The New World screwworm, a parasitic fly that harms both cattle and humans and was once eradicated from the U.S. after more than three decades of effort, is raising new concerns amid an outbreak in Mexico. In response to the looming threat, the U.S. Department of Agriculture (USDA) has announced a halt on imports of cattle, bison, and horses through the southern border.

The New World screwworm produces larvae that feed on the flesh and blood of livestock, pets, wildlife, birds, and, in some cases, humans, causing painful infestations that can lead to potentially fatal damage to their hosts. It was eradicated in the U.S. in 1966 after billions of dollars were spent on an operation that involved releasing hundreds of millions of sterile adult flies, which would mate with wild females and ultimately prevent them from laying viable eggs.

The recent threat emerged after the maggot fly was detected on remote farms in Mexico with minimal cattle movement, reaching as far north as Oaxaca and Veracruz, about 700 miles from the U.S. border.

"Due to the threat of New World Screwworm, I am announcing the suspension of live cattle, horse, & bison imports through U.S. southern border ports of entry effective immediately. The last time this devastating pest invaded America, it took 30 years for our cattle industry to recover. This cannot happen again," the USDA Secretary, Brooke Rollins, announced in an X post.

The first case of infestation in Mexico was reported to the U.S. in November 2024, following which the USDA shut down the border for live animal trade. Trade resumed in February 2025, after the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) and Mexican authorities put in place a thorough inspection and treatment process to safely manage animal movement and reduce the risk of New World screwworm.

The USDA announced that APHIS is deploying sterile flies by air and on the ground at key sites, focusing on southern Mexico and other parts of Central America.

"The protection of our animals and the safety of our nation's food supply is a national security issue of the utmost importance. Once we see increased surveillance and eradication efforts, and the positive results of those actions, we remain committed to opening the border for livestock trade. This is not about politics or punishment of Mexico, rather it is about food and animal safety," Secretary Rollins said in a news release.

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