Collie owners have long known that ivermectin in the treatment of helminths and mites can result in death of some Collies. Until the mutation and its frequency were discovered, the inconsistency in toxic reaction within the breed caused some confusion. Now we know that approximately 30% of collies are homozygous for the mutation, and these are the Collies particularly at risk.
Here's a blog with several articles referenced and a natural herb solution to heartworms and heartworm prevention. Read articles and natural heartworm options.