The U.S. Food and Drug Administration has issued an Emergency Use Authorization allowing the over-the-counter use of Ivomec (ivermectin) injectable solution to help prevent infestations of New World screwworm in certain classes of cattle.
The authorization, announced Feb. 5, 2026, permits Ivomec to be used as a preventive treatment against myiasis — the infestation of wounds by screwworm larvae — when administered within 24 hours of birth, at the time of castration, or when a wound first appears.
New World screwworm (Cochliomyia hominivorax) is a highly destructive parasitic fly whose larvae feed on living tissue. While the pest was eradicated from the U.S. decades ago, recent detections in Central America and concerns about northward spread have prompted heightened federal response efforts, including expanded border surveillance and emergency preparedness measures.
Under the EUA, Ivomec injectable (10 mg/mL ivermectin) may be used only in cattle, excluding female dairy cattle producing milk for human consumption and calves intended for veal production. The product is not approved to treat active screwworm infestations and is authorized solely for prevention.
Key restrictions include:
- A 35-day slaughter withdrawal period
- No established milk discard time — therefore prohibited for lactating dairy cows
- Not authorized for calves processed for veal
- Not approved for treating existing NWS infestations
Ivomec remains approved for other labeled uses in cattle, swine, reindeer, and American bison, but this specific application falls outside its standard approval and is allowed only for the duration of the emergency declaration.
According to the FDA, available scientific evidence supports a reasonable belief that Ivomec may be effective in preventing screwworm infestations when used under the conditions outlined in the authorization.
Dose-confirmation studies conducted in Brazil and Argentina in the early 1990s found that calves receiving a single subcutaneous dose of ivermectin (200 mcg/kg body weight) at birth or castration experienced dramatically lower rates of myiasis compared to untreated controls. In some trials, 30 to 90 percent of untreated calves developed infestations, while treated calves did not require rescue treatment during the first week of exposure.
Additional studies published in the veterinary literature reported prevention rates exceeding 97 percent in naturally exposed calves, though more recent research suggests effectiveness may vary depending on environmental conditions, infestation pressure, and potential antiparasitic resistance.
FDA acknowledged these limitations and emphasized the importance of monitoring effectiveness in the field.
Producer responsibility and resistance concerns
Ivomec will be available over the counter, but FDA officials stressed that producers are responsible for following all labeling, withdrawal periods, and conditions outlined in the EUA fact sheet.
The agency also cautioned that widespread prophylactic use of antiparasitic products could accelerate resistance, potentially reducing effectiveness not only against screwworm but other internal and external parasites.
Producers are encouraged to work with veterinarians to integrate Ivomec into a broader parasite management strategy rather than relying on it as a standalone solution.
FDA reviewed food safety data and concluded that beef products from treated animals are safe for human consumption when the conditions of use are followed. However, because a withdrawal period has not been established for pre-ruminating calves, treated calves — and calves born to treated cows — must not enter the veal market.
Side effects, lack of effectiveness, or product defects associated with Ivomec use under the EUA should be reported to Boehringer Ingelheim Animal Health or directly to FDA’s Center for Veterinary Medicine.
The EUA follows a broader federal effort to prevent the reintroduction of New World screwworm into the United States, including enhanced border controls and increased surveillance in high-risk regions. Officials have warned that an outbreak could have serious animal health, economic, and trade consequences for the U.S. livestock industry.
The emergency authorization will remain in effect until it is revoked or until the U.S. Department of Health and Human Services determines that the public health emergency related to New World screwworm no longer exists.
