Poor, poor Hyper. So misguided... haha. Yes, mosquitos cause heartworms. You probably don't have much of a problem up there, but we sure do down here.
Let's say I never had Toda on Heartgard. He comes down with heartworms today. I can safely give him Heartgard for about 2 months. Heartgard retroactively treats for 2 months. Basically it will kill the heartworms up to 2 months of age then continue to PROactively treat from there on out.
BUT if a dog is seriously infested with the heartworms, then you give them Heartgard, that can be detrimental to their health. Basically before you go on preventative, you test. If your dog is positive, you pay quite a healthy sum of money for treatments. If he's negative, you go on with the Heartgard.
The whole thing for The Heartgard retroactively treating is if you use it, then forget for a couple of months, then remember to give your dog the chewie, it won't harm your dog, it will go back the couple of months, kill any heartworm activity and continue to prevent from there.
Hopefully that made sense.