FDA approves Merck drug for decimated cattle herds to stop screwworm
Cattle kept in the pens of the Chihuahua Regional Livestock Association at the Jeronimo-Santa Teresa border crossing in Ciudad Juarez, Mexico, on Nov. 27, 2024, after the United States halted imports of Mexican cattle due to screwworm.
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U.S. cattlemen will soon have a new way to protect their dwindling herds from the parasitic screwworm threat that has decimated cattle in Mexico.
US Food and Drug Administration conditional approval given A topical treatment for a drug called EXZOLT CATTLE-CA1 Merck Animal Health for the prevention and treatment of New World Screwworm. It can also be used as treatment and control of cattle fever tick.
“The conversation started with the FDA in July, and because there was a food safety element in humans, we had a big data package to generate,” said Holger Lehmann, vice president of pharmaceutical research and development for Merck Animal Health. “This approval is a significant initiative. The United States received the product and Mexico, where it is used, received it in early November,” Lehmann said. he said.
Each dose is effective for 21 days before a new dose needs to be administered. The FDA approved this with a 98-day withholding period to ensure no residue remains in the meat.
Merck Animal Health is prioritizing shipping the product first to U.S. distribution partners located along the U.S.-Mexico border and in areas more likely to be in urgent need by the end of the year. The treatment will be more widely available to veterinarians and producers in mid-January 2026.
Lehmann cautioned that medication alone may not eliminate the parasite anytime soon. “Experts tell us they don’t expect a quick recovery from the screwworm problem in Mexico,” Lehmann said. “They think this is a perennial problem that needs to be solved.”
On December 4, 2025, the U.S. Food and Drug Administration (FDA) granted conditional approval to Exzolt Cattle-CA1, Merck Animal Health’s fluralaner topical solution, for use in beef cattle.
Merck Animal Health
Screwworm is spread by the incubation of fly eggs in the open wounds of cattle feeding on living tissue. Humans can also become infected. To protect the U.S. cattle herd and stop the spread of the parasitic fly, the U.S. Department of Agriculture has closed the Mexican border on and off since 2024 for imports of live cattle, bison and horses.
The border continues to be closed.
Before the closure, Mexico was an exporter of calves to the United States; USDA data showed that the United States imported more than a million cattle annually; this accounted for approximately 3.3% of the total U.S. calf crop.
The screwworm epidemic in Mexico is one of the reasons. Volatility in the cattle futures marketand is behind the high cost of beef, which has become a high-profile issue for the Trump administration amid the president’s falling poll numbers on the economy.
President Trump blamed meat processors and U.S. cattlemen for higher costs. Tariffs on animal feed and farm equipment have been linked to increases in beef prices as well as drought affecting herd size.
Accordingly USDA data, As of November 2025, the number of US cattle on feed was 11.7 million head, a decrease of 2%, or 260,000 head, from 2024. This represents the lowest level of the U.S. cattle herd since 1951.
in November, Tyson Foods announced that it was closes major beef plant Cessation of operations in Lexington, Nebraska, and in Amarillo, Texas, due to cattle shortages.
“As farmers, we are pleased to see the FDA approve new tools like this,” said sixth-generation Texas farmer James Clement III. “When we face outbreaks of tick ticks or screwworms, having effective medications and treatments is not optional—it is essential,” he said. “These products give producers the ability to intervene quickly, protect our herds and protect the livestock industry more broadly,” he said, but added that farmers will have some questions before they start using the drug.
Because it’s winter and temperatures are cooler, the chances of flies carrying screwworms from Mexico to the U.S. are low right now, Lehmann said. “But there is risk in the spring, so we have enough product that we can immediately distribute to cattle breeders for preventive action,” Lehmann said. “From what we know, this treatment is very effective against screwworm, and you want to get that under control. So benefit treatment actually becomes very critical,” Lehmann added.
