Dogs who are allowed to run or play during this time can de-velop life-threatening problems such as a stroke. Treatment makes the heartworms fragile, so the body can break them down slowly over the next 8 weeks. Allowing vigorous activity at any time in these 8 weeks can cause the weakened heart-worms to shatter, causing a clot of worm fragments that blocks off blood flow to the lungs, brain, or other organs (shaking the tree phenomenon). This causes a stroke or sudden death. Conversely, exercise restriction means the worms are slowly broken down by the body until gone, with no symptoms or problems.