FDA Issues Emergency Use Authorization for Generic Drug to Prevent New World Screwworm in Cattle

FDA Authorizes Generic Ivermectin for New World Screwworm in Cattle

The U.S. Food and Drug Administration (FDA) has issued an Emergency Use Authorization (EUA) for Bimectin (ivermectin) injection to prevent New World screwworm (NWS) infestations in cattle. This generic version of Ivomec is authorized for administration within 24 hours of birth, at castration, or at the appearance of a wound. The authorization aims to expand generic drug options, ensuring producers are not reliant on a single manufacturer and safeguarding American agriculture. Important restrictions include a 35-day slaughter withdrawal time and prohibition for use in lactating dairy cows or calves processed for veal. The FDA concluded that the benefits of Bimectin injection for NWS prevention in certain cattle outweigh its known and potential risks.

• The FDA granted an Emergency Use Authorization for Bimectin (ivermectin) injection specifically for the prevention of New World screwworm (NWS) infestations in cattle. This authorization permits its use under specific conditions: within 24 hours of birth, at the time of castration, or upon the appearance of a wound. This targeted application aims to protect vulnerable cattle populations from the parasitic myiasis.
• Bimectin injection is a generic version of Ivomec, which received its own EUA in February 2026 for the same indication. This new authorization by the FDA is a strategic move to expand generic drug options, ensuring that cattle producers have multiple sources for NWS prevention. This reduces dependency on a single manufacturer, enhancing supply chain resilience and safeguarding American agriculture.
• To ensure food safety and prevent drug residues, the EUA for Bimectin injection mandates strict adherence to a 35-day slaughter withdrawal time. Furthermore, the drug is explicitly not authorized for use in lactating dairy cows or calves intended for veal processing. The FDA's decision is based on a scientific assessment concluding that the known and potential benefits of the product outweigh its risks when used as authorized.

Expanding NWS Prevention: Bimectin's Role in Evolving Landscape

The treatment landscape for New World screwworm (NWS) myiasis has been shaped increasingly by concerns over macrocyclic lactone (ML) efficacy. Ivermectin and doramectin have historically represented the cornerstone of ML-based prevention and treatment across livestock species, with MLs employed in approximately 96.5% of treatments targeting gastrointestinal nematodes, ticks, and myiasis in studied herds. However, field data from Argentina have documented meaningful treatment failures: following castration surgery in cattle, both ivermectin 3.15% and doramectin 1% failed to prevent natural Cochliomyia hominivorax infestations, with active myiasis observed in multiple animals from both treatment groups at days 3 and 6 post-administration. At the molecular level, resistance has been linked to the Trp251Ser mutation in esterase E3, a variant known to confer cross-resistance to organophosphates and pyrethroids, raising broader concerns about the durability of established chemical classes.

In companion animal medicine, comparative efficacy data have begun to redefine the therapeutic options available for canine screwworm myiasis. A study of 40 naturally infested dogs evaluated several agents head-to-head: nitenpyram achieved complete larval kill within 6 hours post-consumption, while a spinosad/milbemycin combination demonstrated synergistic activity with full efficacy at 7 hours. Afoxolaner and spinosad monotherapy were both fully effective within 24 hours, whereas milbemycin alone left residual live larvae in two cases at the 24-hour timepoint. These findings position nitenpyram and the spinosad/milbemycin combination as particularly rapid-acting options, with afoxolaner representing a viable though slower alternative.

Standard-of-care protocols across species continue to incorporate multimodal approaches, including mechanical larval extraction — performed surgically in approximately 22% of pediatric cases reported in Uruguay — alongside systemic antibiotics, which were prescribed in 95% of hospitalized pediatric patients. Topical asphaltic products were applied in 94% of pediatric cases, with ether used as an adjunct in 49%. Ivermectin and cephradine were the most frequently documented antimicrobial agents in this population. Taken together, the published data from recent years reflect a field in transition: mounting ML resistance pressure is driving evaluation of novel insecticide classes, while supportive and mechanical interventions remain indispensable components of comprehensive NWS management.