According to the Food and Drug Administration, canine heartworms are not considered to be a contagious infection because they are not transmitted through direct dog-to-dog contact. As the American Heartworm Society explains, heartworms are identified as a communicable parasite because the pathogen is transmitted through mosquito bites.
The American Society for the Prevention of Cruelty to Animals (ASPCA), states that the transmission process for a heartworm infection starts when an infected dog is bitten by a mosquito. Microfilariae, an early-stage larval form of the heartworm is transfered from the misquito to the animal, and then becomes present in the infected animal's bloodstream. These microfilariae then spend a two-week incubation period inside the mosquito, molting twice before migrating to the salivary glands. By then they are in their final larval stage and can be transmitted to another animal when the mosquito bites it. Once they have been transmitted to the new host animal, they mature into adult heartworms.
According to PetMD, once a dog is infected with heartworms, the parasites can live within the lung and heart tissue of the dog for as long as seven years. Symptoms commonly associated with a heartworm infection include lethargy, coughing, nosebleeds and shortness of breath.
Both the ASPCA and PetMD note that heartworms can be prevented by giving dogs a monthly dose of preventive medicine that blocks the heartworm larvae from maturing to adults. This way, even if bitten by an infected mosquito, the dog will not develop a heartworm infection.