Heartworm, or Dirofilaria immitis, is a potentially dangerous parasitic worm spread by mosquitoes. Dogs are the most common hosts for heartworm, although it is also found in cats and other animals, including humans. It commonly lives in the right side of the heart as well as the lungs and the pulmonary arteries. In cats, heartworm is usually only found in the lungs.
Cats are initially more resistant to heartworm than dogs.
Outdoor male pets are much more likely to be infected than indoor pets. Larvae from an infected female mosquito are deposited in the skin usually where the coat is thinnest. They burrow into the animal and change in form, moving into the veins and eventually the heart. In three to four months adult worms emerge. They can survive for about five years in a dog's heart and reach lengths of twelve inches.