Cattle detained within the pens of the Chihuahua Regional Livestock Union , on the Jeronimo-Santa Teresa border crossing in Ciudad Juarez, Mexico on November 27, 2024, after the US stopped imports of Mexican cattle as a consequence of the presence of screwworm.
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U.S. cattle ranchers will soon have a brand new technique to protect their dwindling herds from the specter 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 Recent World Screwworm. It may even be used as a treatment and control for the cattle fever tick.
Farmers may have access to doses of the Merck drug starting on December 20 through their veterinarian.
“The conversation began in July with the FDA, and since there may be a component of human food safety, there was an enormous data package we needed to generate,” said Holger Lehmann, vice president of pharmaceutical research and development for Merck Animal Health. “This approval is a big undertaking. The U.S. has received the product, and Mexico received it in early November, where it’s getting used,” Lehmann said.
Each dose is effective for 21 days before a brand new dose would must be applied. The FDA has approved it with a 98-day withholding period to make sure no residue in meat.
Lehmann cautioned that the drug alone cannot eradicate the parasite any time soon. “Experts tell us in Mexico that they do not expect that they are going to find a way to do away with the screwworm problem quickly,” Lehmann said. “They think it is 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 within the open wounds of cattle, which feed on their living tissue. Humans also can get infected. To guard 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 a million cattle annually, roughly 3.3% of the U.S. total calf crop.
The screwworm outbreak in Mexico is one among the explanations for volatility within the cattle futures market, and behind the high costs of beef, which has turn out to be a high-profile issue for the Trump administration among the many president’s falling poll numbers on the economy.
President Trump has blamed meat processors and U.S. cattlemen for the upper costs. Tariffs related to animal feed and farming equipment have been linked to the rise in beef prices, together with drought impacting herd size.
In accordance with 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, due to cattle shortage.
“As ranchers, we’re glad to see the FDA approving recent tools like this,” said sixth-generation Texas rancher James Clement III. “After we face fever ticks or screwworm outbreaks, having effective medications and coverings readily available is not optional; it’s essential,” he said. “These products give producers the flexibility to reply quickly, protect our herds, and safeguard the broader livestock industry,” he said, though he added ranchers may have some questions before they move ahead with use of the drug.
Since 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 may be risk within the spring, so we now have enough product available that we could deploy immediately to cattle ranchers for preventative motion,” Lehmann said. “Based on what we all know, this treatment may be very effective against screwworm, and you should contain this. So profit treatment actually becomes very critical,” Lehmann added.
