overall larvicidal efficacy and 0% emergence of adult flies. Alejandro Sánchez, a veterinarian and technical manager in MSD Animal Health's Companion Animals Business Unit in Mexico, called the indication "a therapeutic option supported by scientific evidence."

The same release cites Mexican government data on how large the problem is south of the border. SENASICA recorded 37,236 screwworm cases across species from November 2024 through July 2026, including 8,518 in dogs. MSD said it is following regulatory processes in other countries to gain market authorization for the use.

Bravecto joins Credelio, NexGard and Simparica Trio

Elanco's Credelio received a dog authorization on October 24th, 2025. Dr. Ellen de Brabander, Elanco's executive vice president of innovation and regulatory affairs, said in the company's release that "This first ever Emergency Use Authorization for NWS in dogs gives veterinarians and pet owners peace of mind." Boehringer Ingelheim's NexGard followed on February 18th, 2026, with NexGard COMBO authorized the same day for cats.

Zoetis's Simparica Trio came on August 13th, 2026, on a laboratory study in 12 beagles in Brazil, according to FDA's summary. We covered that clearance in our Simparica Trio story. Over-the-counter nitenpyram tablets, including Sergeant's Capstar, were cleared for dogs and cats in September, as we reported in our Capstar story.

FDA's Bravecto summary draws a line against one rival approach. It notes that a conditionally approved screwworm product contains multiple active ingredients, which may raise the chance of adverse events in dogs recently treated with a heartworm preventive or another antiparasitic in the same class. FDA says Bravecto's single active ingredient may minimize that risk.

The class matters for the next three months of a dog's parasite plan. Bravecto's active ingredient, fluralaner, belongs to the isoxazoline class, as do lotilaner in Credelio, afoxolaner in NexGard and sarolaner in Simparica Trio. FDA's fact sheet for veterinarians says fluralaner stays measurable for up to 112 days and advises avoiding routine isoxazoline dosing for at least three months after Bravecto. Isoxazolines have been linked to tremors, loss of coordination and seizures.

Clinics gain a choice and Merck gains a seat

USDA counted 71 confirmed animal cases since June as of October 28th, with 18 still active, in our Arizona coverage. Arizona became the third state with cases, after Texas and New Mexico. Clinics in and near those counties can now choose among four brand-name drugs for an infested dog, and Bravecto requires a prescription.

That prescription step shows up in retail. Chewy lists Bravecto's one-month chew for dogs of 22 to 44 lb at $26.99 and says the product is available by prescription, with Chewy contacting the vet clinic for approval. The listing describes flea and tick protection and does not mention screwworm.

For Merck, the authorization puts a drug it already sells to clinics into a parasite response that Elanco, Boehringer Ingelheim and Zoetis entered earlier. By our math, Bravecto made up 42% of Merck's companion animal sales in the third quarter, as we showed in our Merck Q3 story. Elanco reports its own third-quarter results on November 6th.

The authorizations for Bravecto and its rivals end when the federal emergency declaration does.

Source: FDA Center for Veterinary Medicine