Screwworms mainly affect livestock but can also infest humans, according to the U.S. Centers for Disease Control and Prevention (CDC). Mexico confirmed its first human case in April in a 77-year-old woman, who was treated with antibiotics and is stable.

The US Department of Agriculture has suspended the transport of live cattle, horses and bison through ports of entry along the border with Mexico for at least two weeks. It’s part of an effort to curb the spread of New World screwworm,

The screwworm larva can devastate any mammal or bird it burrows into, raising alarms as flies spread northward.