On June 3, 2026, the USDA confirmed the first U.S. case of New World screwworm (Cochliomyia hominivorax) in decades, after larvae were recovered from the umbilical wound of a calf in Zavala County, Texas.
As of June 8, additional confirmed cases have emerged in multiple Texas counties — including La Salle and Gillespie — alongside the first confirmed case in New Mexico, involving a dog in Lea County that had recently traveled from Mexico.
The New World screwworm, eradicated from the United States in 1966, has advanced steadily northward through Central America and Mexico since 2023, producing extensive outbreaks in livestock, pets, and wildlife.
Unlike ordinary flies, New World screwworm larvae are obligate parasites that enter through open wounds and consume living tissue. As the infestation progresses, they produce expanding lesions that can become severe without prompt intervention.
So what can be done about it? You guessed it… Ivermectin.
Ivermectin: Proven Activity Against New World Screwworms in Animals and Humans
Livestock
Ivermectin has shown remarkable effectiveness against New World screwworms in livestock.
In February 2026, the FDA granted an Emergency Use Authorization (EUA) for Ivomec (ivermectin) 1% injectable as an over-the-counter preventive for cattle exposed to screwworm risk — including after birth, castration, or wound detection.
Why? Because 12 field studies conducted in South America — where New World screwworm outbreaks are common — found that ivermectin provided more than 97% protection against screwworm infestations in wounds under real-world conditions.
Researchers found that a single injection could provide 10–21 days of protection, killing young screwworm larvae before they could burrow deeper into living tissue.
Humans
Published case reports suggest ivermectin can be effective when New World screwworms invade wounds in the mouth, eye socket, skin, or other tissues — a condition known as myiasis.
In a 2011 case series involving six patients, doctors treated individuals suffering from severe oral New World screwworm infestations following major facial trauma, where larvae had invaded damaged tissue in and around the mouth. Mechanical removal alone proved difficult because many larvae were deeply embedded in living tissue. Physicians administered subcutaneous ivermectin (200 μg/kg), which killed the remaining larvae and made them substantially easier to remove. All six patients experienced successful recovery without major complications, leading the authors to conclude ivermectin was an effective adjunctive therapy for severe oral infestation.
In a separate 2006 case series involving two patients, physicians treated severe orbital (eye socket) New World screwworm infestation in individuals with underlying skin cancer, where larvae had penetrated tissue surrounding the eye and posed a serious risk of progressive tissue destruction. After receiving oral ivermectin along with wound debridement, the larvae died and could be removed more completely, helping control the infestation and avoid more aggressive surgical intervention. The authors concluded that ivermectin appeared to play an important therapeutic role in severe cases of New World screwworm myiasis.
“Horse paste” wins again.
Epidemiologist and Foundation Administrator, McCullough Foundation
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