FDA approves Merck drug for decimated cattle herds to combat snailworms
On November 27, 2024, cattle were held in Chihuahua Regional Livestock Union stables at the Jeronimo-Santa Teresa border crossing in Ciudad Juarez, Mexico, after the United States stopped imports of Mexican cattle due to the presence of screwworms.
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U.S. cattle farmers will soon have a new way to protect their dwindling herds from the threat of the screwworm parasite that is decimating cattle in Mexico.
The U.S. Food and Drug Administration has granted conditional approval to a drug called EXZOLT CATTLE-CA1, a topical treatment for Merck Animal health for the prevention and treatment of New World screwworm. It can also be used to treat and control the cattle fever tick.
“The conversation started in July with the FDA, and because there is an element of human food safety, we had to create a large data package,” said Holger Lehmann, vice president of pharmaceutical research and development at Merck Animal Health. “This approval is a significant undertaking. The US received the product, Mexico received it in early November, where it is being used,” said Lehmann.
Each dose is effective for 21 days before a new dose must be given. The FDA approved it with a shelf life of 98 days to ensure there is no residue in the meat.
Merck Animal Health is prioritizing shipping the product by the end of the year initially to U.S. distributors in areas along the U.S.-Mexico border where there is a greater likelihood of immediate need. The treatment will be more widely available to veterinarians and producers from mid-January 2026. Merck previously announced that it would be more widely available starting December 20th.
Lehmann warned that the drug alone could not eradicate the parasite so quickly. “Experts tell us in Mexico that they don’t expect to be able to get rid of the screwworm problem quickly,” said Lehmann. “They think it’s a perennial problem that needs to be solved.”
On December 4, 2025, the U.S. Food and Drug Administration (FDA) granted conditional approval for Exzolt Cattle-CA1, a topical fluralaner solution from Merck Animal Health, for use in cattle.
Merck Animal Health
The screwworm is spread by hatching fly eggs in the open wounds of cattle that feed on their living tissue. People can also become infected. In order to protect the US cattle population and stop the spread of the parasitic fly, the US Department of Agriculture has temporarily closed the border with Mexico to the import of live cattle, bison and horses since 2024.
The border remains closed.
Before the shutdown, Mexico was an exporter of calves to the United States. Data from the USDA shows that the U.S. imported over a million cattle annually, accounting for about 3.3% of the U.S. total calf crop.
The screwworm outbreak in Mexico is one of the reasons for this Volatility in the livestock futures marketand behind the high cost of beef, which has become a major issue for the Trump administration amid the president’s declining poll numbers on the economy.
President Trump blames meat processors and U.S. ranchers for the higher costs. The tariffs on animal feed and farm equipment are linked to rising beef prices and drought affecting herd sizes.
According to the USDA, the number of U.S. cattle on feed was 11.7 million head in November 2025, a decrease of 2% or 260,000 head compared to 2024. That represents a U.S. cattle population at its lowest level since 1951.
In November, Tyson Foods announced that the company would close its large beef plant in Lexington, Nebraska, and limit operations in Amarillo, Texas, due to the cattle shortage.
“As ranchers, we are pleased to see the FDA approve new tools like this,” said James Clement III, a sixth-generation Texas rancher. “When we are struggling with fever ticks or snailworm infestations, the availability of effective medications and treatments is not optional; it is essential,” he said. “These products give producers the ability to respond quickly, protect our herds and protect the entire livestock industry,” he said, but added that ranchers will have some questions before moving forward with using the drug.
Because it is winter and temperatures are cooler, the likelihood of flies transmitting screwworms from Mexico to the U.S. is currently low, according to Lehmann. “But there is a risk in the spring, so we have enough products available that we can immediately provide to ranchers for preventive measures,” said Lehmann. “As far as we know, this treatment is very effective against screwworm, and you want to control it. So the benefit of treatment actually becomes very important,” Lehmann added.
