Dogs could get sick or even die if placed on heartworm prevention when they have heartworm disease, with symptoms including vomiting and diarrhea. Even if they have been on heartworm prevention year-round there is always the possibility that the product may have failed for various reasons (your pet spit out the pill, did not absorb the pill appropriately, topical medicine was not applied properly, forgot to administer medication on time, etc.). The earlier we can treat your pet for heartworm disease the better the prognosis. When starting heartworm prevention, it is important that you perform an initial heartworm test.