A dry unproductive cough. Once heartworms make their way into the lungs and begin reproducing in the lungs and surrounding veins, your dog will begin exhibiting a dry cough, typically after exercising. Inactivity or laziness, if your pet seems more tired than usual, doesn’t want to go outside, or partake in physical activity, there is a good chance that your pet is sick. Weight loss and loss of appetite, if you notice weight loss and a lack appetite in your dog, then you should take him to the vet immediately to rule out heartworms and other illnesses. Shallow and rapid breathing, when worms inhabit your dog’s lungs and the surrounding veins, respiratory problems will also occur. Distended chest, a swollen, bulged, or distended chest is typically a symptom of an adult heartworm infection. Allergic reactions, while symptoms of allergic reactions are more common in cats with heartworms, dogs may also sometimes exhibit asthmatic symptoms similar to an allergic reaction in response to heartworm infection. Collapsing or fainting, once a large population of heartworms enter the heart and causes a blockage of blood flow, known as caval or vena cava syndrome, fainting will occur.