New World screwworm (NWS) is a parasitic fly that lays its eggs in warm-blooded animals’ exposed soft tissue, flesh wounds, and body orifices, including the eyes, ears, nose, mouth, and genital areas. The eggs hatch into maggots that burrow into and feed on the flesh.
The parasitic infection can occur in people, though it’s uncommon in the U.S.; the biggest risk is to livestock and wild deer populations. The U.S. eradicated NWS from the U.S. and Mexico in the 1960s by breeding millions of male flies, sterilizing them with radiation, and then releasing them for females — who only mate once in a lifetime — to mate with and lay unfertilized eggs.
But NWS began creeping back north from Central America in 2023, and the first U.S. case since eradication was reported in June 2026.