06/18/2026
There have been calls for the use of ivermectin in feed as a tool to combat the spread of New World Screwworm. AABP understands the need to use all tools available while also avoiding unintended consequences that could negatively impact cattle health. The following statement from AABP Executive Director Dr. Fred Gingrich addresses these potential negative impacts of using ivermectin in feed for New World Screwworm, including the development of resistance, treatment failures, and missing active infestations that require treatment since ivermectin has FDA authorization for prevention only in the injectable formulation. Inspecting cattle every 24 hours and reporting suspected cases is the primary control method that is needed for combating this pest.
AABP members can also find this statement under the Members Resources tab on the website, then the New World Screwworm resources page, or directly at https://aabp.org/resources/screw_worm/2026-6-16-AABP_NWS_Ivermectin_feed_statement.pdf.
June 16, 2026
The American Association of Bovine Practitioners (AABP) is an international association of over 4,500 cattle veterinarians, veterinary technicians and veterinary students. The mission of the AABP is to provide support, continuing education, and advocacy for current and future veterinary professionals and the cattle they serve. The vision of AABP is to serve society as leaders in cattle health, welfare and productivity.
AABP members will be on the front lines to address the recent incursion of New World Screwworm (NWS). Although many of the products that may be used to treat these infestations in cattle are over-the-counter, AABP encourages all producers to work with their veterinarian to develop identification, treatment and prevention protocols for their operations.
The Food and Drug Administration Center for Veterinary Medicine (FDA CVM) has authorized the use of several products for cattle to treat and prevent NWS through Conditional Approvals or Emergency Use Authorization.
It is critical that veterinarians work with producers to ensure that animals are inspected daily to identify cattle infested with NWS. Identification of infected animals will allow for individual animal treatment as well as reporting to state and federal animal health authorities so that prevention measures based on the USDA New World Screwworm Playbook can be implemented. Reporting affected premises will allow for sterile fly dispersals and other interventions to control the spread of NWS.
AABP has been made aware of calls for FDA to authorize the use of ivermectin in feed or other products meant to be consumed on a free-choice basis for range cattle. Currently, AABP does not support the use of this delivery method.
There is a dramatic reduction in bioavailability of ivermectin delivered orally versus injectable ivermectin. In-feed ivermectin available free choice will also result in increased systemic ivermectin concentration variability due to the combination of variation in both oral bioavailability and feed intake differences between individuals. This may result in animals receiving a subtherapeutic dose which can result in therapeutic failure.
There is an increasing resistance to anthelmintics in cattle. Wide-spread use of ivermectin may increase resistance pressure of parasites to ivermectin and the development of resistance to ivermectin for NWS. This resistance development to ivermectin may potentially select for cross resistance to other NWS therapeutics which may limit our ability to treat individual animals with current drug approvals. There are numerous studies that have demonstrated widespread resistance to ivermectin in Central and South America.
AABP supports the use of the products that have been conditionally approved or are under emergency use authorization for NWS. In-feed ivermectin does not currently have this authorization and it is prohibited to use drugs extralabel in feed. If animals on open pastures have access to ivermectin, it may create a false sense of security resulting in underreporting of cases. Individual animal inspection and treatment are necessary to combat the spread of this fly.
AABP supports continuing research on treatment modalities for NWS in cattle, including research on the effectiveness and consequences of delivery of these medications through feed and other delivery methods.
References:
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2. Anziani OS, Flores SG, Moltedo H, Derozier C, Guglielmone AA, Zimmermann GA, Wa**er O. Persistent activity of doramectin and ivermectin in the prevention of cutaneous myiasis in cattle experimentally infested with Cochliomyia hominivorax. Vet Parasitol. 2000 Jan;87(2-3):243-7. doi: 10.1016/s0304-4017(9 9)00167-3 PMID: 10622617.
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5. Muchiut S, Miro MV, Anziani O, Nava S, Lifschitz A. Failure of doramectin and ivermectin in preventing Cochliomyia hominivorax myiasis in a subtropical region: A pharmacokinetic-pharmacodynamic study. Vet Parasitol. 2025 Feb;334:110384. doi: 10.1016/j.vetpar.2024.110384. Epub 2024 Dec 24. PMID: 39733599.
6. Benitez Usher C, Cruz J, Carvalho L, Bridi A, Farrington D, Barrick RA, Eagleson J. Prophylactic use of ivermectin against cattle myiasis caused by Cochliomyia hominivorax (Coquerel, 1858). Vet Parasitol. 1997 Oct;72(2):215- 20. doi: 10.1016/s0304-4017(97)00018-6. PMID: 9404848.
K. Fred Gingrich II DVM
Executive Director
American Association of Bovine Practitioners
[email protected]
419-606-3558 (cell)
