What are signs of heartworms in dogs?

Puppies are usually not even tested for heartworms if they are under six months of age; vets will begin heart worm preventive meds then (also a good age for spay/neuter). Unfortunately, older dogs that do have heartworms will not show symptoms until the parasites are well established within the heart, such as coughing, malaise, rapid breathing, etc. The actual blood test is for the larvae produced by adult heartworms. A puppy that is infested with other worms (round, whip and tapewoms) may have similar symptoms such as the rapid breathing and tiring easily, and will also have very pale gums due to the anemia from blood loss because of the parasites. Hope this helps with your question!