The adult worms live in the heart. If the dog is infested with heartworms and giving a medicine to kill the worms (the preventive), the worms can strangle the heart. It is a very bad situation. When I was a girl, my mother didn't understand this and got one dog tested and split the meds with our other dogs. Apparently one of the dogs was positive and died very painful. It was horrible to watch :(
When we were in Ohio, if the dog was on prevention year round they tested every two years. Here in Louisiana, they test every 6 months unless on prevention year round and then once a year. Our vet informed us that even on year round prevention, many dogs will still test positive here. It is extremely bad in this area. Those preventions are guarenteed. If your dog becomes positive, their treatment is paid for. Our dogs won't be missing any treatments ;)