Screwworm

Screwworms were last seen in USA in the 1970s and had been considered eradicated. If it returned, its effect could be catastrophic to wildlife and livestock. In 2016, however, this parasite made reappearance, with first signs that seemed baffling. Infected deer were stumbling around in confusion, howling in pain, found with their heads hideously disfigured as if eaten alive.