I have two fosters right now who are going through HW treatment and they are doing well; and I have had others who have done very well.
The treatment now is different and the risks are less than years past. They usually break it up into two treatments and they give steroids which are very important to reduce inflammation. The shots are painful and they usually give pain meds for the first couple of days following the injections. Keeping them quiet is of utmost importance!
The following website has information which might help you ....
American Heartworm Society | Canine Heartworm Disease
Best wishes to you and to your little Cooper! I am sure he will do well. It is hurtful to me to hear these stories. We see so many pups with Heartworms now; and it all could be prevented for a very small amount of money.
Also important to know is to have your pups tested yearly even IF they are on preventatives. Some people skip these tests and don't realize that even IF they are on preventatives they can come up positive. Highly unlikely, but possible.
Linda