STORY: Hummer, a young mixed-breed dog, walks around a variety of samples until he finds what he’s searching for – the scent of screwworm.The canine is part of a select group chosen for a three-month intensive training program from a special unit of Mexico's health safety agency Senasica.

TECAMAC, Mexico - With tail wagging, tongue out and ears perked, Hummer - a young mixed breed dog with a slick black coat - sits down in front of a sample of screwworm scent, promptly accepting praise and treats from his handler.

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.