We use Heartgard as well and have since the problem with Ivermectin was discovered. At that time we had some mutts on the farm, and unsure about their parentage, we switched immediately. We test yearly and our dogs come off Heartgard Dec, Jan, Feb. It's expensive, but the alternative is an even more expensive and painful treatment for heart worm.
Thanks for the heads upon the Wormshield. I'm going to look it up right now.