Topical drug for screwworm, cattle fever tick receives conditional approval

The Food and Drug Administration has conditionally approved Exzolt Cattle-CA1 (fluralaner) topical solution for cattle for two parasites: New World screwworm (NWS) and cattle fever tick.

The December 4 approval is for the prevention and treatment of infestations of NWS (Cochliomyia hominivorax) larvae, and the treatment and control of cattle fever tick (Rhipicephalus microplus) in beef cattle 2 months of age and older and replacement dairy heifers less than 20 months of age.

This comes after recent emergency use authorizations for two animal drugs for NWS: Credelio and Credelio Cat for dogs and cats, respectively.

"The FDA is leveraging every resource and authority at its disposal to ensure producers and veterinarians have the critical products they need to protect their livestock and safeguard our food supply against NWS and cattle fever tick," said Timothy Schell, PhD, director of the FDA Center for Veterinary Medicine.

Exzolt Cattle-CA1, a pour-on topical solution, is available only by prescription from a licensed veterinarian. Available in the first quarter of 2026, it will initially come in two bottle sizes-1 L and 5 L-including 50 mg of the active ingredient fluralaner per mL. A third size of 250 mL has also been approved.

The ready-to-use topical formulation is intended for single use only and can be directly applied to the hair and skin in a narrow strip extending along the dorsal midline from the withers to the base of the tail. The recommended rate of administration is 1 mL per 44.1 lbs (1 mL/20 kg) body weight, equivalent to 1.13 mg of fluralaner per lb body weight (2.5 mg/kg).

It is not approved for use in bulls intended for breeding (1 year and older), dairy calves, and veal calves. There is a 98-day withdrawal period.

Exzolt Cattle-CA1 is sponsored by Merck Animal Health. Fluralaner, the active ingredient in Exzolt Cattle-CA1, belongs to a class of parasiticides called isoxazolines.

The conditional approval was "based on demonstration of safety and a reasonable expectation of effectiveness when administered according to label directions based on global studies conducted by Merck Animal Health in recent years," according to the company, which has also received market authorization for the product in Mexico for the prevention and treatment of NWS to help control the spread of the parasite in cattle.

Screwworm's northward migration from Central America has prompted a massive response from animal health officials, who are calling for heightened vigilance from small and large animal veterinarians to ensure the parasite does not regain a foothold in the U.S.

More recent active cases of NWS infestation recorded were an 8-day-old cow reported on November 29 in the Mexican state of Veracruz and a nearly 2-year-old cow on November 28 in the state of Nuevo León.