Australian shepherds, border collies, shelties, and any mixed breeds that have these
dogs in their genotype are susceptible to ivermectin
toxicity. Without going into confusing specifics, Ivermectin is able to cross the blood brain barrier in collies and collie crosses, which causes Ivermectin toxicity. Since collies almost all have white feet, vets used to have an adage they went by that said "white feet, don't treat".