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, suggesting that the U.S. is exaggerating the threat to its beef industry from the parasite, the New World screwworm fly. The female flies lay eggs in wounds on warm-blooded animals, hatching larvae that are unusual among flies for feeding on live flesh and fluids instead of dead material.