Heartworm in dogs is caused by Dirofilaria immitis, a parasite that lives in the chambers of the heart and in the major blood vessels. Mosquitoes transmit the heartworm microfilaria which they get when feeding off an infected dogs’ blood. Adult canine heartworms are usually lodged inside blood vessels that run between the heart and lungs. When worms exist in large numbers within the pulmonary artery, there is a consequent thickening of the blood vessel walls that can result in an increased resistance in the flow of blood. This abnormality increases the likelihood of pulmonary hypertension that can possibly end up in heart failure. The signs manifested by a dog harboring heartworms and the severity of the infection will depend on the number of adult worms which are present in the major blood vessels of the heart and lungs. Usually, the larger ... Read more »