The New World screwworm, a parasitic fly whose larvae feed on the flesh of warm-blooded animals, has reemerged in Texas, threatening the U.S. cattle industry for the first time in roughly 50 years. The parasite was initially detected in a calf in Zavala County in early July, with confirmed cases now exceeding 30, largely concentrated in southern Texas. The infestation has raised concerns about how the screwworm entered the country and how it is spreading.

The screwworm’s larvae infest wounds on livestock, leading to potentially lethal infections. Although the fly itself can only travel limited distances, infections are believed to spread primarily through movement of infected animals, either domesticated or wild. However, no wild screwworm flies have been caught to date, and there have been no reported cases among wildlife.

Authorities have established quarantines in parts of more than a dozen Texas counties to control the outbreak, and multiple states have restricted the movement of livestock from Texas. The U.S. Department of Agriculture (USDA) is collaborating with the Department of Homeland Security to monitor the U.S.–Mexico border and is exploring innovative methods such as drones and artificial intelligence for wildlife surveillance.

One puzzling case outside Texas involved an infected dog in New Mexico, which recovered after treatment. Initial assumptions that the dog contracted the parasite in Mexico were later disputed when state officials confirmed the animal had not crossed the border, suggesting local transmission and hinting at a wider, undetected presence of the screwworm in the region. No further cases have been reported in New Mexico.

The debate over how the screwworm entered the U.S. has become politically charged. U.S. Agriculture Secretary Brooke Rollins attributed the parasite’s spread to what she described as the Biden administration’s “failed immigration policies” and “wide open border.” In contrast, Representative Teresa Leger Fernández (D-N.M.) criticized such claims as misinformation, cautioning against blaming immigrants for the outbreak. The USDA has also faced criticism, mostly from Democratic lawmakers, over budget cuts and agency restructuring, which some say may have impaired surveillance and response efforts.

Experts acknowledge significant uncertainty about the exact pathways of the screwworm’s introduction. Kevin Esch, executive vice president at Zoetis Inc., a company producing screwworm treatments, said much of the discussion is speculative. He noted that as cases in Mexico drew closer to the U.S. border, the parasite may have naturally migrated northward due to insufficient control efforts.

The resurgence of screwworm in the region traces back to a sharp increase in infections across Central America and Mexico beginning in 2023. This expansion followed decades of successful eradication programs led by the U.S., Mexico, and Central American countries, which had confined the parasite south of the Darién Gap—a dense 60-mile stretch of land between Panama and Colombia serving as a natural barrier. The main tool against the screwworm has long been the production and release of sterile flies in Panama to disrupt the parasite’s reproductive cycle.

Experts believe that disruptions caused by the COVID-19 pandemic, including reduced animal health surveillance, and illegal cattle trade throughout the region contributed to the screwworm’s resurgence and northward spread. Paolo Tizzani, a veterinary epidemiologist with the World Organization for Animal Health, noted that pandemic-related restrictions likely impaired ongoing prevention efforts. Jeremy Radachowsky of the Wildlife Conservation Society emphasized that illegal cattle movements along smuggling routes in Central America have sustained reinfestation cycles, complicating eradication efforts.

In response, the U.S. and Mexico recently opened a new facility in Metapa, Mexico, to mass-produce sterile screwworm flies, aiming to limit the parasite’s reproduction. Still, officials warn that containment and eradication are likely to take years and will require coordinated efforts across borders to address both animal health measures and illegal livestock trade.