Cattle detained in the pens of the Unión Ganadera Regional de Chihuahua, at the Jerónimo-Santa Teresa border crossing in Ciudad Juárez, Mexico, on November 27, 2024, after the United States stopped imports of Mexican cattle due to the presence of screwworm.
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American ranchers will soon have a new way to protect their dwindling herds from the threat of the parasitic screwworm, which is decimating livestock in Mexico.
The U.S. Food and Drug Administration gave conditional approval to a drug called EXZOLT CATTLE-CA1, a topical treatment for merck Animal Health for the prevention and treatment of the New World screwworm. It can also be used as a treatment and control of cattle fever tick.
“The conversation started in July with the FDA, and because there is a human food safety element, we had to generate a large data package,” said Holger Lehmann, vice president of pharmaceutical research and development at Merck Animal Health. “This approval is an important task. The United States received the product and Mexico received it in early November, where it is being used,” Lehmann said.
Each dose is effective for 21 days before a new dose is necessary. The FDA approved it with a 98-day retention period to ensure there are no residues in the meat.
Merck Animal Health is prioritizing shipping product first to U.S. distribution partners located in areas along the U.S.-Mexico border that are most likely to have an immediate need, before the end of the year. The treatment will be more widely available to veterinarians and producers in mid-January 2026. Merck had previously said it would be more widely available on December 20.
Lehmann warned that the drug alone cannot eradicate the parasite in the short term. “Experts tell us in Mexico that they don't expect to be able to quickly get rid of the screwworm problem,” Lehmann said. “They think it's a problem that will take several years to solve.”
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 use in cattle.
Merck Animal Health
Screwworm is transmitted by incubating fly eggs in the open wounds of cattle, which feed on their living tissue. Humans can also become infected. To protect the American cattle herd and stop the spread of the parasitic fly, the United States Department of Agriculture has closed the border with Mexico to imports of live cattle, bison and horses, intermittently, since 2024.
The border remains closed.
Before the shutdown, Mexico was an exporter of calves to the US, and USDA data showed that the US imported more than one million head of cattle annually, about 3.3% of the total US calf crop.
The screwworm outbreak in Mexico is one of the reasons for volatility in the cattle futures marketand behind high beef costs, which has become a high-profile issue for the Trump administration amid declining poll numbers on the president's economy.
President Trump has blamed American meat processors and ranchers for the higher costs. Tariffs associated with feed and farm equipment have been linked to rising beef prices, along with drought affecting herd sizes.
According to USDA data, in November 2025, the US cattle herd destined for feed was 11.7 million head, down 2%, or 260,000 head, from 2024. That represents a US cattle herd at its lowest level since 1951.
In November, Tyson Foods announced that it would close its main beef plant in Lexington, Nebraska, and reduce its operations in Amarillo, Texas, due to a livestock shortage.
“As ranchers, we're happy to see the FDA approve new tools like this,” said James Clement III, a sixth-generation Texas rancher. “When faced with febrile ticks or screwworm outbreaks, having effective medications and treatments on hand is not optional; it is essential,” he said. “These products give producers the ability to respond quickly, protect our herds and safeguard the livestock industry as a whole,” he said, although he added that ranchers will have some questions before moving forward with using the drug.
Because it's winter and temperatures are colder, Lehmann said, the likelihood of flies carrying screwworm from Mexico to the U.S. is currently low. “But there is a risk in the spring, so we have enough product available to be able to send it immediately to farmers for preventative measures,” Lehmann said. “Based on what we know, this treatment is very effective against screwworm, and it is important to contain it. Therefore, treatment with benefits actually becomes very critical,” Lehmann added.
