More than 30 men and women are competing in the 51st Iditarod Sled Dog Race in Alaska this year. In the competition that officially started on Sunday, the competitors have to cover a distance of more than 1000 kilometers for more than a week. Iditarod is considered the toughest dog sled race in the world. This year, with 33 teams - led by eight women and 25 men - it is the smallest field in history after several previous champions retired. The defending champion is the American Brent Sass. The Iditarod leads through the Alaskan wilderness - often in snow storms and extremely sub-zero temperatures. Established in 1973, the competition takes its name from a river and now-abandoned place in western Alaska. The race commemorates a dog-sled expedition in 1925 to bring vaccine to Nome to combat an outbreak of diphtheria. Goods and mail were also transported along the Iditarod Trail in the 20th century.