Animal health companies are repurposing old drugs to kill the New World screwworm, a flesh-eating parasite affecting cows, sheep, and other mammals that was detected in the US last week. The New World screwworm is a type of fly that lays eggs in wounds on animal skin. When the larvae hatch, they eat the animal’s flesh. An infestation can seriously injure or even kill cows or other mammals. Prevalent in South America, the screwworm can also feed on wild animals, domestic pets, and sometimes humans.