[theBCnut]

The amount of Ivermectin in Heartgard, if dosed properly should not be a problem for Shelties, Collies, Border Collies, etc. They can handle about 10 times the proper dose before reacting if they are positive for that particular gene, unless they are in the extreme minority. And there is a genetic test for that gene now, which affects an average of only 1-2% of the dogs within those breeds.