During blood feeding, the worms move out of the stylets and into the wound. In dogs and other canines, the worms molt to the adult stage and migrate to the large blood vessels and heart where mating and reproduction occur. Disease in dogs occur when the number of worms becomes sufficiently large enough to impede blood flow and heart function. In humans, immature worms frequently become encapsulated in the lungs where they are detectable by chest x-ray. Heartworm disease in cats is less frequent than in dogs.
Cats are susceptible but appear to be poorer hosts than dogs.
The most prominent clinical signs include coughing, dyspnea, vomiting, lethargy and anorexia. Acute collapse and death can occur. Development of heartworm in the vector starts when microfilariae are ingested by the female mosquito during blood feeding on an infected dog. This larval form is followed by the first stage larva and the first molt occurs in the Malpighian tubule cells at 8 days. During the second larval stage, the internal organs of the worms are formed.