Flesh-Eating Screwworms Have Returned to the US: What You Need to Know

Flesh-Eating Screwworms Have Returned to the United States

In a development that has alarmed agricultural officials, veterinarians, and public health experts alike, the United States Department of Agriculture (USDA) has confirmed the first known domestic infection of the New World screwworm since the pest was eradicated from American soil in the 1960s. The carnivorous fly larvae, which feed on the living flesh of warm-blooded mammals, represent one of the most destructive agricultural pests ever documented — and their reappearance on US territory is raising urgent questions about biosecurity, livestock safety, and the durability of decades-old eradication victories.

What Exactly Is a Screwworm?

The New World screwworm (Cochliomyia hominivorax) is not your average household fly. This parasitic insect has earned its gruesome nickname through a particularly horrifying biological behavior: its larvae, after hatching from eggs deposited in open wounds or natural body orifices of living animals, burrow deep into flesh and feed on living tissue. Unlike many fly species whose larvae consume dead or decaying matter, screwworm maggots require living host tissue to survive and develop.

The name itself comes from the螺旋-shaped burrowing motion the larvae use as they drill deeper into the wound — resembling a screw being driven into wood. Left untreated, an infestation can kill a large animal within days. Even minor wounds, including those caused by ticks, barbed wire, or routine ear-tagging, can serve as entry points for egg-laying female flies.

How Do Screwworms Spread?

Female screwworm flies are opportunistic and aggressive. They are attracted to the smell of blood and fresh wounds, and a single female can lay between 200 and 400 eggs at a time. Once hatched, larvae develop rapidly, feeding for five to seven days before dropping to the ground to pupate. In warm climates, the entire life cycle from egg to adult fly can be completed in as little as three weeks, allowing populations to explode with terrifying speed if left unchecked.

The pest is endemic to parts of South America, Central America, and the Caribbean. Animals — both domestic and wild — can carry larvae across borders, making international trade and wildlife migration key vectors for reintroduction.

The Historic Eradication: A Scientific Triumph Now Under Threat

The United States fought a decades-long battle against the New World screwworm throughout much of the 20th century. At its peak, the pest was causing catastrophic losses to the American cattle industry, with some estimates suggesting hundreds of millions of dollars in annual damage. The screwworm was a defining agricultural crisis of its era.

The solution came in the form of one of the most innovative pest-control programs in scientific history: the Sterile Insect Technique (SIT). Developed by USDA entomologists Edward Knipling and Raymond Bushland — work that would later earn them the World Food Prize — the technique involved mass-rearing male screwworm flies, sterilizing them with radiation, and releasing them into the wild in enormous numbers. When sterile males mated with wild females, no viable offspring were produced. Over successive generations, wild populations collapsed.

By 1966, the screwworm had been eradicated from the continental United States. The program was subsequently extended southward through Mexico and Central America, eventually pushing the pest's established range below the Darién Gap in Panama, where a biological barrier has since been maintained through continuous sterile fly releases.

Why the 2025 Reappearance Is So Alarming

The USDA's confirmation of a new US infection signals that the biological barrier maintained at the Panama-Colombia border may have been breached — or that the larvae arrived via an alternative pathway, such as an infected animal entering through a port of entry. Officials have not yet disclosed the precise location of the confirmed case or the species of animal affected, but the implications are far-reaching.

• The US livestock industry, which was effectively screwworm-free for over half a century, may lack the institutional preparedness and field-level awareness needed to detect and respond to cases quickly.
• Wildlife populations, including white-tailed deer, bears, and other native mammals, could serve as reservoirs that make eradication far more complicated than it was in the 1960s.
• Climate change has expanded the geographic range where screwworm flies can thrive, potentially allowing them to establish in states where they previously could not overwinter.
• In rare cases, screwworm larvae can infect humans — a condition known as myiasis — particularly affecting vulnerable individuals with open wounds or compromised immune systems.

What Is the USDA Doing About It?

Following the confirmed detection, USDA Animal and Plant Health Inspection Service (APHIS) officials are expected to mobilize rapid response teams, increase surveillance at ports of entry, and coordinate with state veterinarians to issue guidance to livestock producers and pet owners. The agency has a long history of managing the screwworm threat at international borders and retains the capacity to scale up sterile fly production if a wider outbreak is detected.

Livestock producers are being urged to inspect their animals regularly for signs of infestation, which include restlessness, foul odor from wounds, and visible larvae in wound tissue. Early detection is critical, as infestations can be treated effectively with approved parasiticides when caught in time.

What Livestock Owners and Pet Owners Should Do Now

Whether you raise cattle, keep horses, or simply live in a rural area, awareness is your first line of defense. Agricultural experts recommend the following precautions in light of the confirmed reintroduction:

• Inspect all animals daily, paying close attention to any existing wounds, recent surgical sites, or areas prone to tick attachment such as ears, the groin, and around the eyes.
• Treat all wounds promptly and keep them clean and covered where possible to reduce attractiveness to egg-laying flies.
• Report any suspected screwworm infestations immediately to your state veterinarian or local USDA APHIS office — do not wait to see if a wound resolves on its own.
• Be cautious when importing animals from regions where screwworm remains endemic, and ensure all regulatory health certifications are current and verified.

A Wake-Up Call for Biosecurity

The return of the New World screwworm to American soil is a stark reminder that eradication is never truly permanent. The natural world does not respect borders, and biological pest-control victories require ongoing vigilance to maintain. The 1960s eradication was a monumental scientific achievement — but sustaining that achievement in an era of increasing global trade, climate change, and shifting wildlife corridors demands continuous investment and preparedness.

For now, federal and state agencies are working to assess the scope of the reintroduction and determine the most effective containment strategy. The screwworm's return does not mean an outbreak is inevitable, but complacency at this stage would be a serious mistake. Farmers, veterinarians, wildlife managers, and ordinary pet owners all have a role to play in catching and containing this pest before it reestablishes a foothold on American soil.

Stay informed, inspect your animals, and report anything suspicious. The battle against the screwworm was won once — with vigilance, it can be won again.