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

Flesh-Eating Screwworms Have Entered the US: Everything You Need to Know

For the first time in more than six decades, the United States is facing a confirmed case of New World screwworm infestation. The U.S. Department of Agriculture (USDA) officially announced this week that the carnivorous fly larva — insects infamous for feeding on the living flesh of warm-blooded mammals — have been detected on American soil. The news has sent shockwaves through agricultural communities, veterinary circles, and public health agencies across the country. Here is a comprehensive breakdown of what screwworms are, why their return is so alarming, and what is being done to contain the threat.

What Exactly Is a Screwworm?

The New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae are among the most destructive agricultural pests ever recorded. Unlike most fly species whose maggots feed on decaying matter, screwworm larvae feed exclusively on the living tissue of warm-blooded animals — and they do so aggressively and relentlessly.

The adult screwworm fly is roughly the size of a common housefly, featuring a metallic blue-green body and distinctive orange eyes. What makes it uniquely dangerous, however, is not the adult insect but its offspring. Female screwworm flies lay their eggs — sometimes several hundred at a time — directly into open wounds, scratches, insect bites, navels of newborn animals, or any other exposed area of skin on a living host. Once the eggs hatch, the larvae burrow headfirst into the surrounding flesh, feeding and growing as they go.

The name "screwworm" comes from the way the larvae move through tissue, rotating in a corkscrew-like motion as they burrow deeper. Left untreated, a screwworm infestation can kill a large animal such as a cow, deer, or dog within a matter of days. In rare but documented cases, screwworms have also infested humans.

Why This Is Such a Big Deal: A Look Back at History

To understand why the USDA's announcement has triggered such concern, it helps to know just how catastrophic screwworm infestations once were in the United States. Prior to eradication efforts, screwworms caused hundreds of millions of dollars in losses to the American livestock industry every single year. Cattle ranchers in the southern United States lived in constant fear of the fly, and no animal — wild or domesticated — was safe from infestation during warm months.

The economic damage was so severe that the USDA launched one of the most ambitious pest eradication campaigns in agricultural history. Starting in the late 1950s and continuing through the 1960s, the government employed the Sterile Insect Technique (SIT) — a method involving the mass rearing and release of sterile male screwworm flies. When sterile males mated with wild females, no viable offspring were produced, gradually collapsing the wild population.

By 1966, screwworms had been completely eradicated from the continental United States. It was hailed as one of the greatest victories in the history of applied entomology and agricultural science. Keeping them out has required decades of sustained effort along the US–Mexico border, including ongoing sterile fly release programs maintained in cooperation with Central American nations.

How Did Screwworms Get Back Into the US?

The precise details of how the current infestation occurred are still being investigated by USDA officials. However, screwworm populations have never been fully eliminated from parts of South America, and the insects have periodically re-emerged in Central America and Caribbean nations in recent decades. Livestock movement, wildlife migration, and gaps in border biosecurity are among the most commonly cited pathways through which such pests can re-enter previously cleared regions.

The detection of the current case follows earlier alerts from border states and reports of unusual wound infestations in animals brought to veterinary clinics. The USDA's Animal and Plant Health Inspection Service (APHIS) confirmed the diagnosis after laboratory analysis of larval samples. Officials are now working urgently to determine the geographic scope of the infestation and whether additional cases exist that have not yet been identified.

Which Animals Are Most at Risk?

• Livestock: Cattle, sheep, goats, and pigs are among the most vulnerable animals due to their prevalence in agricultural settings and their frequent exposure to minor wounds during handling, branding, or routine veterinary procedures.
• Wildlife: Deer, bears, and other wild mammals can also be heavily impacted, with screwworm infestations capable of decimating local wildlife populations if left unchecked.
• Pets: Dogs and cats with open wounds, especially those that spend time outdoors, are also susceptible. Pet owners in affected areas are being advised to check their animals regularly for unusual wounds or signs of distress.
• Humans: While rare, human cases have been documented. Individuals with open wounds who are in affected areas should exercise heightened caution and seek medical attention immediately if they notice signs of infestation.

Recognizing the Signs of a Screwworm Infestation

Early detection is critical. A screwworm infestation typically begins as a wound that appears to worsen rapidly despite normal care. The affected area may emit a foul odor, appear unusually swollen or discolored, and upon close inspection, small cream-colored larvae may be visible moving within the wound cavity. Infested animals often exhibit extreme distress, restlessness, loss of appetite, and separation from the herd or group. Any animal displaying these symptoms should be examined by a licensed veterinarian immediately.

What Is Being Done to Contain the Outbreak?

The USDA has activated emergency response protocols and is coordinating with state agriculture departments, border authorities, and international partners to assess and contain the situation. Sterile insect release programs — the same technology that led to the original eradication — are expected to be a central component of the response strategy.

Livestock producers and pet owners in potentially affected regions are being asked to report any suspicious wound cases to their local veterinarian or state animal health official as soon as possible. Early reporting is essential to mapping the infestation and preventing further spread.

What Livestock Owners and Pet Owners Should Do Right Now

• Inspect all animals daily for open wounds, particularly around the face, ears, navel, and genitals.
• Treat any wound — no matter how minor — promptly and keep it clean and covered where possible.
• Contact a veterinarian immediately if you notice rapidly worsening wounds, unusual odor, or visible larvae.
• Report suspected cases to USDA APHIS or your state's department of agriculture without delay.
• Avoid unnecessary movement of livestock across state lines until officials issue further guidance.

The Bigger Picture: Biosecurity in a Changing World

The return of the screwworm to the United States is a sobering reminder that the victories of the past are never permanently guaranteed. Climate change, increasing global trade, shifting wildlife migration patterns, and complex international supply chains all create new opportunities for previously eradicated pests and diseases to re-enter regions where they once caused tremendous harm. Agricultural biosecurity must remain a national priority — not just a reactive measure deployed in moments of crisis, but a proactive, well-funded, and continuously maintained system of vigilance.

For now, authorities are urging calm but swift action. The screwworm has been beaten before, and the scientific tools to fight it still exist. Whether those tools can be deployed quickly enough to prevent widespread damage remains to be seen. Stay informed, stay vigilant, and report anything suspicious to the appropriate authorities without hesitation.