Heartworm disease is spread by a bite from an infected mosquito. It doesn’t matter whether the mosquito finds your dog outside or on your sofa. Additionally, most dogs are exposed to mosquitos, especially during morning and evening walks. Therefore, you should have your dog tested for heartworm and if the test is negative, immediately begin preventive treatment. Most heartworm preventives also protect against intestinal parasites, which your dog can get from the feces of an infected animal. Better safe than sorry.