70 days after a single bite from an infective mosquito, adult heartworms can reach your dog’s lungs. And then the real trouble starts: Serious and potentially fatal, heartworm disease can cause lasting damage to the blood vessels of the heart and lungs. If your dog has heartworms, you might notice signs, including coughing, difficulty breathing, sluggishness, reduced ability to exercise, and even loss of consciousness. Or you might not notice anything, as not all dogs exhibit symptoms—which is why prevention is recommended over expensive treatment that can be a prolonged, painful, and not always successful ordeal for your dog.