Heartgard doesn't work

My dog has been taking Heartgard well over a year, this medication is supposed to prevent a variety of parasites, including roundworm. While my dog was on Heartgard, he contracted roundworm. Heartgard failed to prevent this from happening.

In the company's defense, they are offering to pay for the treatments made by the vet in order to remove the roundworm from my dog. However, this shouldn't have happened in the first place.

I pray that whoever reads this, does more research into the various prevention medicines out there for their pets, before deciding on one that doesn't work. The last thing any pet owner would want, is for something bad to happen to their beloved animal(s).