I have fostered multiple dogs afflicted with heartworm and it's terrible to keep an active animal still for months to prevent them from having a pulmonary embolism. It's widespread, difficult to cure, and otherwise fatal.

You don't note if your dog was an indoor dog, but on that front, some of us do not like to have fleas and ticks in our houses.

edit: also, if you have dog and cats, you really need to have your animals on heartworm prevention. Heartworms are much more dangerous to cats than dogs.