The threat posed by New World screwworm to U.S. Cattle production is prompting new preventative measures, including the recent conditional approval of Exzolt Cattle-CA1, a topical solution developed by Merck Animal Health. Approved by the U.S. Food and Drug Administration in December 2025, this pour-on medication offers both prevention and treatment options for infestations caused by the larvae of this parasitic fly, as well as providing treatment and control for cattle fever tick.
New World screwworm, though not currently detected within the United States, remains a significant concern due to its northward movement in Mexico. The FDA’s conditional approval reflects a proactive approach to safeguarding the U.S. Cattle industry. Cattle fever tick, however, *is* present in the U.S., making Exzolt Cattle-CA1 a timely intervention.
Exzolt Cattle-CA1 utilizes fluralaner, an isoxazoline-class ectoparasiticide, representing a breakthrough in parasite management. This is the first such drug approved for use in U.S. Beef cattle, offering a novel mode of action and, crucially, no currently known resistance. According to John Hutcheson, director of beef cattle technical services for Merck, the solution provides “fast-acting protection” against these potentially deadly parasites.
The conditional approval is based on a “reasonable expectation of effectiveness,” supported by existing research and Merck Animal Health’s own studies. Fluralaner has demonstrated safety and efficacy across multiple species in over 170 clinical studies globally and is already approved for use in 93 markets. The FDA’s conditional approval process allows for the introduction of critical treatments while further efficacy data is collected.
Specifically, Exzolt Cattle-CA1 is approved for use in beef cattle aged 2 months and older, and in replacement dairy heifers under 20 months of age. This proves *not* approved for use in bulls intended for breeding that are 1 year of age or older, dairy calves, or veal calves. A critical consideration for producers is the withdrawal period: cattle treated with Exzolt Cattle-CA1 must not be slaughtered for human consumption within 98 days of treatment.
The recommended dosage is 1 mL of the solution per 44.1 pounds (or 20 kg) of body weight, delivering 1.13 mg of fluralaner per pound (2.5 mg/kg). Accurate weight determination and dosing are essential for optimal effectiveness. The product is designed for direct application along the dorsal midline, extending from the withers to the base of the tail.
The FDA’s conditional approval process requires Intervet, Inc. (a subsidiary of Merck) to generate additional effectiveness data over the next five years to support full FDA approval. This ongoing data collection will further solidify the drug’s efficacy and safety profile.
This approval highlights the FDA’s commitment to protecting the U.S. Livestock industry and ensuring a safe food supply. As Timothy Schell, Ph.D., director of the FDA’s Center for Veterinary Medicine, stated, the agency is “leveraging every resource and authority” to provide producers and veterinarians with the tools needed to combat these parasitic threats.
The emergence of Exzolt Cattle-CA1 represents a significant step forward in cattle parasite control, offering a new layer of defense against New World screwworm and cattle fever tick. Producers should consult with their veterinarians to determine if this product is appropriate for their specific needs and to ensure proper administration and adherence to withdrawal times.
