Cattle detained in the pens of the Chihuahua Regional Livestock Union , at the Jeronimo-Santa Teresa border crossing in Ciudad Juarez, Mexico on November 27, 2024, after the United States stopped imports of Mexican cattle due to the presence of screwworm.
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U.S. cattle ranchers will soon have a new way to protect their dwindling herds from the threat of the parasite screwworm, which is decimating cattle in Mexico.
The U.S. Food and Drug Administration granted conditional approval for a drug called EXZOLT CATTLE-CA1, a topical treatment from Merck Animal Health for the prevention and treatment of New World Screwworm. It can also be used as a treatment and control for the cattle fever tick.
Farmers will have access to doses of the Merck drug starting on December 20 through their veterinarian.
“The conversation started in July with the FDA, and because there is an element of human food safety, there was a big data package we had to generate,” said Holger Lehmann, vice president of pharmaceutical research and development for Merck Animal Health. “This approval is a significant undertaking. The U.S. has received the product, and Mexico received it in early November, where it is being used,” Lehmann said.
The FDA has approved it with a 98-day withholding period to ensure no residue in meat.
Lehmann cautioned that the drug alone cannot eradicate the parasite any time soon. “Experts tell us in Mexico that they don’t expect that they’re going to be able to get rid of the screwworm problem quickly,” Lehmann said. “They think it’s a multi-year problem to get resolved.”
On December 4, 2025, the U.S. Food and Drug Administration (FDA) granted conditional approval to Exzolt Cattle-CA1, a fluralaner topical solution from Merck Animal Health, for the use in beef cattle.
Merck Animal Health
Screwworm is spread by hatching fly eggs in the open wounds of cattle, which feed on their living tissue. Humans can also get infected. To protect the U.S. cattle herd and to stop the spread of the parasitic fly, the U.S. Department of Agriculture has closed the border to Mexico for imports of live cattle, bison, and horses, on and off, since 2024.
The border continues to be closed.
Before the closure, Mexico was an exporter of calves to the U.S., with USDA data showing the U.S. imported over one million cattle annually, approximately 3.3% of the U.S. total calf crop.
The screwworm outbreak in Mexico is one of the reasons for volatility in the cattle futures market, and behind the high costs of beef, which has become a high-profile issue for the Trump administration among the president’s falling poll numbers on the economy.
President Trump has blamed meat processors and U.S. cattlemen for the higher costs. Tariffs associated with animal feed and farming equipment have been linked to the rise in beef prices, along with drought impacting herd size.
According to USDA data, as of November 2025, the U.S. cattle on feed were 11.7 million head, down 2%, or 260,000 head, from 2024. That represents a U.S. cattle herd at its lowest level since 1951.
In November, Tyson Foods announced it was closing its major beef plant in Lexington, Nebraska, and cutting back operations in Amarillo, Texas, because of the cattle shortage.
“As ranchers, we’re glad to see the FDA approving new tools like this,” said sixth-generation Texas rancher James Clement III. “When we face fever ticks or screwworm outbreaks, having effective medications and treatments on hand isn’t optional; it’s essential,” he said. “These products give producers the ability to respond quickly, protect our herds, and safeguard the broader livestock industry,” he said, though he added ranchers will have some questions before they move ahead with use of the drug.
Because it’s winter and the temperatures are cooler, Lehmann said the likelihood of flies carrying screwworm from Mexico into the U.S. is currently low. “But there is risk in the spring, so we have enough product available that we could deploy immediately to cattle ranchers for preventative action,” Lehmann said. “Based on what we know, this treatment is very effective against screwworm, and you want to contain this. So benefit treatment actually becomes very critical,” Lehmann added.
