One of the most successful eradication programs in the history of American public health and agriculture achieved something remarkable: the complete elimination of the New World Screwworm fly (Cochliomyia hominivorax) from the United States in 1966. The screwworm — named for the screw-like motion its larvae use to burrow into the living flesh of warm-blooded animals — is considered one of the most economically and medically devastating livestock parasites in the Western Hemisphere. For decades, its absence from U.S. soil has protected American cattle, horses, wildlife, and, in rare cases, humans, from a condition that can be fatal if untreated.
That protection was breached in 2026. USDA Animal and Plant Health Inspection Service (APHIS) confirmed the detection of New World Screwworm in cattle in Maverick County, Texas — a county along the Rio Grande in southwest Texas that borders Mexico’s Coahuila state. The detection followed confirmation of the parasite’s re-emergence in livestock in Mexico in 2024 and 2025, where USDA and Mexican authorities have been engaged in emergency sterile insect technique (SIT) operations to prevent northward spread. USDA closed the US-Mexico border to cattle imports in November 2024 following the initial Mexican detections, citing the risk of screwworm importation — a decision that proved prescient.
CDC confirmed on June 12, 2026 that it is supporting the USDA and Texas Department of State Health Services in an ongoing investigation — a statement that underscores both the agricultural and the human health dimensions of this re-emergence.
What New World Screwworm Actually Does
The New World Screwworm fly (C. hominivorax) is a parasitic fly species that lays eggs exclusively in fresh wounds, natural body openings, or moist, warm skin surfaces of living warm-blooded animals. After hatching, the larvae burrow deeply into living tissue — spiraling as they feed, which is the source of the screwworm name. Unlike common maggots that decompose already-dead tissue, screwworm larvae consume only living flesh. The infestation, called myiasis, grows rapidly: a single wound can attract multiple female flies, each laying 200 to 400 eggs, and the resulting mass of larvae can destroy large quantities of living tissue in days.
In livestock, screwworm infestations kill cattle, horses, deer, pigs, sheep, and goats when untreated. The initial wound — which can be as small as a tick bite, a scratch, or a natural orifice — becomes a rapidly enlarging, foul-smelling cavity teeming with larvae. Animals in screwworm-endemic regions without adequate veterinary attention can die within 7 to 14 days of initial infestation. The economic destruction during the era of active US screwworm infestation in the early 20th century was estimated at hundreds of millions of dollars annually.
For humans, screwworm myiasis is less common but well documented. Cases have been reported in international travelers returning from Latin America and in individuals with open wounds in areas of screwworm activity. Infestations can occur in wounds, nasal passages, ear canals, and in severe immunocompromised patients, in ulcerated skin. Human screwworm myiasis is treatable with topical and mechanical removal of larvae plus appropriate wound care, but requires prompt recognition and intervention.
The Eradication Program That May Need to Restart
The original US screwworm eradication campaign, conducted jointly between the United States and Mexico from the 1950s through the 1960s, used a technique called the Sterile Insect Technique (SIT): mass rearing of screwworm flies followed by radiation-induced sterilization and mass aerial release of sterile male flies into the environment. Sterile males compete with wild males for females, but their matings produce no offspring. As the proportion of sterile males in the environment increases, the wild population collapses. The same SIT approach is now being applied in an emergency fashion in Mexico to create a biological barrier against northward spread.
USDA APHIS maintains a standing New World Screwworm national response plan for exactly this scenario. The detection of screwworm in Texas cattle triggers immediate tracing of animal movements, quarantine of affected premises, investigation of the wound site and larvae, laboratory confirmation, and deployment of emergency SIT operations to the affected area.
Any cattle producer, rancher, veterinarian, or wildlife manager in Texas and neighboring states who observes a wound in a warm-blooded animal — cattle, horses, deer, dogs, goats — with unusual tissue destruction, foul odor, or visible larvae should contact USDA’s Animal Disease Hotline at 1-866-536-7593 immediately.
Frequently Asked Questions
Q: What is the New World Screwworm?
A: Cochliomyia hominivorax is a parasitic fly that lays eggs in the living flesh of warm-blooded animals. Its larvae burrow into and consume living tissue, causing infestations that can be rapidly fatal in livestock and occasionally affect humans.
Q: When was screwworm eradicated from the United States?
A: 1966, through a decades-long program using the Sterile Insect Technique — mass release of radiation-sterilized male flies to collapse the wild population. The program was one of the most successful pest eradication campaigns in US history.
Q: How did screwworm return to Texas in 2026?
A: Following re-emergence in Mexican livestock in 2024 and 2025, cattle crossing the US-Mexico border into Texas served as an importation pathway. The US had closed the border to cattle imports in November 2024 in response, but the detection occurred despite those measures.
Q: Can humans get screwworm?
A: Yes, though rarely. Human myiasis occurs when fly larvae infest open wounds, natural body openings, or skin surfaces. It is treatable but requires prompt identification and removal. Travelers to Latin America with open wounds should take particular care.
Q: What should ranchers or farmers do if they suspect screwworm?
A: Contact the USDA Animal Disease Hotline immediately at 1-866-536-7593. Do not treat the wound without veterinary guidance, as proper sample collection for laboratory confirmation is essential before the wound is cleaned.
