Dogs could get sick (vomiting, diarrhea, and/or death) if placed on heartworm prevention when they have heartworm disease. Even if they have been on heartworm prevention year-round, there is always the possibility that the product may have failed for various reasons; the earlier we can treat your pet for heartworm disease, the better the prognosis. All heartworm medication manufacturers guarantee their product, providing you use the heartworm prevention year-round and are performing yearly heartworm tests.
When starting heartworm prevention, or if your pet has not been on heartworm prevention year-round, it is important that you perform a heartworm test six months after starting the prevention to rule out the pre-patent period. The pre-patent period refers to the time in which a dog has early developmental larvae which cannot be detected on a heartworm test, even though your dog is already harboring heartworm infection. If you do not do this, it is possible the product manufacturer may not cover your pet’s treatment should they test positive for heartworm disease in the future.