The Flesh-Eating Parasite Creeping Toward the U.S.—And Why Vets Are the Only Ones Who Can Stop It

USDA Sounds the Alarm: New Screwworm Threat Could Devastate U.S. Animals—Veterinarians Urged to Prepare Now

A dangerous flesh-eating parasite is inching closer to the United States—and veterinarians may be the last line of defense.

The New World screwworm (NWS), a deadly fly whose larvae consume the living tissue of warm-blooded animals, has breached a once-reliable barrier in Panama and is rapidly moving north. Now, animal health officials warn that U.S. veterinarians must be ready to detect and respond before the parasite spreads unchecked.

To help, the U.S. Department of Agriculture (USDA) has launched a new training module—“Module 41: New World Screwworm: A 21st Century Perspective”—to educate veterinary professionals on how to identify, report, and stop NWS in its tracks.

Why This Parasite Is So Dangerous

The New World screwworm fly, Cochliomyia hominivorax, lays eggs on open wounds—no matter how small. The resulting larvae burrow into flesh, often going undetected until thousands have infested the animal. Infections cause severe pain, decay, and, if untreated, death.

Just one female fly can lay up to 3,000 eggs in her short 10- to 30-day lifespan. That means a single unnoticed case could spark an outbreak capable of devastating livestock, pets, and wildlife alike.

Veterinarians on the Front Line

According to the USDA’s new training module, veterinarians and veterinary technicians are uniquely positioned to spot the first signs of infection—especially in animals traveling across borders or presenting with mysterious wounds.

“You are our first line of defense,” the USDA module states. “You may be the first one to see an NWS larvae that’s entered the country. Early recognition and reporting are essential.”

What to Watch For

Veterinary professionals are urged to look for:

• Unusual wounds, often near the eyes, nose, mouth, ears, or genitals

• Foul-smelling discharge, pus, or blood from lesions

• Visible larvae by day three post-infestation, measuring 2 mm to 1.5 cm

• Behavior changes like head shaking, isolation, or excessive rubbing

• Animals standing in water or rubbing against trees or fences

If a suspected case is identified, veterinarians must report immediately to both state and federal animal health authorities. The USDA’s after-hours emergency line is (866) 536-7593.

The Threat Is Real—and Getting Closer

After crossing Panama’s Darien Gap in 2022, screwworm cases exploded from 25 to over 6,500 in 2023. Since then, it has been confirmed in Costa Rica, Nicaragua, Honduras, Guatemala, and now Mexico. In some regions, the parasite has even infected people—30 human cases have been reported in Nicaragua alone.

Mexico reported a positive NWS case to U.S. authorities in November, prompting the USDA to temporarily suspend cattle and bison imports. Imports have resumed under strict new protocols, including pre-clearance and mandatory treatment for screwworm.

USDA’s Message to Veterinary Teams: Be Ready

The USDA, along with partners like APHIS and international animal health agencies, is racing to contain the outbreak. But if NWS crosses the U.S. border, it could take months—and millions—to control.

That’s why every veterinary professional, regardless of specialty, is being called to act. Review the USDA’s training. Know the signs. Report anything suspicious.

Because in the fight against this parasite, early detection isn’t just important—it’s everything.