I know the collie breeds can't have ivermectin and I remember hearing that Australian Shepherds can't either. So, is it dogs with the merle gene? I'm asking because I want to start my daschund on ivermectin heartworm preventative, but he is a double dapple. The dapple gene is the same thing as merle in collies and aussies. I don't want to take any chances with him. I know there are a couple of products that don't contain ivermectin, but they aren't as effective and atleast one of them causes sickness in alot of dogs.