Heartworms (or even a single adult heartworm) can cause a lot of damage to an infected dog. Adult heartworms can grow as long as twelve inches when they are full grown. At that point, they live in the heart and lungs which cause heart and lung failure.
The treatment can be just as deadly. When the adult worms are killed, they travel throughout the bloodstream and can cause all kinds of problems. For this reason, dogs who are receiving treatment need to be monitored closely. There are times when a dog may need additional treatment to get rid of all of the heartworms.
It is much easier to test your dog for heartworm and use a preventative than to lose your dog to this deadly, but easily preventable, disease.