There is a huge poster at my vet's office about Heartguard and indoor cats getting it. Our vet inforces Heartguard strongly!

Mosquitoes get in your house...they feed on your cat...your cat gets heartworms.

According to that poster, Wisconsin is in the high-ranking spot for indoor cats and heartworms. I can't remember exactly what it said, next time we to go the vet (June 5th), I'll take another look.