A Parasite That Eats Living Flesh Is Back in Texas. Here's What That Means

The Problem

U.S. federal agricultural inspectors have confirmed that a parasite called the New World screwworm has been found in a calf in Texas. This is the first time the fly has shown up in the continental United States since the 1960s, when a coordinated U.S.-Mexico eradication program wiped it out.

What makes this different from just a curiosity: the fly has been moving northward through Mexico for months. As of May 2026, it was within 60 miles of the U.S. border. The U.S. government has a system in place to fight it, but that system was never designed for a crisis this close to home, and it is being assembled as the threat unfolds.

The screwworm is native to South America. Over the past few decades, the biological barrier the U.S. and Mexico built together — essentially a line of defense — has been deteriorating. That barrier has now broken.

What the Screwworm Actually Does

A female screwworm fly lays eggs on open wounds or tender skin. The larvae burrow into living tissue and feed on it. This condition is called myiasis. Untreated, it can kill livestock in weeks, according to Reuters.

The fly does not just affect cattle. Pets, wildlife, and humans can catch it too. In August 2025, the U.S. confirmed its first-ever human case — a person who had traveled to an affected area and brought the infection home.

The economic stakes are real. Texas has one of the largest cattle herds in the country. If the screwworm spread widely, it would kill more calves, drive up veterinary costs, damage ranchers' profits, and create problems selling beef to countries that require strict pest-free certification. The American Veterinary Medical Association flagged this as a national threat, not just a regional one.

Medications: A Faster Response Than Before

Unlike in 1966, when the first eradication campaign began, producers now have pharmaceutical options. As of mid-2026, about a dozen government-approved drugs exist for screwworm treatment in cattle. Several were approved recently, using emergency pathways:

• Zoetis received approval for Dectomax-CA1 (doramectin) in September 2025. It is the only over-the-counter product specifically approved for screwworm treatment in cattle.
• Boehringer Ingelheim secured emergency authorization for an over-the-counter product in February 2026.
• FDA approved Cattle-CA1 (a topical liquid) in December 2024 for both prevention and treatment.
• Merck Animal Health added EXZOLT CATTLE-CA1, a pour-on product, in June 2026.

The older medications — permethrin sprays and coumaphos — are still used alongside these newer options. Texas A&M AgriLife Extension Service recommends coordinated use of sprays, aerosols, and other topical treatments to disrupt the fly's life cycle.

If a rancher suspects infestation, the first step is to call a veterinarian and state animal health officials, and preserve samples for diagnosis and tracking.

The Real Solution: Sterile Flies

Pharmaceuticals manage the problem. But the core solution — the one that actually stopped the screwworm in 1966 — is the Sterile Insect Technique, or SIT.

Here's the idea: scientists breed millions of male screwworms in the lab and sterilize them. Those sterile males are then released by airplane across the affected area. When a sterile male mates with a wild female, no offspring result. Do this at scale, generation after generation, and the wild population collapses.

The math works. But the logistics are brutal. You need industrial-scale breeding facilities running continuously. You need aircraft to spray millions of flies across hundreds of square miles, month after month. It is expensive, slow, and operationally demanding.

Yet it has a proven track record. Starting in 1972, the U.S. and Mexico worked together to push the screwworm's range steadily southward, eventually establishing a barrier in Panama. That took decades and enormous investment. Now the question is whether the same system can be mobilized quickly enough to stop the fly in Texas before it gets a foothold.

The Labor Crunch

There is a harder problem that no pill solves: ranches are short on workers.

Screwworm management is hands-on. Ranchers must spot infested wounds early, manually remove larvae, and apply treatments. This happens in open fields, often across thousands of acres. Technology cannot yet do this automatically at scale.

Texas A&M recommends that ranchers plan ahead: schedule calving, branding, and other procedures that create wounds during seasons when the fly is least active. That advice is actually old — it was standard practice before 1966. The fact that it is being revived shows how real this threat has become.

What Happens Next

As of June 2026, the confirmed case is still in Texas. Mexico is mounting its own containment efforts. The USDA is coordinating a response across animal health, veterinary, and public health channels.

The treatment toolkit is better than it was 60 years ago. But the real outcome depends on three things: whether the federal government can ramp up sterile fly production and release fast enough, whether the U.S.-Mexico program can hold the line together, and whether this is a one-time introduction or the start of an established population.

The USDA has not yet said which scenario it thinks is happening. When it does, that answer will matter most to ranchers and the people and institutions that depend on affordable beef.