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  1. Product Safety Information

Letter to Horse Owners and Veterinarians about FDA-Authorized Drugs to Prevent New World Screwworm in Horses

July 15, 2026

Dear Horse Owners and Veterinarians,

As part of the ongoing federal response to New World screwworm (NWS), the U.S. Food and Drug Administration has issued Emergency Use Authorizations (EUAs) for two products for the prevention of NWS infestations in horses: Ivermectin Liquid for Horses (ivermectin oral solution) and Dectomax/Dectomax-CA1 (doramectin injection).  

FDA recommends veterinarians and horse owners use these drugs only in horses susceptible to NWS myiasis (e.g., horses with wounds from injury or surgery, newborn foals with healing umbilical area) and take into consideration the risks and benefits of using a drug in the context of each specific circumstance. Due to a narrower margin of safety in horses than other species, these products are not intended for prevention in healthy horses without wounds or expectation to develop wounds.  Additionally, these products are not authorized to treat horses with NWS myiasis.  If a wound is already infested, or an infestation develops after administration, seek treatment.

This letter outlines important considerations when selecting which drug to use in a given situation. The information provided is not a substitute for the judgment of a licensed veterinary professional.

Authorized Uses

  • Ivermectin Liquid for Horses is authorized for the short-term prevention of NWS myiasis in newborn foals (within 24 hours of birth) or in horses of any age at the time of initial wound care.
  • Dectomax/Dectomax-CA1 is authorized for the prevention of NWS myiasis in numerous species, including horses that are one year of age or older.  Per the product’s directions for use in horses, this product is for administration at the time of wound appearance or time of castration.

Duration of Effect

  • Ivermectin Liquid for Horses is estimated to provide protection for up to 24 hours after a single dose.
  • Dectomax/Dectomax-CA1 may provide protection beyond the first 24 hours after a single dose in horses; however, the duration of effect after 24 hours in horses is unknown.

For horses with wounds expected to remain open and vulnerable for an extended time, either product needs to be used in conjunction with additional fly control measures such as wound bandaging, fly repellants, and other barrier methods.

Marketing Status & Route of Administration

  • Ivermectin Liquid for Horses requires a veterinary prescription, particularly because administration of the drug using a nasogastric tube is a medical procedure requiring professional skill, experience, and anatomical knowledge.
  • Dectomax/Dectomax-CA1 is available over the counter because it is an injectable drug that can be safely used by laypeople by following the directions on the label for the prevention of NWS infestations in horses.

Supporting Evidence

Neither product has clinical trial data in horses specifically demonstrating effectiveness in prevention of NWS myiasis. Both EUAs meet the standard that it is reasonable to believe the product may be effective under the specified conditions of use.  

  • The Ivermectin Liquid for Horses EUA is supported by cattle studies conducted in South America in the 1990s using injectable ivermectin, combined with pharmacokinetic bridging data comparing drug blood levels in cattle versus horses. A 2019 Brazilian study showed considerably lower effectiveness in cattle than the previous studies. There are no clinical data from horses, and the oral formulation in horses has never been directly tested against NWS in horses.
  • For the Dectomax/Dectomax-CA1 EUA, in addition to pharmacokinetic bridging of cattle studies using injectable Dectomax, three dose confirmation studies were conducted in horses in the United States using the same route of administration (intramuscular injection) and the same dose (200 mcg/kg) authorized under this EUA. Although these studies evaluated antiparasitic activity against gastrointestinal parasites rather than NWS specifically, they demonstrate that the drug can be administered safely and properly in horses at the authorized dose.

Margin of Safety

Margin of safety for a drug is the difference between the dose that produces the intended effect and the dose at which adverse effects may occur. A wider margin means there is more room between the authorized dose and a dose that could cause adverse effects; a narrower margin means that difference is smaller, making precise dosing especially important. A narrow margin of safety does not mean a product is unsafe; it simply means that it is critical to carefully follow dosing instructions and not to exceed the authorized dose.

For both products, FDA determined that the authorized doses are appropriate for use in horses. As with any medication, it is important to use these products only as directed at the authorized dosage and to seek prompt veterinary attention for unexpected changes in a horse’s behavior or health after administration.

  • Ivermectin Liquid for Horses was not evaluated in a new horse-specific safety study because FDA relied on a previously approved oral ivermectin horse product with an established margin of safety.
  • Dectomax/Dectomax-CA1 was evaluated in a dedicated horse safety study. Mild adverse effects were observed at doses only modestly above the authorized dose (1.5X or 1.5 times the authorized dose), indicating a relatively narrow margin of safety.  At doses of 4.5X and above, doramectin was associated with significant clinical abnormalities and resulted in humane euthanasia of two horses administered 7.5X due to moribundity.

Judicious Use

FDA recommends veterinarians and horse owners use these drugs only in horses susceptible to NWS myiasis (e.g., horses with wounds from injury or surgery, newborn foals with healing umbilical area) and take into account the risks and benefits of using a drug in the context of each specific circumstance.  Animal drugs are only one tool to address the risk of NWS. Veterinarians or horse owners may wish to also consider management and care approaches as part of an integrated approach to manage the risk of NWS myiasis in horses.  

A Note on EUA Status

Both products are authorized under EUAs in response to the HHS determination of a significant potential for a public health emergency related to NWS. They will remain authorized for these specific uses in horses while the HHS determination continues, unless the EUAs are revised or revoked. FDA actively monitors the effectiveness and safety of products authorized for NWS and will make changes as needed. Neither product is approved for use in horses.  

Because Dectomax has a full approval for other indications (treating and controlling internal and external parasites in cattle and swine), veterinarians may use it in an extralabel manner, subject to the requirements of the Animal Medicinal Drug Use Clarification Act.  Ivermectin Liquid for Horses is not a fully approved product and cannot legally be used extralabel.

Serious adverse events must be reported to the product’s sponsor as a condition of the EUA. FDA also encourages reporting of non-serious adverse events and lack of effectiveness:

  • Ivermectin Liquid for Horses: Contact Solvet at 1-877-456-2755, submit FDA Form 1932a at fda.gov/reportanimalae, or call FDA at 1-888-FDA-VETS.
  • Dectomax/Dectomax-CA1: Contact Zoetis at 1-888-963-8471, submit FDA Form 1932a at fda.gov/reportanimalae, or call FDA at 1-888-FDA-VETS.

For more information on Ivermectin Liquid for Horses, see the Fact Sheet , FOI Summary , and Letter of Authorization.

For more information on Dectomax/Dectomax-CA1 for horses, see: the Fact Sheet – Horses, FOI Summary, and Letter of Authorization.

FDA appreciates the vigilance of U.S. horse owners and veterinarians in helping protect animals against New World screwworm.

Sincerely,

U.S. Food and Drug Administration’s Center for Veterinary Medicine

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