Heartworm can be fatal in dogs if it goes untreated. Unfortunately, a dog can have heartworm for years without any obvious symptoms manifesting themselves, though as the heartworm gets worse, your dog's symptoms will get worse and more obvious. If you want to catch heartworm in your dog early (and you do), pay attention to the following symptoms that appear as the heartworm continues to develop.
Your dog can get heartworm from getting bitten by an infected mosquito. Once bitten, heartworm larvae get under your dog's skin and these larvae eventually travel through your dog's bloodstream to the right side of his heart. In his heart, the larvae grow into mature heartworms. The adult heartworms then create microfilaria, which are offspring that circulate back into your dog's bloodstream. In turn, the microfilaria develop larvae, which mosquitoes ingest when they bite an infected dog -- and so the cycle of infection continues.
As more adult heartworms begin to grow in your dog's heart, you'll start to hear soft, deep coughs in your dog, which will become worse as your dog exerts himself. The mass of heartworms will begin to block the right side of your dog's heart, causing slow heart failure (and potentially other organ failure, as well).
As the heartworm disease continues to grow, your dog may show signs of lethargy and weight loss, and may even start to cough up blood. As heartworm moves into its most dangerous and final phase, your dog will have trouble breathing and his chest will likely start to bulge. When these symptoms start to appear, your dog is close to congestive heart failure.
At the earliest symptoms, take your dog to the vet; he has a simple blood test that can tell if your dog is infected. You can even give your dog some preventive medicine in case you live in an area with high infection rates.