The reason for testing your dog for heartworm disease each year is simple: you cannot be 100% sure that all the monthly heartworm pills stayed in your pet long enough to be fully digested. All of us have experienced our dogs vomitting for no reason. They eat grass, garbage and toys, so who knows when you may give your pet his/her pill and moments later, without your knowledge, he spits it up and is now unprotected. By testing annually we cover the bases and know for sure that your pet is not infected with heartworm disease. Another reason is that most of the heartworm preventions on the market today carry a guarantee. That is, they will pay for your pets treatment if it becomes infected with heartworm or intestinal parasites...THE CATCH?....you have to have proof from your veterinarian that an annual heartworm test and fecal test were preformed.