All of my animals are on heatworm preventatives and I will tell you why. I worked for a vet for two years and I saw a LOT of dogs diagnosed with HW disease. Some of them, by the way, were not diagnosed until after they were dead--a horrible way to die in case your wondering. Others were diagnosed when the owners brought them in for a complaint like coughing or exercise intolerance, or when an owner came in for a routine annual exam and requested a HW test. The animals who were treated had to endure a round of poison to their system about 10 times as bad as any HW prevention out there. We kept them overnight because it often made them very ill--some dogs with the worst cases of worms would even go into seizures. Even the ones who handled it well had to endure a lot. HW treatment is a time consuming process that involves treating, testing, retreating, retesting for several weeks, during which time exercise must be restricted. It involves blood draws--not just the tiny amount needed for a standard Occult test, but a testtube full (what they call a "knox" test, which was once the only way to diagnose the problem.) Dogs with HW disease are not always healthy even after successful treatment--the worms can permanantly damage the heart, causing complications which last for the rest of their life. A life which, by the way, may now be shortened significantly.
I had the unhappy experience of watching a dog die from HW disease once. The owner was playing ball with the dog (a boxer) when he just keeled over and began gasping. By the time he reached the clinic the dog had foam and blood pouring from his nose; he was choking; his tongue was blue. We put him on oxygen, but his lungs were so filled with fluid from his heart failure it didn't help. He died. When the doctor performed a necropsy he found a heart completely filled with worms--like the picture Jean posted. The dogs owners requested that we keep the heart and put it on display...to educate people.