Heartworms are a contagious, blood-borne parasite carried by mosquitoes.  Adult heartworms can live in your dog's heart and number from as few as 2 to several hundred.  In the early stages, heartworms are assytomatic, i.e., they show no symptoms which are chronic weight loss, coughing, gagging, tiring easily, heavy breathing and heart failure, which we can see in the latter stages.  These adult worms, which are 2-4 inches long, produce baby heartworms (microfilaria) which is the form carried by the mosquito.

There have been an increasing number of incidents of heartworm disease in cats.  However, cats typically only carry a few adult worms and no baby heartworms.  Cats experience anything from an occasional cough to asthma symptoms to heart failure as well.