This thread help you:
http://www.yorkietalk.com/forums/gen...edication.html
Ivermectin (Heartgard, Triheart and a couple other brands) shouldn't be given to Collies-type breeds because they hae a sensitivity to it. It can be argued that the dose in heartworm meds is so low that it won't hurt them but if I had a Collie, I'd use something else.