Flesh-Eating New World Screwworm Advances Through Mexico, Prompting U.S. Border Response

Federal and state agricultural authorities are mobilizing a large-scale response as the New World screwworm, a parasitic flesh-eating fly capable of killing livestock and infecting humans, continues its northward advance through Mexico. Texas Agriculture Commissioner Sid Miller recently confirmed the pest has been detected roughly 70 miles south of the U.S. border, reigniting fears of an outbreak that the United States spent decades and billions of dollars to eradicate.

The New World screwworm is among the most destructive livestock pests in the Western Hemisphere. Unlike most flies, whose maggots feed on decaying tissue, screwworm larvae consume living flesh, boring deep into wounds on cattle, wildlife, pets, and occasionally humans. An untreated infestation, known as myiasis, can kill a full-grown cow within one to two weeks. A single female fly can lay up to 400 eggs at a time, and multiple females are often drawn to the same wound, accelerating tissue destruction. The pest can also infect humans, particularly those with open sores, immunocompromised conditions, or who sleep outdoors in affected regions.

The current northward spread marks the most serious screwworm threat to the United States in nearly half a century. The parasite had been pushed back to a biological barrier in Panama through a cooperative U.S.-Panama program known as COPEG, which released sterile male flies to collapse wild populations. However, outbreaks reported in Panama in 2023 allowed the species to breach that barrier, moving through Central America and into southern Mexico by late 2024. Since then, cases have steadily progressed north through Mexican states including Chiapas, Oaxaca, Veracruz, and most recently further toward the Rio Grande. U.S. officials have warned that a domestic outbreak could devastate the cattle industry — Texas alone produces roughly $15 billion in annual cattle revenue — and disrupt the broader North American food supply.

In response to the current threat, the USDA has deployed a multi-pronged strategy. Secretary of Agriculture Brooke Rollins announced plans to build a new sterile fly dispersal facility at Moore Air Base in South Texas, supplementing the existing COPEG plant in Panama, which currently produces around 100 million sterile flies per week. Officials have acknowledged that capacity will need to increase dramatically — potentially to 400 million or more weekly — to push the outbreak back south. Import restrictions on Mexican livestock remain in place, and surveillance has intensified along the border, with inspectors examining animals for signs of infestation. Texas officials are also coordinating with ranchers to report any suspicious wounds on livestock and wildlife.

Officials are urging the public, ranchers, hunters, and pet owners in border states to remain vigilant. Early detection is considered critical: a single undetected case in the United States could seed a localized population that would take years and significant funding to contain. The USDA, the Centers for Disease Control and Prevention, and state veterinarians are watching closely as the warmer months approach, when fly activity peaks. The coming weeks are expected to prove decisive in determining whether enhanced surveillance and sterile fly deployment can halt the parasite’s advance before it crosses into Texas.