We have alot of mosquitos here in Florida. Heartworm preventative is a must. It's a very ugly disease that no one wants to witness. I lost my Pittie, Bully, to it many years ago. He had it when I found him wandering through my yard one day. I rescued him, got him the treatment, but still lost him to a miserable death. It was awful. As others have said, do not give your dog heartworm preventative without proper testing beforehand. Whatever you do.....do the test first. It could lead to more severe problems or possibly death. Thank God for the newer cures for the disease. As My Peanuts said, years ago your dog would almost always die if they had a positive test. Please have your dog/s tested and start the treatment as soon as you can. It will be well worth it