FDA Issues Emergency Use Authorization for Drug to Treat New World Screwworm in Dogs and Puppies

FDA Authorizes Simparica TRIO for New World Screwworm in Dogs

The U.S. Food and Drug Administration (FDA) granted an Emergency Use Authorization (EUA) on August 13, 2026, for Simparica TRIO (sarolaner, moxidectin, and pyrantel chewable tablets) to treat New World screwworm (NWS) infestations in dogs and puppies. This authorization, issued to Zoetis Inc., aims to provide a crucial treatment option in the event of NWS exposure, particularly in identified risk areas like Texas and New Mexico, despite the current low overall risk in the U.S. The FDA concluded that the product's known and potential benefits for NWS myiasis treatment outweigh its risks.

• EUA Scope and Efficacy Basis: The Emergency Use Authorization for Simparica TRIO specifically targets the treatment of New World screwworm myiasis in dogs and puppies, explicitly stating it is not authorized for prevention. The FDA's decision is based on a comprehensive evaluation of scientific evidence, leading to the conclusion that it is reasonable to believe the product may be effective, with its benefits outweighing potential risks in this emergency context.
• Drug Composition and Safety Profile: Simparica TRIO is a combination product containing sarolaner, moxidectin, and pyrantel. Sarolaner, an active ingredient, belongs to the isoxazoline class of antiparasitic drugs. While generally safe, isoxazoline products have been associated with neurologic adverse reactions, including muscle tremors, loss of coordination, and seizures, necessitating that Simparica TRIO be available only by prescription from a licensed veterinarian.
• Public Health Preparedness and Awareness: This EUA reflects the FDA's proactive strategy to ensure readiness for animal health emergencies, even in situations of low current risk. Acting FDA Commissioner Kyle Diamantas emphasized the goal of providing multiple treatment options. Officials encourage dog owners to stay informed about NWS signs and consult veterinarians, rather than being alarmed, reinforcing the ongoing success in containing the spread of NWS in the U.S.

Addressing Unmet Needs in New World Screwworm Treatment

Current treatment of New World screwworm (NWS) myiasis in dogs and puppies is constrained by the absence of purpose-approved therapeutics and the biological urgency imposed by the parasite itself. Open wounds are susceptible to reinfestation, and untreated cases can be fatal, placing a premium on interventions that are simultaneously rapid, persistent, and fully efficacious.

• Off-label reliance as standard of care: NWS myiasis caused by Cochliomyia hominivorax has traditionally been managed with extra-label use of ivermectin, reflecting a lack of approved therapeutic options specifically indicated for this condition.

• Variable speed of larvicidal action across available agents: Comparative data reveal meaningful differences in onset of activity — nitenpyram achieved 100% larval kill at 6 hours post-consumption and spinosad/milbemycin at 7 hours, while afoxolaner and spinosad achieved full kill within 24 hours. Milbemycin performed more slowly, with live larvae persisting in two cases at the 24-hour mark, underscoring that not all available agents meet the speed-of-kill threshold required to prevent reinfestation.

• Diagnostic misclassification delaying intervention: Furuncular myiasis is frequently misdiagnosed as cellulitis or abscess due to nonspecific presenting symptoms. This leads to initiation of anti-inflammatory and antibiotic regimens that are ineffective against larval infestation, prolonging morbidity and deferring appropriate parasiticide treatment.

• Multimodal management requirements adding therapeutic complexity: Parasiticide administration alone is insufficient; effective case management requires mechanical or surgical larval extraction, wound debridement, and ongoing monitoring for secondary infection — increasing the clinical burden and resource demands associated with each case.

Simparica TRIO: Efficacy and Safety Considerations

Published safety and tolerability data for the sarolaner, moxidectin, and pyrantel combination (Simparica Trio) consistently demonstrates a favorable profile across its studied indications. In a randomized, placebo-controlled, double-masked field study conducted over 10 months in endemic areas of Denmark and Italy, no treatment-related adverse events were observed among 622 client-owned dogs administered the combination at 30-day intervals. The product achieved 100% efficacy in preventing natural Angiostrongylus vasorum infection, with clean tolerability throughout the study duration. Similarly, across six laboratory studies evaluating tick control efficacy against four common European species — Dermacentor reticulatus, Ixodes ricinus, Ixodes hexagonus, and Rhipicephalus sanguineus — no treatment-related adverse events were recorded in any study. Dogs in these trials received minimum dosages of 1.2 mg/kg sarolaner, 24 µg/kg moxidectin, and 5 mg/kg pyrantel (as pamoate salt).

Beyond individual study endpoints, retrospective analyses have evaluated the real-world implications of the combination formulation on treatment adherence. Dogs receiving Simparica Trio demonstrated increased compliance relative to those managed on separate flea/tick and heartworm preventive regimens, translating into greater cumulative time under parasitic protection. This compliance advantage carries meaningful clinical relevance, as gaps in preventive coverage directly increase the risk of parasitic infection and vector-borne disease transmission. Taken together, the published evidence base supports the conclusion that the sarolaner, moxidectin, and pyrantel combination is well tolerated, with no serious adverse events reported across its studied indications in dogs.