One thing we do know that heartworm treatment is costly and not without dangers:
http://pets.webmd.com/dogs/treating-...m-disease-dogs
For me, one good reason for the yearly blood test is to see if my dog has the early stages of heartworm( no symptoms), and it can then be dealt with early.
HEARTGARD Plus used monthly as directed can help prevent heartworm infection in your dog , but not a perfect product.