The U.S. has closed its southern border again to livestock imports, saying a flesh-eating parasite has moved further north in Mexico than previously reported. Mexico's president was critical Thursday,

The sterile fly technique, first used by the U.S. in the 1960s and now in Panama, prevents female screwworms from reproducing by mating them with sterilized males. The United States is hoping to stop a flesh-eating parasite from crossing the Mexican border with the aid of a few million flies.

The U.S. has shut its southern border to livestock imports again after a flesh-eating parasite was found further north in Mexico than previously reported, raising alarm among American agriculture officials.