Dogs can become extremely sick if placed on heartworm prevention while they have a severe 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 (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.) and the earlier we can treat your pet for heartworm disease, the better the prognosis. Some companies will guarantee their product providing that you use the heartworm prevention year-round and are performing yearly heartworm test.
