I have a Golden that has never been on a heartworm preventitive and I am going to bring him in next month to gt his thyroid medicine refilled. When I am there I want to ask the vet about this but I am so scared to find out he may have heartworms. He is 3 and only goes out at night to go to the bathroom. Is it possible he could have them? What is is all involved with the testing and what is a good heartworm preventitive? Does he have to get tested before he starts the medicine?