New World screwworm reaches the United States as North American response intensifies

New World screwworm (Cochliomyia hominivorax) continues to advance northward through Mexico and into the United States, where animal infestations have been confirmed since June 2026. The parasite threatens cattle in particular, but horses, small ruminants, pigs, dogs, cats and wildlife are also susceptible. Control efforts now rely heavily on surveillance, movement restrictions and a major expansion of sterile-fly production and release capacity.

The scale of the regional outbreak remains substantial. As of 7 October, countries in Central America and Mexico had reported more than 209,400 animal cases and more than 2,700 human cases. In the United States, however, the CDC had recorded no locally acquired human infestation by that date and continued to assess the risk to the general population as very low.

United States: animal cases since June

The first U.S. animal case of the current episode was confirmed by USDA on 3 June 2026 in Zavala County, Texas. The affected animal was a three-week-old calf with larvae in the umbilical region. Additional detections followed rapidly. By 8 June, USDA had confirmed cases in calves and a dog, including an infested dog in Andrews County that had reportedly recently been in Mexico. A further calf case in La Salle County brought the national total to six confirmed cases by 9 June.

The outbreak has since expanded geographically. According to the Texas Animal Health Commission, the 2026 outbreak had involved 17 Texas counties and 43 premises with at least one confirmed detection. These figures refer to affected counties and premises, not the number of individual animals, which is tracked separately by USDA.

Which animals are at risk?

New World screwworm larvae feed on the living tissue of warm-blooded animals, usually entering through wounds. Cattle are a major concern because calves may be infested through the umbilicus, while branding, castration, dehorning, calving injuries and other skin wounds can create entry points.

But the host range is broad. USDA and state authorities warn that the parasite can affect livestock, pets and wildlife. Horses, donkeys and mules are susceptible, as are sheep, goats and pigs. Dogs and cats can develop rapidly progressive lesions if infestations are not detected and treated early. Wildlife surveillance is also critical because infected free-ranging mammals could maintain transmission beyond livestock premises.

Humans are also susceptible to myiasis, although infections are much less common than in animals.

Surveillance is becoming increasingly intensive

The U.S. response combines veterinary examination, wound inspection, fly trapping and wildlife surveillance. Since the first Texas detection, USDA reports conducting more than 1,000 drone surveillance flights, observing over 20,000 animals, while more than 130 trail cameras have recorded nearly 17,000 animals. Confirmed animal detections and wild-fly captures are recorded in a national USDA dashboard by state, county, species and confirmation date. Individual cases remain classified as active while treatment and wound management continue.

Movement restrictions and infested zones

Texas has established regulated areas around affected locations. As of 12 October, movement restrictions remained in place in parts of Brewster, Crockett, Edwards, Jeff Davis, Pecos, Presidio, Schleicher, Starr, Sutton, Terrell, Val Verde and Zapata counties. Warm-blooded animals cannot be moved out of infested zones without prior authorisation and inspection. The objective is to prevent infested animals from transporting larvae into areas where adult flies could establish new populations.

Sterile flies remain the cornerstone of eradication

The main biological control strategy is the sterile insect technique: enormous numbers of sterilised male flies are released so that wild females mate without producing viable offspring. The operational facility at Pacora, Panama, currently produces and disperses about 100 million sterile flies per week. A renovated production facility at Metapa, Mexico was completed in June 2026 and is being scaled toward another 100 million sterile flies per week. USDA is also building a major domestic production facility at Moore Air Base in Texas, designed for a projected capacity of 300 million sterile flies per week. A separate dispersal facility at Moore is already operational and can release up to 100 million flies per week.

Additional dispersal capacity is operating in Mexico, including Hermosillo, Tuxtla and Tampico, allowing sterile flies to be deployed across a progressively larger part of the outbreak zone.

A major animal-health challenge despite low human risk

The return of New World screwworm to the United States represents an important epidemiological change after decades of successful eradication. For livestock production, the immediate risks are animal suffering, reduced productivity, mortality in severe untreated cases, movement restrictions and potentially major trade consequences. Wildlife also creates a difficult surveillance challenge because infestations may occur outside managed livestock systems.

The public-health picture is currently more reassuring. No locally acquired human case had been reported in the United States by 7 October, and CDC considers the overall population risk very low, although it acknowledges that human infections could occur where infected flies are circulating.