Imagine standing at the fence line of your ranch, watching your herd shrink year after year, knowing that somewhere just across the border a flesh-eating parasite is waiting for its chance to jump the line. For American cattle producers, that nightmare just got a little less terrifying.
On December 4, 2025, the Food and Drug Administration handed down some rare good news to an industry that’s been taking body blows for years. They granted conditional approval to a new topical treatment that promises to stop the New World screwworm in its tracks – and just maybe keep what’s left of America’s cattle herd from disappearing altogether.
A New Defense in a Long-Running War
The drug goes by the name Exzolt Cattle-CA1, developed by Merck Animal Health. It’s a pour-on solution containing fluralaner – yes, the same active ingredient you’ve probably seen in flea and tick treatments for dogs – but now reformulated specifically for beef cattle.
Ranchers will be able to get their hands on it through veterinarians starting December 20. That’s cutting it close for an industry already on edge, but as one Texas rancher told me recently, “When you’re fighting for survival, any lifeline matters.”
Why This Approval Actually Matters
Let’s be brutally honest – the U.S. cattle herd is in its worst shape since the Eisenhower administration. As of November 2025, cattle on feed numbers sit at just 11.7 million head. That’s down 260,000 from last year and represents the smallest herd since 1951.
Every single animal counts right now.
The screwworm threat isn’t theoretical. This isn’t some distant possibility that might happen someday. The parasite has already exploded through Mexico, forcing repeated closures of the U.S. border to live cattle imports. Before the closures began, Mexico shipped over a million calves north every year – calves that helped keep U.S. feedlots full and beef prices from going completely through the roof.
Those imports have essentially stopped.
Understanding the Enemy
If you’ve never seen what screwworm does to cattle, count yourself lucky. The adult flies lay eggs in open wounds – even small ones from branding or dehorning. When the larvae hatch, they don’t just feed on dead tissue like most maggots. These monsters eat living flesh.
I’ve talked to producers who’ve found calves with infestations so severe the animals had to be euthanized. The pain is excruciating. Untreated, mortality approaches 100%. And yes, before you ask – humans can get it too, though that’s mercifully rare in modern times.
“When we face fever ticks or screwworm outbreaks, having effective medications and treatments on hand isn’t optional; it’s essential.”
– James Clement III, sixth-generation Texas rancher
Clement isn’t exaggerating. The last major screwworm outbreak in the United States was eradicated in the 1960s through the sterile insect technique – releasing millions of sterilized male flies to crash the population. That program cost hundreds of millions of dollars and took decades.
Mexico’s current outbreak suggests that program has broken down somewhere along the line.
How the New Treatment Actually Works
Exzolt Cattle-CA1 represents a completely different approach. Rather than trying to control the fly population across millions of square miles, it makes the cattle themselves deadly to screwworm larvae.
The fluralaner gets absorbed through the skin and distributes throughout the animal’s body. When screwworm larvae start feeding, they ingest the compound and die. Simple, brutal, and apparently very effective based on the data Merck submitted.
The treatment also works against cattle fever ticks, which is a nice bonus for producers in the permanent quarantine zone along the Texas-Mexico border. Two birds, one stone.
- Single application provides protection against screwworm
- Also controls cattle fever ticks
- 98-day meat withholding period (long, but necessary for food safety)
- Available through veterinarians starting December 20, 2025
- Already being used successfully in Mexico
The Bigger Picture for Beef Prices
Anyone who’s bought steaks lately doesn’t need me to tell them beef is expensive. What they might not realize is how precarious the supply situation has become.
When Tyson Foods announced the closure of their Lexington, Nebraska beef plant – one of the largest in the country – the writing was on the wall. There simply aren’t enough cattle to keep these massive facilities running at capacity.
The screwworm situation in Mexico removed what had been a crucial pressure valve for the U.S. industry. Those Mexican calves weren’t just filler – they represented about 3.3% of America’s annual calf crop. In a business where margins are measured in pennies per pound, that’s enormous.
Add drought, high feed costs, and years of herd liquidation, and you get the perfect storm we’re living through right now.
What Happens Next
Merck’s people are cautious – refreshingly so – about what this approval actually means. They’re clear that this drug won’t eradicate screwworm in Mexico. Experts there are talking about a multi-year battle to get the parasite back under control.
What it can do is protect what’s left of the U.S. herd if (when) the parasite makes its way north. Winter cold currently keeps fly activity low, but come spring? Every producer along the border knows the risk skyrockets.
Having product already in the country and ready to deploy matters more than people outside agriculture probably realize. When you’re dealing with biological threats, speed is everything.
“We have enough product available that we could deploy immediately to cattle ranchers for preventative action.”
– Holger Lehmann, Merck Animal Health
That’s not marketing speak. That’s the kind of preparation that can make the difference between containing an outbreak and watching it explode across multiple states.
The Human Element
Perhaps the most interesting aspect of this whole situation is watching how quickly government agencies can move when they really need to. The conversations between Merck and regulators started in July. Five months from first discussion to conditional approval is lightning fast by FDA standards.
That tells you everything about how seriously they view the threat.
For ranchers, the approval brings something that’s been in short supply lately: options. When your herd is your life’s work, and every year seems to bring a new existential threat, having another tool in the toolbox means more than any city person can probably understand.
Will this solve America’s cattle shortage? Of course not. Will it help prevent a parasitic disaster that could make the current situation look mild by comparison? Absolutely.
Sometimes that’s enough.
The beef on your plate next year might cost more than you’d like. But thanks to some quick work by regulators and a pharmaceutical company that saw the writing on the wall, there’s at least a fighting chance that there will still be beef on that plate at all.
In an industry that’s been losing ground for years, that’s not nothing.
