Some estimates show that over 5 percent of roughly 650 million military dogs deployed by US forces suffer from PTSD. The concept of canines being affected by the mental strains of battle is a relatively new one, and is up for debate. But the role these dogs play in the military has become increasingly important — dogs are considered to be the most effective means of tracking down improvised explosive devices (I.E.D's), which have become the main cause of casualties in Afghanistan over the past three years.